WO2002063768A1

WO2002063768A1 - Circuits with dynamic biasing

Info

Publication number: WO2002063768A1
Application number: PCT/US2001/003852
Authority: WO
Inventors: Nagendra Krishnapura; Yannis P. Tsividis
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 2001-02-05
Filing date: 2001-02-06
Publication date: 2002-08-15

Abstract

Techniques are provided for the implementation of dynamically biased circuits. In these circuits, bias currents are varied according to signal amplitude. Benefits include reduced power dissipation, reduced noise, and increased dynamic range. The techniques can be employed in various types of circuits such as, for example, amplifiers, log-domain circuits, and filters.

Description

CIRCUITS WITH DYNAMIC BIASING

TO ALL WHOM IT MAY CONCERN:

Be it known that WE, NAGENDRA KRISHNAPURA and YANNIS P. TSINIDIS, citizens of INDIA and GREECE, respectively, whose post office addresses are 722 Willow Avenue, Apt. 1, Hoboken, NJ 07030; and 410 Riverside Drive, Apt. 52A, New York, NY 10025, respectively, have made an invention entitled

CIRCUITS WITH DYNAMIC BIASING

of which the following is a

SPECIFICATION

BACKGROUND OF THE INVENTION [0002] In order to conserve energy in electronic circuits, particularly in battery-operated electronics, it is preferable to use bias currents which are no larger than necessary. Therefore, because the minimum required bias current tends to depend on signal amplitude, it is often desirable to use actual bias currents which are dependent on the amplitude of the signal. An additional advantage of amplitude-dependent biasing is that, if the bias current is only as large as needed, it will produce the least possible amount of noise (e.g., shot noise). These advantages have been discussed in the electronics literature with respect to at least one specific log-domain circuit. D.R. Frey and Y.P. Tsividis, "Syllabically Companding Log Domain Filter Using Dynamic Biasing," Electronics Letters, vol. 33, no. 5, Aug. 28, 1997. Amplitude-dependent biasing can used in other circuits, e.g., amplifiers. However, one potential problem is that the bias can, in some cases, interact with the signal. Accordingly, there is a need for circuits in which the bias control and the signal properties are "orthogonal" — i.e., do not interact with each other.

SUMMARY OF THE INVENTION [0003] It is therefore an object of the invention to provide a circuit which can accommodate signals of various amplitudes in an energy-efficient manner. [0004] It is a further object of the invention to provide a circuit which can accommodate signals of various amplitudes while maintaining a high signal-to-noise ratio. [0005] It is yet another object of the invention to provide a circuit which can accommodate signals of various amplitudes while avoiding excessive interaction between the bias control and the signal. [0006] These and other objects are accomplished by a circuit having a bias which can be adjusted according to a signal which is received, generated, or transmitted by the circuit. [0007] In accordance with one aspect of the invention, a signal is processed using an apparatus comprising: (1) a selected one of a class- AB circuit and a class-B circuit, the selected one having at least one input and at least one bias, the at least one input being adapted to receive at least one input signal, and the selected one being configured to process the at least one input signal to thereby generate at least one output signal related to the at least one input signal by an input-output characteristic having a crossover region which exhibits distortion; and (2) an amplitude detector configured to perform the operations of: (a) receiving the at least one input signal; (b) detecting at least one amplitude of the at least one input signal, and (c) dynamically adjusting the at least one bias in accordance with the at least one amplitude, wherein the at least one bias controls a level of the at least one output signal such that the at least one output signal avoids the crossover region.

[0008] In accordance with another aspect of the invention, a signal is processed using a filter having at least one input and at least one bias, wherein the at least one input comprises: (1) a first input for receiving a first input signal; and (2) a second input for receiving a second input signal, wherein the filter is configured to perform the steps of: (a) applying a first filtering operation to the first input signal, thereby generating a first output signal which is communicated to at least one output of the filter, the first filtering operation having a first frequency characteristic in which low frequencies are suppressed, and (b) applying a second filtering operation to the second input signal, the second input signal controlling the at least one bias, the second filtering operation having a second frequency characteristic in which low frequencies are passed, and the second input signal being adjusted in accordance with an amplitude of the first input signal.

[0009] hi accordance with an additional aspect of the invention, a signal is processed using a filter having at least one input and first and second biases, wherein the at least one input comprises first and second inputs, the first input being adapted to receive a first input signal and a first bias signal related to an amplitude of at least one of the first and second input signals, the first bias signal being for controlling the first bias, the second input being adapted to receive a second input signal and a second bias signal, the second bias signal being for controlling the second bias, the second bias signal being approximately equal to the first bias signal, and the filter being configured to filter a difference of first and second input signals, thereby generating a filter output signal.

[0010] In accordance with another aspect of the invention, a signal is processed using a combined filter comprising: (1) a first filter having a first filter configuration, a first bias input for receiving a first bias, a first input for receiving a first input signal, and a first output for providing a first output signal; (2) a second filter having a second filter configuration, a second bias input for receiving a second bias, a second input for receiving a second input signal, and a second output for providing a second output signal, the second filter configuration matching the first filter configuration, the first bias and the second bias being adjusted in accordance with at least one amplitude of at least one of the first input signal and the second input signal, and the first bias and the second bias being adjusted to be approximately equal; and (3) a combined filter output configured to provide a combined output signal comprising a difference of the first output signal and the second output signal.

[0011] In accordance with yet another aspect of the invention, a signal is processed using an apparatus comprising: (1) a first transistor, comprising a first signal-receiving terminal, a first current-carrying terminal adapted to be connected to a voltage source, and a second current-carrying terminal connected to the first signal-receving terminal; (2) a second transistor, comprising: a second signal-receiving terminal connected to the first signal-receiving terminal, a third current-carrying terminal adapted to be connected to the voltage source, and a fourth current-carrying terminal; (3) a first adjustable current source in communication with the second current-carrying terminal and allowing a first bias current to flow through the second current-carrying terminal; (4) a second adjustable current source in communication with the fourth current-carrying terminal and allowing a second bias current to flow through the fourth current-carrying terminal, the second bias current being approximately equal to the first bias current, and the first and second adjustable current sources being adjusted in accordance with an amplitude of a first input signal coupled into at least one of the second current-carrying terminal and the fourth current-carrying terminal; and (5) an output connected to the fourth current-carrying terminal.

[0012] In accordance with an additional aspect of the invention, a signal is processed using an apparatus comprising: (1) a dynamically biased signal-processing circuit having an input and an output; and (2) a feedback path providing a feedback signal from the output to the input.

[0013] In accordance with a further aspect of the invention, a signal size is detected by a detector comprising: (1) a differencing block configured to perform the operations of: (a) receiving a first input signal, (b) receiving a second input signal, and (c) generating a difference signal comprising a difference of the first and second input signals; (2) an exponentiator configured to exponentiate a signal comprising the difference signal, thereby generating an exponentiated signal, wherein an output signal of the detector comprises the exponentiated signal; and (3) a filter configured to perform low-pass filtering of a signal comprising the difference signal, thereby generating a filtered signal, wherein the output signal further comprises the filtered signal, and wherein the second input signal comprises the output signal. 14] In accordance with yet another aspect of the invention, a signal size is detected by a detector comprising: (1) first, second, third, fourth, and fifth nodes, wherein an input signal is received by the first node; (2) a first transistor, comprising: (a) a first signal- receiving terminal connected to the second node, (b) a first current-carrying terminal connected to the third node, and (c) a second current-carrying terminal adapted to receive a first bias current; (3) a second transistor, comprising: (a) a second signal-receiving terminal connected to the fourth node, (b) a third current-carrying terminal connected to the third node, and (c) a fourth current-carrying tenninal adapted to receive a second bias current, the fourth current-carrying terminal being connected to the fourth node; (4) a high-frequency shunt connected between the fourth node and a first voltage node, the first voltage node being adapted to be connected to a first voltage source; (5) a third transistor, comprising: (a) a third signal-receiving terminal connected to the fourth node,

(b) a fifth current-carrying terminal connected to the fifth node, and (c) a sixth current- carrying terminal adapted to receive a third bias current; and (6) a fourth transistor, comprising: (a) a fourth signal-receiving terminal adapted to be connected to a second voltage source, (b) a seventh current-carrying terminal connected to the fifth node, and

(c) an eighth current-carrying terminal connected to the first node. BRIEF DESCRIPTION OF THE DRAWINGS [0015] Further objects, features, and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention, in which:

Fig. 1 is a schematic diagram illustrating a transconductor having dynamic biasing in accordance with the invention;

Fig. 2 is a schematic diagram illustrating an amplifier output stage having dynamic biasing in accordance with the invention;

Fig. 3 is a block diagram illustrating a feedback amplifier having a dynamically biased output stage in accordance with the invention;

Fig. 4 is a voltage graph illustrating the use of dynamic biasing to avoid crossover distortion in a circuit in accordance with the invention;

Fig. 5 is a block diagram illustrating the use of dynamic biasing applied to a low-pass input of a circuit in accordance with the invention;

Fig. 6 is a block diagram illustrating the use of dynamic biasing applied to the input of a circuit having an internal low-pass node in accordance with the invention;

Fig. 7 is a block diagram illustrating the use of an auxiliary circuit for dynamic bias in accordance with the invention;

Fig. 8a is a schematic diagram illustrating a first-order log-domain filter in accordance with the invention;

Fig. 8b is a schematic diagram illustrating a replica of the circuit of Fig. 8a, in accordance with the invention; Fig. 9a is a block diagram illustrating a circuit having a differential output in accordance with the invention;

Fig. 9b is a block diagram illustrating a circuit having a single-ended output in accordance with the invention;

Fig. 10a is a graph of input current, and the envelope thereof, being received by a circuit in accordance with the invention;

Fig. 10b is a graph of differential output of a circuit in accordance with the invention;

Fig. 10c is a graph of voltage at a node within a circuit in accordance with the invention, wherein the circuit is dynamically biased;

Fig. lOd is a graph of voltage at a node within a circuit in accordance with the invention, wherein the circuit has a constant bias;

Fig. lOe is a graph of noise current of a circuit in accordance with the invention, wherein the circuit is dynamically biased;

Fig. lOf is a graph of noise current in a circuit in accordance with the invention, wherein the circuit has a constant bias;

Fig. 11 is a block diagram illustrating an envelope detector in accordance with the invention;

Fig. 12 is a schematic diagram illustrating a current-mode envelope detector in accordance with the invention;

Fig. 13 is a schematic diagram illustrating a current mirror circuit in accordance with the invention: Fig. 14 is a schematic diagram illustrating a class- AB log-domain filter in accordance with the invention;

Fig. 15a is a block diagram illustrating a linear, lossy-low-pass filter;

Fig. 15b is a block diagram illustrating a companding low-pass filter having input-output characteristics similar to those of the filter of Fig. 15 a;

Fig. 16 is a schematic diagram illustrating a circuit including the envelope detector of Fig. 12 coupled to a current mirror circuit in accordance with the invention;

Fig. 17a is a schematic diagram illustrating a band-pass filter;

Fig. 17b is a schematic diagram illustrating a band-pass filter having an auxiliary input for the introduction of dynamic bias in accordance with the invention;

Fig. 18 is a graph of simulated frequency response of the band-pass filter of Fig. 17b;

Fig. 19 is a schematic diagram illustrating an exemplary Tow-Thomas biquad circuit having band-pass and low-pass outputs;

Fig. 20a is a block diagram illustrating a log-domain filter;

Fig. 20b is a block diagram illustrating a log-domain filter with an input stage omitted;

Fig. 21 is a block diagram illustrating the use of a multiple-stage auxiliary circuit in accordance with the invention;

Fig. 22 is a schematic diagram illustrating a compensation circuit in accordance with the invention;

Fig. 23 is a schematic diagram illustrating a feedback arrangement in accordance with the invention; Fig. 24 is a schematic diagram illustrating an amplifier in accordance with the invention; and

Fig. 25 is a schematic diagram illustrating a log-domain filter in accordance with the invention.

[0016] Throughout the figures, unless otherwise stated, the same reference numerals and characters are used to denote like features, elements, components, or portions of the illustrated embodiments. Moreover, while the subject invention will now be described in detail with reference to the figures, and in connection with the illustrative embodiments, changes and modifications can be made to the described embodiments without departing from the true scope and spirit of the subject invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION [0017] Fig. 1 illustrates an example of a transconductor circuit which is dynamically biased in accordance with the invention. The circuit of Fig. 1 is powered by voltage sources Vcc and VLI_OO- It is to be noted that voltage (i.e., electrical potential) is inherently relative, and accordingly, the term "voltage source," as used herein, is defined to include ground (i.e., a voltage source producing a voltage of zero). In particular, in the circuit of Fig. 1, either of Vcc and V_LIOO can be a connection to ground. The same is true for at least one voltage source in each of the circuits disclosed herein. [0018] In the circuit of Fig. 1, transistors Q o₂ and Q o₄ form a current mirror which sends current through transistors O and Q₂, respectively. The emitters of transistors O and Q₂ are connected by a resistor R₁₀₆. Each of transistors Q and Q₂ is biased with a bias current IE which flows through its current-carrying terminals — specifically its emitter and collector. The transconductor of Fig. 1 is operated in a differential mode in which the input voltage N is applied across the respective signal-receiving terminals (i.e., the base terminals) of the transistors Q and Q₂. The transconductor produces an output

current Io-

[0019] If the transconductance of the bipolar transistors is much larger than

the transconductance of the stage, L/Vi, becomes approximately equal to l Rιo₆, independently of I_E- Accordingly, I_E can be set at the minimum value required for a given signal. Specifically, a high value of i_Ecan be used for large signals, and a low value of I_E can be used for small signals.

[0020] In order to establish the most suitable bias of a signal processing circuit — such as, for example, a transconductor, an amplifier, or a filter — it can be desirable to base the bias upon a signal representing the amplitude or envelope of the signal being processed. The amplitude or envelope signal can be received from an external source, or can be generated using an envelope detector. A low-pass-filtered rectifier, well-known for use in many other applications, is one example of a circuit which can be used as an envelope detector.

[0021] It is to be noted that the circuit of Fig. 1 can also be reconfigured to have a topology in which the current sources I_E are connected to Ncc, the current mirror is connected to N_Lioo, the ΝPΝ transistors are replaced with PΝP transistors, and the PΝP transistors are replaced with ΝPΝ transistors.

[0022] The technique of the invention can also be employed in the output stage of an amplifier, an example of which is illustrated in Fig. 2. The output stage of Fig. 2 is powered by two voltage sources V_DD and Vuoo- The circuit includes p-channel field effect transistors ("FETs") F₂₀₂ and F₂₀₄ which serve as current sources and are controlled by a bias voltage Vc. The bias voltage Vc is applied to the signal-receiving terminal — in this case, the gate — of each of p-channel transistors F202 and F₂o₄. The bias currents flowing through the current-carrying terminals — in this case, the sources and drains ■ — of p-channel FETs F₂₀₂ and F₂₀₄ are fed into respective drain terminals of n-channel FETs F₂₀₆ and F₂₀₈. The gates of n-channel transistors F₂oe and F₂₀₈ are connected by a resistor R₂₁₀. The gate and drain of FET F₂₀₆ are connected together. An input voltage V; is coupled to the gate of n-channel FET F₂₀₈ through a capacitor C₂₁₂. An output voltage Vo and an output current Io are generated at the connected drains of n-channel transistor F₂₀₈ and p-channel transistor F₂₀₄. In accordance with the invention, the bias voltage V_c can be adjusted according to the input signal, such that F₂₀₂ and F ₀₄ produce higher bias currents for larger signals and lower bias currents for smaller signals. 23] One method of feeding a signal into a circuit is through alternating current ("AC") coupling — for example, through a capacitor, as illustrated in Fig. 2. However, other techniques can also be used. In the circuit of Fig. 2, the transconductance of the stage will depend on the bias current. If there is high gain in front of this stage, and the entire circuit is operated in a closed-loop (ie., feedback) mode, such bias-dependent transconductance need not have a large effect on the transfer function of the entire circuit. In addition, the stage can be reconfigured by using p-channel FETs in the circuit mirror and n-channel FETs to control the bias current; in such a reconfigured circuit, the sources of the p-channel FETs of the current mirror would be connected to Nj_d, and the sources of the n-channel biasing FETs would be connected to V_L2oo- [0024] In accordance with the invention, dynamically biased circuits can be designed as shown in Fig. 3. In the circuit of Fig. 3, an input signal u₃ passes through the positive input of a differencing block 302, from which the difference signal c ₃ passes to a gain stage 304 where it is amplified to produce an amplified signal w₃. In this example, the

gain stage 304 is assumed to have a very large gain — ideally oo. The amplified signal

w>₃ enters a dynamically biased circuit 306 which generates an output signal^. The bias of the dynamically biased circuit 306 is controlled by a bias control 310. A feedback path 308 connects the output of the dynamically biased circuit 306 to the negative input

of the differencing block 302. The difference signal f₃ seen by the gain stage is u_ - βy₃,

where β is the feedback factor (β < 1 for an amplifier). In the steady state of the feedback

loop, the difference signal dι = u_ — βy₃ is 0. This implies that the output attains a value

y_ =^■ w₃/β. It is to be noted that the value of ys is independent of any quantity other than

the input w₃. Therefore, it can be seen that the bias of the dynamically biased circuit has no effect on the output. As a result, disturbances due to bias changes are reduced due to the application of feedback. [0025] In particular, when the bias of the dynamically biased circuit 306 is changed, the output tends to change. However, the changed output^, through the negative feedback loop 308 and the gain stage 304, causes a change in the input w₃ of the dynamically biased circuit 306 in such a manner as to counteract the influence of the bias 310 and restore the output^ to its original value. An advantage of the circuit of Fig. 3, as compared to a class-B circuit, is that the circuit of Fig. 3 produces no crossover distortion. [0026] There are several ways in which one may configure dynamically biased circuit topologies. For example, a low-pass class B or class AB circuit can be dynamically biased to avoid the crossover region, where large distortion usually occurs. An exemplary voltage characteristic of such a circuit is illustrated in Fig. 4. In this example, the average value of the input signal V;_n is varied, so that V_n(t) always stays clear of the high-distortion region of the voltage characteristic. By this technique, the bias is controlled to be sufficient to preserve the linearity of the circuit, but otherwise to be as small as possible so that low power dissipation — and in some circuits, low noise — is achieved.

[0027] Additional examples of circuits in accordance with the invention are illustrated in Figs. 5 and 6. The circuit 502 of Fig. 5, which can be, for example, a filter or an amplifier, is not a low-pass circuit. Therefore, because the bias tends to be slowly varying- it can be beneficial to apply the bias to a separate, low-pass input 504, and to apply the signal being processed to the main input 506, as shown in Fig. 5. The circuit generates an output signal 508.

[0028] Figs. 17a and 17b illustrate an example of atype of filter, in this case aband-pass filter, which can be dynamically biased using a low-pass input in accordance with the invention. Fig. 17a illustrates an exemplary band-pass filter having an input u,__p and an output y\η. The filter includes two resistors R₁₇₃ and R₁₇₄ and a capacitor C₁₇2 which serve as an input network. The filter also includes an amplifier 175 with a gain of -k, where A: is a positive number — e.g. , a positive integer. Feedback is provided by a feedback capacitor C₁₇₁. [0029] Fig. 24 illustrates an example of an amplifier which can be used as the amplifier 175 in the circuit of Fig. 17. The exemplary amplifier 175 includes transistors Q_{2 01} and Q₂₄o₂ and resistors R_Lι, RE_I, and R_E2- The signal-receiving terminal (i.e., the base) of Q₂₄oι receives an input voltage N₂₄. The transistor Q₂₄oι has current-carrying terminals — a collector and an emitter. The collector of Q₂₄oι is connected to the signal-receiving terminal (i.e., the base) of transistor Q_2402> and is also connected to a voltage source VH24 through a resistor R_LI. The emitter of Q ₄₀ι is connected to another voltage source V_L24 through an additional resistor REI. In the specific example illustrated in Fig. 24, _H24 has a higher voltage than V_L2₄. Q24₀1, RL_I, and R_EI form an emitter degenerated amplifier stage having a gain of-k, where k = R_LI /R_EI- The output voltage of this stage is the

collector voltage V_C240i ofQ₂₄₀₁.

[0030] In the illustrated amplifier 175, transistor Q2₄o2 and resistor R_E2 form an emitter follower stage having a gain of 1. The base of Q₂₄₀₂ receives the amplified voltage Vc₂₄oi from the collector of Q2₄oι- The collector of Q₂₄o2 is connected to voltage source VL₂₄- The emitter of Q₂₄₀₂ is connected to voltage source V_H24 through resistor R_E2. The output voltage Vo₂₄ of the emitter follower — which is also the output voltage of the entire amplifier 175 — is the voltage at the collector of Q2402-

[0031] The gain -k of the amplifier 175 does not strongly depend on the bias currents I_E24OI and IE₂4₀₂ flowing through Q_{2 01} and Q₂₄o₂, respectively. However, the bias currents I_E24oι and I_E2402 affect the size of the input voltage Vj₂₄ that can be accommodated by the amplifier 175. Furthermore, a direct current ("DC") voltage component V_mDC of the input voltage N₂₄ can affect the bias currents IE_24OI and I_E2402, as is demonstrated below.

[0032] The bias current I_E24oi flowing the Q2401 is: IE2401 - (VinDC - Nbe24)/ E1, where V_be₂₄ is the base-emitter voltage of the transistors Q2401 and Q₂₄₀2- The bias current

IE2402 flowing through Q_{2 02} is:

ΪE2402 ⁼ [(VinDC" V e24)R l EI ~ Vbe24]/ E2-

[0033] It can thus be seen that the bias currents IE2401 and L3₂40₂ of the amplifier 175 can be

controlled by adjusting the DC component VJ_ΠDC of the input voltage N₂₄ received by the

amplifier 175. For example, V;_nDc can be reduced if the AC amplitude of N₂₄ is small,

thereby reducing the bias currents IE24OI and IE2 02- Because of the reduced bias currents

IE240I and IE2402, the amplifier 175 has reduced power consumption.

[0034] For the application of dynamic biasing to the circuit of Fig. 17a, it might not be

effective simply to add a bias to the input U_bp, because any DC components of W_b are

blocked, by Cι ₂, from reaching the input of the amplifier 175. In fact, in the circuit of

Fig. 17a, the DC gain from the input «_bp to the output y_\η is essentially zero. However, in accordance with the invention, this circuit can be reconfigured to provide another input

through which DC signals can reach the amplifier 175. For example, as illustrated in Fig.

17b, the originally grounded end of R₁₇₄ can be disconnected from ground and used as an

auxiliary input w_lp. From this auxiliary input w_lp to the output y_\η, the filter has a low-

pass characteristic. Accordingly, a dynamic bias — which tends to be slowly varying —

can be applied to this input «_lp. Fig. 18 illustrates exemplary simulated transfer functions

of the filter of Fig. 17b, from the main and auxiliary inputs «_bp and u_\v to the output j/₁₇.

The component values used for the circuit simulation of Fig. 18 are: R ₇₃=R₁₇₄=lkΩ,

C₁₇₁=Cι₇₂=0.1 μF, and k=8. As can be seen from Fig. 18, the transfer function from the input U_bp of the filter to the output ₁ vanishes at both low and high frequencies. However, the transfer function from the auxihary input u_\v to the output y_η vanishes only for high frequencies. Specifically, in this example, it can be seen that the DC (i.e., very low frequency) gain is essentially zero for the main input U_bP and 1 for the auxiliary input

U_\v.

[0035] If a circuit comprises a low-pass circuit 602, as illustrated in Fig. 6, it is possible to have the same input 604 for both the bias 610 and the signal 612, yet separate outputs 606 and 608. The bias 610 and the signal 612 can, optionally, be combined using a voltage adder 614, to thereby generate the input signal 604. Furthermore, although the intended output of the circuit 602 may not be low-pass, some internal portions of the circuit can, in some cases, be adjusted even if the bias control is itself low-frequency. Such a technique can be used, for example, in topologies derived from the Tow-Thomas biquad.

[0036] An example of such a biquad circuit is illustrated in Fig. 19. The circuit receives an input voltage w₁₉ and generates aband-pass output voltage y__v and a low-pass output voltage y_\p. The input signal w₁₉ is fed through an input resistor R^ to the negative input terminal of a first amplifier 1918, which produces the band-pass output voltage - A feedback circuit including a resistor R₁₉o₂ and a capacitor C₁₉₁₄, connected in parallel, provide coupling between the output and negative input of the first amplifier 1918. The band-pass output signal > _p is fed through a resistor R₁₉₁₂ into the negative input of a second amplifier 1920, which generates the low-pass output voltage y\_p. A feedback capacitor Cι₉₁₆ connects the output and negative input of the second amplifier 1920. [0037] Finally, a feedback circuit connects the low-pass output y_\v with the negative input terminal of the first amplifier 1918. The feedback circuit includes a third amplifier 1922 and three resistors R₁₉ιo, Rι₉o₈, and R₁₉o₆. Resistor R₁₉ιo connects the low-pass output y_p with the negative input of the third amplifier 1922. The output of the third amplifier 1922 is connected, through R₁₉₀₆, to the negative input of the first amplifier 1918. Resistor R₁₉₀₈ connects the output and negative input of the third amplifier 1922. It is to be noted that any or all of the amplifiers 1918, 1920, and 1922 shown in Fig. 19 can comprise the amplifier 175 illustrated in Fig. 24. The amplifier 175 of Fig. 24 has been discussed extensively above for use in the circuits of Figs. 17a and 17b.

[0038] In accordance with the invention, a dynamic bias can be applied to the band-pass output — which can also serve as a low-pass input — of the circuit of Fig. 19. Such a technique allows adjustment of the low-pass portion of the circuit (which includes the second amplifier 1920), thereby providing benefits such as increased energy efficiency, reduced noise, and increased dynamic range, as discussed above.

[0039] In some filters it may not be possible to adequately control multiple points within a filter from a single bias input, because the individual transfer functions of various portions of the circuit may be different. In such a case, it can be beneficial to use an auxiliary circuit such as shown in Fig. 7, and feed individual bias control signals 706 to multiple points in the main circuit. The auxiliary circuit 702 can be approximately similar to the main circuit 704. Individual envelope or mean value extraction circuits can, optionally, be used to generate the various outputs 706. The auxiliary circuit 702 can, optionally, be a low-pass equivalent of the main circuit 704, and can be fed by the envelope (or mean value, etc.) of the input 708, such that the individual bias control signals 706 are delayed by suitable amounts before being fed to the main circuit 704. In particular, if the main circuit 704 is a filter, there are typically phase shifts at the various internal nodes. The auxihary circuit 702 preferably mimics these phase shifts such that the bias control signals 706 adjust the respective internal nodes of the main circuit 704 using the correct phases.

[0040] Fig. 21 further illustrates the use of such an auxiliary circuit. The auxiliary circuit 702 of Fig. 21 includes multiple stages 2102 which can, optionally, be essentially identical to the multiple stages 2106 of the main circuit 704. Each of the stages 2102 of the auxiliary circuit produces an output signal 2108 which can be essentially identical, in both amplitude and phase, to the intermediate signals 2110 present between the respective stages 2106 of the main circuit 704. Each of the output signals 2108 is sent into its own envelope detector 2104 which generates a bias control signal 706 for the appropriate portion of the main filter 704. Because the auxiliary circuit 702 matches the main circuit 704, any phase or time shifts present in the main circuit 704 are also present in the respective outputs 2108 of the auxiliary circuit 702. As a result, each of the bias control signals 706 is phase or time shifted by the proper amount.

[0041 ] It may be desirable to use two matching versions of a signal-processing circuit, each fed by different polarity signals, with the outputs of the two versions being subtracted so that the bias component cancels out, as described in further detail below with respect to a particular log-domain circuit. In another embodiment, the signal and bias can be fed to one circuit, while the second circuit receives only the bias.

[0042] In accordance with an additional embodiment of the invention, an externally linear time-invariant filter — which can be internally non-linear — can be biased dynamically (i.e., variably) in accordance with the signal so that large signals do not overload the filter, and small signals are not buried under noise. For example, a log-domain filter can be biased in such a manner, and dynamic biasing can be used for other types of filters as well.

[0043] Fig. 8a illustrates an example of a first-order, log-domain, low-pass filter. Such a filter generally operates by performing a logarithm operation upon an input signal, filtering the resulting logarithmic signal, and performing an exponential (i.e., anti- logarithm) operation upon the filtered signal to restore the filtered, logarithmic signal to an output signal which is linearly related to the input signal. A log-domain filter is considered a "companding" filter because it first compresses the signal and then expands it. Generally, companding filters are internally non-linear, yet they can be designed to be externally linear — i.e., the output being linear with respect to the input.

[0044] The concept of companding is further illustrated by Figs. 15a and 15b. Fig. 15a illustrates an exemplary linear first order filter. In the filter of Fig. 15a, an integrator 1502 having a gain constant k is connected in a negative feedback loop with an amplifier 1504 having a gain of a/k. Negative feedback is provided using a differencing block 1506. The resulting circuit is a low-pass filter having the following transfer function:

_ffW.ZC!⁾.JL

U(s) s + a which generally describes a low-pass filter having a bandwidth of a rad/s. Fig. 15b illustrates a general companding equivalent of the low-pass filter in Fig. 15a. A nonlinearity block 1512 having a non-linear function f(v) is used to provide the output y, and an amplifier 1508 having a gain of l/f '(v), where/ '(v) is the derivative of/fv , is used at the input. f(v) serves as an expander,^" and the amplifier with gain l/f'(v) serves as a compressor. For example, in a log-domain filter, f(v) would be an exponential function. The compressor and expander, together with a modified feedback path 1510, form a low- pass filter that is equivalent to the linear filter of Fig. 15a and realizes the transfer function H(s) given above. The relation between the input u and the intermediate variable v is nonlinear in Fig. 15b.

[0045] In the case of the circuit of Fig. 8a, assuming that all of the transistors are ideal (i.e. , that their base currents are zero or negligible), the input portion of the circuit, formed by transistors Q_lp and Qz_v, has a logarithmic voltage/current characteristic. Specifically, the base-emitter voltage of Q_2p, N_be2p, is approximately constant, and the base-emitter voltage of Q_lp, N_beι_p, is proportional to the logarithm of the normalized input currents:

N_beip=Ntln[(iin+Ibias) Is]- Therefore, the base voltage of Q_2p, V 2_P, is:

V_b2p ⁼⁼V_be2P+V_tlni(ii_n-i-I_bi_as)/I_s], where V_t is the thermal voltage of Q2_P and I_s is the saturation current of Q_2p. [0046] The filter uses transistor Q_3p to send the logarithmic component of V_b2P into the base of transistor Q_4p. At low frequencies, the output portion of the circuit, formed by transistor Q_4p, produces a current i _p, into the collector of Q_4p, which is exponentially related to the base voltage of Q_4p : i_4p=Kexp {In [(iin+Ibi_as) I_s] } = [(ii_n+I_bias)/I_s] . where K is a constant. [0047] Accordingly, the relationship between the input signal i_in and the output signal i_outp is ultimately linear. Low-pass filtering is provided by a high-frequency shunt — in this case, the capacitor C_lp — which shorts out high-frequency signals at the base of Q_4p. Ideally, in a log-domain filter, the relationship between the large signal currents ti_p and f_4p in the input and output transistors O_p and Q _p, respectively, is linear and time invariant — assuming that t_lp is always positive. Assuming that the base currents of Q_2p and Q_3p are negligible, i_lp is the sum of an AC input signal , and a bias current /bi_as- -t_bi_as s typically constant. The output i_outp is obtained by subtracting

from _v. I__VII__V is the DC gain of the filter.

[0048] In accordance with the invention, dynamic biasing can be applied to circuits such as the filter of Fig. 8 a, by varying / i_as in accordance with the envelope of the input i_\n so that /i_as is slightly larger than the minimum value required to keep i\_v positive at all times. Such dynamic biasing lowers the power consumption and the output noise of the filter for small inputs, while enabling the circuit to accommodate very large inputs without excessive distortion.

[0049] Dynamic biasing also alters the "gain" from the input current to the internal voltages. Gain alteration has also been used for syllabic companding, which involves slowly varying the gain of an input amplifier in order to accommodate varying signal sizes and to maintain a relatively constant-amplitude output signal. However, dynamic biasing is simpler to implement than syllabic companding. On the other hand, in dynamic biasing systems, the time varying / ia_s is filtered along with the input signal, and is also included in the output signal. Accordingly, i_oυtp is no longer merely a filtered version of , but also includes a filtered version of /bia_s. Consequently, it can be preferable to use a compensation circuit for some applications, in order to compensate for the presence of the filtered Iw_as signal in the output signal. [0050] An example of such a compensation circuit is illustrated in Fig. 22. The circuit of Fig. 22 is similar to the circuit for which compensation is desired — i.e., the circuit of Fig. 8a. In particular, the compensation circuit of Fig. 22 includes transistors Q₂₂₀₁, Q₂₂₀₂, Q₂₂₀₃, and Q₂₂₀₄, which behave similarly to the transistors Q_lp, Q_2p, Q_3p, and Q_4p, respectively, of the filter of Fig. 8a. Capacitor C2210 of Fig. 22 provides low-pass filtering similarly to capacitor C_lpof Fig. 8a. However, the compensation circuit of Fig. 22 includes an additional transistor Q22oe which mirrors the current flowing through Q₂₂₀₃- The emitter current I_x of Q220₆ is fed into the node 82 (in Fig. 8a) to which the emitter of Q_3p is connected. A current source I,™ provides bias current into the collector of Q₂₂₀i-

[0051] The compensation circuit receives, into the collector of Q₂₂o_1? the envelope I_E of ij_n, rather than ij_π itself. For larger amplitude input signals, I_E increases, causing an increase in the current flowing through Q₂₂03- The increased current in Q₂₂o₃ causes an increase in the current I_x which flows into node 82 of the filter of Fig. 8a, thereby increasing the base voltage of Q_4p. As a result, the quiescent (i.e. bias) current flowing through Q_4p is increased, thereby enabling the exponentiator stage of the filter of Fig. 8a to accommodate the larger input signal ij_n which is being received.

[0052] If the amplitude of ij_n decreases, IE decreases, which reduces I_x. The voltage at node 82 drops, thereby decreasing the bias current flowing through Q _p. Consequently, power consumption and shot noise are reduced for input signals having smaller amplitudes.

[0053] Moreover, there is an additional method for distortionless dynamic biasing. In accordance with the invention, a single-ended filter such as the circuit illustrated in Fig. 8 a is duplicated. The duplicate circuit is operated with the same bias /_bi_as but an inverted input -i\_n, as shown in Fig. 8b. [0054] The duplicate circuit, an example of which is illustrated in Fig. 8b, includes transistors Q_ln, n, Q_3n, and Q_4n which correspond to transistors Q_lp, Q_2p, Q_3p, and Q_4p of the original circuit, illustrated in Fig. 8a. The circuit of Fig. 8b also includes current sources /_2n and /_3n — which are of approximately equal value to /_2P and /_3p, respectively, of the original circuit. Capacitor C_ln of the duplicate circuit is approximately equal in value to C_lp of the original circuit. The output transistor currents t_4p and z_4n in the respective filters of Figs. 8a and 8b can be written as:

*4_P( = Ci„(0 + /bias( ) * (t); i_Aτi(t) = Hin(t) + /bias( ) * ^(t), (1) where h(t) is the impulse response of each filter (i.e., the impulse response between i_\p and z^' _4p, and between ι_ln and z _n, where all base currents are assumed to be zero or negligible) and "*" denotes convolution. In the differential output tout = Up — Un, the bias dependent term /_bi_as (0 * h(f) cancels out, giving the result: tout (t) = 2tj_n (t) * h(t). The relation between t_out and i_\n is therefore linear and time invariant, and is the same (except for a factor of 2) as that between to_utp and ij_n in the original log-domain filter (the circuit of Fig. 8a) operating with a constant bias. No extra circuitry is required to compensate for the effect of I ias, because I i_as is not present in the output.

[0055] In accordance with the invention, a dynamically biased log-domain filter can be operated pseudo-differentially to cancel the effects of time varying bias, as illustrated in Fig. 9a. For example, two matching circuits 902 and 904 — which can be, for example, the circuits of Figs. 8a and 8b — can be used in the differential configuration illustrated in Fig. 9a. The input signal of such a configuration would be 2ij_n, and the output signal would be i_4P-i_4n. Such a configuration can eliminate the need to provide a bias current of

(l_2p/I_3p)Ibias into transistor Q_4p, or a bias current of ( ti Ibias into transistor Q_4n. Furthermore, structures that operate using differential input (e.g. certain class- AB circuits) can also be used in Fig. 9a. Such a differential circuit 908 is represented by the dotted lines of Fig. 9a.

[0056] In addition to cancellation of the bias dependent terms, pseudo-differential operation has benefits such as cancellation of even-order non-linearities and common mode interferences. For example, if the elements of a circuit are non-ideal — e.g., if the transistors in a log-domain filter have characteristics which deviate from ideal logarithms and exponentials — the input and bias signals can interact with the non-idealities to generate harmonics, especially even-order harmonics. Because even-order harmonics have the same sign and approximately the same values in both halves of a pseudo- differential circuit, these harmonics cancel, thereby providing improved signal quality.

[0057] Furthermore, if I_bi_as contains noise, approximately the same noise signal, with the same sign, is present in each half of the circuit. Consequently, noise signals introduced by I i_as are cancelled in the differential output. In contrast, the input signal tj_nis present with opposite signs in the respective halves of the circuit. Therefore, the input signal is -not canceled in the differential output. As a result, the circuit of Fig. 9a provides an improved signal-to-noise ratio.

[0058] In accordance with another aspect of the invention, the scheme shown in Fig. 9b, in which the second filter 904 receives only the bias signal, can be used. The technique of supplying the input signal to only one of the filters can be advantageous for applications in which single-ended input is desired. Either of the arrangements of Figs. 9a and 9b can, optionally, include a differencing block 906 at the output, which can be advantageous for applications in which single-ended output is desired. Single-ended input and/or output can be desirable for, e.g., for proper interfacing with other circuits.

[0059] The linear time-invariant relation between the input and output transistor currents in a log-domain filter enables the cancellation of time varying bias components at the output. In contrast, if time varying gains are placed before and after a classical linear filter, pseudo-differential operation does not result in a linear time-invariant system.

[0060] The base emitter voltage of Qj_p in Fig. 8a is given by: b_eι_p = V_t ln[(/^'i_n + /bia_s) /_s] • An

increase in the envelope of i_n by a factor causes bias to increase by the same factor

because /_bias is derived from the envelope of tj_n. In other words, ij„i becomes

and

I_bi_asi becomes

where ij_nι and Ibiasi are the initial values of ij_n and I_Was, and i_in2 and I_bi_as2 are the new values. Therefore, b_eι_pl (the initial value) becomes _beip2 (the new value):

[0061] It can thus be seen that F_b undergoes only a DC shift equal to V_t In (α). Because

of the linearity between ή_p and z^' _4p, it can be seen that _be4p also undergoes only a DC shift. Therefore, the AC signal applied to the voltage-mode filter 802 between the input and output transistors (enclosed by dashed fines in Fig. 8a) remains the same regardless of the input signal strength if dynamic biasing is used. This confirms the analogy of dynamic biasing to syllabic companding. Like syllabic companding, dynamic biasing also increases the dynamic range of a log-domain filter.

[0062] Fig. 14 illustrates an exemplary class- AB instantaneous companding log-domain filter which can be dynamically biased in accordance with the invention. The filter of Fig. 14 incorporates log-domain filters similar to those of Figs. 8a and 8b, in accordance with the invention. The left half of the filter includes transistors Q_lp and Q_2P which perfonn a logarithm operation upon the signal u_p entering the left half. Transistors Q_3p and Q_4p restore the left half of the signal to linearity by performing an exponential operation upon the logarithmic signal. Capacitor C_lp, which serves as a high-frequency shunt, provides low-pass filtering. Q_2P is biased with a bias current I_2p, and Q _p is biased with a bias current I_3p. The right half of the filter includes components Q_ln, Q_2n, Qj_n, Q_4n, and Ci_n which perform the same functions — in the right half — as Q _p, Q_2p, Q_3p, Q_4p and Ci_p perform in the left half. The two halves of the filter are cross-coupled using transistors Q_5p and Q_5n.

[0063] The difference current u_p-u_n is the input to the filter, and the difference current y_v-y_n is the output. The filter can operate in a class-AB mode in which the left half of the circuit handles positive portions of the input signal — i.e., when u_p is positive and u_n is negative — and the right half handles negative portions of the input signal — i.e., when u_p is negative and w_n.is positive..

[0064] Figs. 10a- 1 Of illustrate the results of a simulation, in accordance with the invention, of an exemplary pseudo-differential configuration of the low-pass filters of Figs. 8a and 8b, where the combined filter was configured to have a -3dB frequency of 100kHz (/_2p = /_3p = 1 μA, C_lp = 61.5 pF). The input was a sinewave with a changing envelope (Fig. 10a). The circuit was simulated in two different modes of operation: (i) with a dynamic bias 10% larger than the changing envelope, and (ii) with a constant bias 10% larger than the largest envelope (the largest envelope being 2μA, as illustrated in Fig. 10a). The constant bias case corresponds to classical class- A operation. The outputs of the filter in the two different modes are plotted in Fig. 10b and are identical in this simulation. Figs. 10c and lOd show the base emitter voltage of Q_4p (a voltage internal to the filter) in the two cases. Syllabic companding is clearly seen in Fig. 10c — the internal voltage swing is constant regardless of the input amplitude. With a constant bias, the amplitude of the internal voltage varies with the input current, as can be seen in Fig. lOd. The results of transient noise simulations are shown in Figs. lOe and lOf. It is evident from these figures that dynamic biasing provides noise reduction for small input signals. These results demonstrate the external linearity and syllabic companding nature of the dynamically biased filter. 65] The technique of the invention provides several advantages over conventional circuits. First, compensation circuits used in conventional circuits require extra design effort, in some cases as much as for the main filter, and add to the power consumption and noise of the overall filter. In contrast, the method of the invention — illustrated by example in Fig. 9a — has no additional design overhead because the required filter is simply duplicated. Furthermore, the technique of the invention introduces no additional power consumption or noise because there is no "extra" circuitry other than the filter used to differentially process the input signal. [0066] Fig. 25 illustrates an additional example of a log-domain filter which can be dynamically biased in accordance with the invention. Similarly to the log-domain filter of Fig. 8a, the log-domain filter of Fig. 25 can be used in one or both of filter blocks 902 and 904 of the circuits of Figs. 9a and 9b. The filter of Fig. 25 receives an input signal «_25θo which, if the filter is used in block 902 of one of the circuits of Figs. 9a and 9b,

equals ii„ + Transistors Q₂₅oι, Q2502, and Q₂₅03 are biased with currents Ibias, l2S02, and I₂₅₀₃, respectively. The output signal ^₅₀₀ is the collector current of transistor Q₂₅o₄- The emitters of Q2S01 and Q2₅o3 are connected to each other, as are the emitters of Q2so2 and Qj₅₀4- A bias voltage -sbias fixes the base voltages of Q2501 and Q2so4- The bases of Oj₅₀₂ and Q₂₅o3 are connected to each other, and are also connected to V₂₅bias through a capacitor C2₅₁o-

[0067] A FET F₂₅o₆ is used, in a feedback arrangement, to control the current flowing through Q2₅oι- The FET F2₅o₆ serves as a regulated current source. The source terminal of F₂₅o₆ is connected to a voltage source V_L25- The drain of F2so₆ is connected to the emitter of Q2₅oι- The gate of F2₅o6 is connected to the collector of Q2₅oi- If Q2S01 is in a region of its operating characteristic — i.e., its current- voltage characteristic — in which its collector current would tend to exceed w₂₅oo₅ the collector voltage of soi drops, causing the gate voltage of F ₅₀₆ to drop. The drop in gate voltage causes the drain current of F₂₅o₆ to decrease, which increases the

The increase in emitter voltage .decreases the .base-emitter voltage V_b-asoi of Qjsoi, which tends to cause a decrease in the collector current of Q₂₅oι- If, on the other hand, Qjsoi is in a region of its operating characteristic in which its collector current would tend to be less than soo, the opposite result occurs: Vbe25oι is increased, which tends to cause an increase in the collector current of Q₂₅oι- In equilibrium, the collector current and base-emitter voltage V_be25oι of Q2₅oi are thus regulated to maintain the transistor Q2501 in a region of its operating characteristic in which the collector current of Q₂₅oι is exponentially dependent

upon the base-emitter voltage Vbe25oι, and in which the base-emitter voltage Vb_e25oi s logarithmically related to the collector current of Q2501.

[0068] As a result, Q2₅₀₁ performs a logarithm operation on M2_S00- thereby generating V_be250i- Because the base voltage of Q₂₅oι is fixed by V2Sbias, the resulting logarithm signal is present at the emitters of Q₂₅₀1 and Q2₅03- Because the base and collector of Q₂₅₀₃ are connected together, Q250₃ acts as a diode which communicates the logarithm signal to the base of Q₂₅o₂- High-frequency signal components are suppressed by a high- frequency shunt — in this case, capacitor C2₅₁o — connected between the base of Q2₅o₂ and voltage source N₂sbias- Q2S02 is biased by a current l2₅02- The collector current and base-emitter voltage of Q₂₅₀₂ are regulated by a FET F2₅o₈ which operates similarly to the FET F₂₅o₆ which regulates the collector current and base-emitter voltage N_be2soι of Q₂₅oι-

[0069] Transmitter Q₂₅o₂ communicates the low-pass-filtered, logarithm signal from the base of Q₂₅02 to the emitter of Q25₀2, this emitter being connected to the emitter of Q₂₅o₄- Because the base voltage of Q ₅o₄ is fixed by voltage source N₂₅bi_as5 the filtered, logarithm signal is induced in the base-emitter voltage N_be2S04 of Q2₅04- Because the output signal ^₂₅₀₀ is exponentially related to Nbe2₅04, transistor Q2₅o4 exponentiates the filtered, logarithm signal whichispresent in Vbe250₄, thereby restoring the signal to linearity. Consequently, jV2₅oo is linearly related to «2₅oo- The transfer function between ^₂₅oo and

«2₅oo is:

H₂50θ(s) = Y250θ(s)/U₂50θ(s) = (l2₅02/T2503) (l + sC 51θV_t/l2₅03)₅ where V is the thermal voltage of the various transistors in Fig. 25.

[0070] Even if the current gains of the transistors are finite, the only base current that significantly affects the operation of the circuit is that of Q2so₂- However, the base

current of Q2₅₀₂ is a constant 1₂₅02 β which is subtracted from L:₅0₃. Consequently, no significant additional nonlinearity is introduced into the circuit, and the only effect of the

finite β — assuming that β is constant with respect to the collector currents — is a

reduction of the bandwidth of the filter. If this bandwidth reduction is undesirable, it can

be counteracted either by injecting a current 12502 β, as illustrated in Fig. 25, or by using automatic tuning techniques known to those skilled in the art.

[0071] Instantaneous companding via class- AB or class-B operation is another technique which has been used to realize high dynamic range log-domain filters. In this technique, a differential filter receives an input signal which equals the difference of half- wave rectified or geometrically split currents.

[0072] However, the technique of the invention provides several advantages over class- AB instantaneous companding. For example, in a preprocessing circuit in accordance with the invention, the accuracy of the envelope detector is less important, provided that its output is larger than the actual envelope. In contrast, a class-AB splitter generally must accurately reproduce the input signal in the splitter's difference output in order to avoid added distortion. For at least this reason, the envelope detector of the invention is simpler to design than a^'class-AB splitter." Furthermore, inxonventional-circuits; mismatch of circuit elements can lead to distortion because of internal non-linearity (in class-AB filters) and incomplete cancellation of bias components (in dynamically biased filters). For example, various frequency components of the input signal can interact with circuit nonlinearities to cause intermodulation distortion, i.e., spurious signals at various sum and difference frequencies of the various frequency components, til fact, in a conventional companding filter, if internal components deviate from their ideal nonlinear (e.g., ideal logarithm or ideal exponential) characteristics, such deviation can also result in distortion. In contrast, circuits in accordance with the invention tend to produce slowly varying bias components which, in many cases, can be more acceptable than intermodulation distortion. In addition, noise from the envelope detector of the invention cancels at the output of the filter. In contrast, the two outputs of a conventional class-AB sphtter contain noise in opposite phases of the input for large signals; such noise does not cancel at the filter's output, and the uncanceled noise can degrade the signal-to-noise ratio of the filter.

[0073] The bias /_bias in Figs. 8a, 8b, 9a, 9b, and 25 can be generated using an envelope detector which can be, for example, a current mode envelope detector in accordance with the invention. Fig. 11 provides a block diagram of such a circuit. The output y of the detector is subtracted from the input u of the detector using a differencing block 1106. The output u-y of the differencing block 1106 is fed into an exponentiating block 1102 to

produce an exponentiated error u_. A low pass filter 1104 having a cutoff frequency ω_p

filters the exponentiated error U_f to produce the output y.

[0074] To better understand-the-operation of-the envelope detector-of Fig. 11 , it is useful to consider a case in which the input u is a sine wave having an angular frequency much

larger than ω_p, and the output; is less than the envelope of u. During the portions of the input cycle in which u exceeds v, the output «_f of the exponential becomes extremely large. Because of the large signal entering the low-pass filter, the output; rapidly increases to reach u. As the cycle proceeds, the input u falls below the output . The exponentiating block 1102, whose input u-y is now negative, reduces its output U_f to a very small value, close to zero, which in turn causes the output of the low-pass filter 1104 to drop exponentially at a rate determined by its time constant. Since the low-pass filter's time constant is much longer than the input period, y does not drop appreciably in one cycle of the input u. Therefore, in steady state, the output; stays very close to the peak value of the input u, with a small drop between successive input peaks. If the input amplitude drops appreciably, the error u-y is constantly negative and the input u_ of the low-pass filter is therefore essentially zero. The output; falls exponentially until it reaches the new, reduced, peak value of the input u. On the other hand, an increase in the input amplitude causes the input U_f of the low pass filter to be very large due to the exponentiation of a positive quantity, andj therefore rises rapidly to reach the new peak value. This "fast attack" behavior is desirable, since, in a dynamically biased filter, the bias is preferably kept larger than the input in order to avoid distortion.

[0075] Fig. 12 illustrates an example of a circuit realization, in accordance with the invention, of the envelope detector of Fig. 11. In the detector 1200 of Fig. 12, the input signal and the output envelope are current-mode signals. However, voltage-mode signals can also bejreceived and-generated-by, e.g.„.adding simple. current- voltage converters. For example, a transconductor such as the circuit of Fig. 1 can be used to convert a voltage-mode signal to a current-mode signal.

[0076] The emitter voltage ^₁₂₁ of transistor Qm in Fig. 12 can be written as: _el21 = Ferr - Ftln (Il2l/Is), where I_s is the saturation current of Q121.

[0077] The circuit comprising transistors Q₁₂ι, Q122, Q123, and Q₁₂₄, the capacitor C12₆, and the bias sources /₁21./₁₂₂, and /₁₂₃ acts as a low-pass filter governed by the following equation:

dl ^L 124 ___ I______ I ^L 123 V, - V - bias

I ' x 1,24. + ^■ I_m exp ( dt C V C ^\26 V^r t v_t

[0078] The last term in the above equation denotes the input to the low-pass filter. F_en- appears in the argument of the exponential. This circuit can therefore perform the combined functions of the exponentiator 1102 and low-pass filter 1104 of Fig. 11 if V_sn is made proportional to the error between the input and the output.

[0079] The output /y₂₄ is subtracted from the input /_n at the collector node of Q₁₂₄. If /i_n is larger than/_/24, the collector voltage of Q₁₂₄ increases, and if Ij_n is smaller than I₁₂₄, the collector voltage of Q ₂₄ decreases. The voltage swing at the collector of Q is limited by a voltage-limiter. In the particular circuit of Fig. 12, the voltage-limiting function is performed by diodes D_\ and D₂. The error voltage thus generated at the collector is inverted by the amphfier A — in order to obtain the correct sign for feedback — and fed to the base of Q₁₂₁ as V_β„.

[0080] In order to tap the output, the bases of Qi_23a and Qπ^a^e^bmected to^" the bases of

Qi₂₃ and Q₁₂4. respectively. Transistor Qma is fabricated with a cross-sectional area α times larger than Q₁₂₄ in order to ensure a safety margin in the bias current fed to the log- domain filters. The term "cross-sectional area," as used herein, can include the collector area and/or the emitter area of a transistor, depending on the particular device- fabrication technology used to form the transistors. PNP transistors Qι₂₇ and Qπ₈ are used to mirror

αlπ₄ as required, thereby providing dynamically controlled bias currents for one or more

nodes of a main circuit for which dynamic biasing is desired. An exemplary embodiment of the inverting amplifier A is illustrated in inset 1204 of Fig. 12. The amplifier A includes p-channel FETs Fi_2a and Fi_2b which form a current mirror, as well as amplifying n-channel FETs F₁₂ and F_12d. The sources of Fι_2c and

are connected by a resistor Ri_2a- The drain of F^_c is connected to the bias voltage Vbias of the envelope detector through a resistor Ri_2b which serves as an output load for the amplifier. FETs F^_c and Fπd are biased by bias currents Isi₂a and Isi2 , respectively.

[0081] When the amplifier A is used as part of the envelope detector of Fig. 12, the collector voltage N_n of transistor Q^₄ is fed into the gate of n-channel FET Fj_2C- Because the amplifier A operates in a differential mode, its output N_err is proportional to the difference between Nbi_as and Ni_n.

_[0082] Fig- 12 also illustrates an exemplary embodiment of a feedback arrangement 1202 which can be used to drive bias currents /₁₂₁ and ₁₂₃ through Q₁₂₁ and Q₁₂₃ in a controlled manner. The transistor in Q_llain the feedback arrangement 1202 represents a transistor

through which a regulated current is to be driven — e.g., one of the transistors Q₁₂₂, Q₁₂₃,

or Qi₂₄ ^"i Figrl2r The^_operation-of the feedback arrangement 1202 can be readily understood by considering Fig. 23, in which the n-channel FET Fπ, the ΝPΝ transistor Q_llb, and current source In are modeled as a voltage-controlled current source !«,- If the control voltage v/b increases, the current through I_ft increases. If the collector current I_cn of the transistor Qi ι_a tends to be smaller than the current I₁₂, the collector voltage of Q _la> which is also the control voltage v b, tends to increase. This increased v b increases the current through L_b, which in turn draws a larger current through Q _la. The opposite effect — i.e., a decrease in v/b and the current through I_β — occurs when the collector current I_d i tends to be larger than In. The circuit settles at a point where I_cπ = 1_\2.

[0083] Fπ In, and Qπ_b emulate 1^. Fn and In form a source follower with near-unity gain that simply translates the collector voltage to a suitable level for driving Qπ_b- The level- shifted voltage is converted into a current using the transistor Qπ_b. The circuit settles to a

point where Icn J12.

[0084] Fig. 13 illustrates an example of an inverting current mirror which can be used to connect a dynamic bias control circuit (e.g., an envelope detector) to a signal-processing circuit (e.g., an amplifier, a transconductor, or a filter) which requires bias current to flow out of, not into, the signal processing circuit. For example, the current mirror of Fig. 13 can be used to connect the envelope detector of Fig. 12 to the transconductor circuit of

Fig. 1.

[0085] The current mirror of Fig. 13 is powered by voltage sources Nm3 and V_LI_OO- Transistor Q₁₂₇ (also illustrated in Fig. 12) is driven by output current αl^ of the

envelope detector of Fig. 12. The base of PΝP transistor Q_12S (also illustrated in Fig. 12)

is connected-to-the-base of-P-ΝP transistor Q₁₂₇j and-accordingly, the current Pi₃Ii₂₄

flowing through Q_{1 8} (where β₁₃ is a constant) is proportional to the current odι₂₄ flowing

through Q ₂₇- The current β₁₃Ii24 from Qι₂₈ flows into the collector of a diode-connected (i.e., base and collector of transistor connected together) NPN transistor Q₁₃o2- The base and collector of Q ₃₀₂ are connected to the respective bases of output transistors Q^_M-

The output currents γι₃Ii24 and δι₃I]24 of the output transistors Q₁₃o₄ mirror the current

P₁₃I₁₂₄ flowing through Qι₃02, which in turn mirrors the current 0.1^4 flowing through

Q₁₂₇ — OCI₁₂₄ being the output of the envelope detector of Fig. 12. It can therefore be seen

that the output currents γι₃Iι₂₄ and δι₃I_]24 of the current mirror of Fig. 13 ultimately

mirror the output current odι₂₄ of the envelope detector of Fig. 12. Furthermore, output

currents γ₁₃Iι₂₄ and δι₃Iι₂4 flow in the proper direction — i.e. , with the current flowing in,

not out — to provide the bias current I_E to a circuit such as the transconductor of Fig. 1.

The constants γι₃ and δ₁₃ depend upon the device characteristics — e.g., the relative

cross-sectional areas — of transistors Q124, Qi24a, Q127, Q128, Q1302, and Q_{1 0} . 86] In order to utilize the envelope detector of Fig. 12 to control the bias of a differential circuit such as the transconductor of Fig. 1, it can be desirable to feed only the positive side, or only the negative side, of differential voltage signal Vj into the non- differential input current signal Ij_n of the envelope detector. For example, one side of the voltage-mode signal N can be fed into the input of the detector through a resistor, in order to produce the current-mode signal Ij_n. Alternatively, Vj can be converted to a current-mode signal using a transconductor. For example, the conversion can be performed using a non-dynamically biased version of a circuit having a topology similar to the transconductor of Fig. 1, but in which IE is kept constant, rather than being adjusted as described above. Such a circuit can be particularly useful, because it can convert a differential, voltage-mode signal into a non-differential, current-mode signal. The aforementioned non-dynamically biased circuit — which can send a signal into the input Ii_n of the envelope detector of Fig. 12 — is not to be confused with the dynamically biased version of the circuit of Fig. 1, in which the bias current I_E can be adjusted by an

output current — e.g., γ₁₃Ii₂₄ or δ₁₃I₁₂₄ — of a current mirror receiving the output current

ai of the envelope detector of Fig. 12.

[0087] The envelope detector of Fig. 12 can also be utihzed to control the bias of a filter such as the low-pass filters of Figs. 8a and 8b. For example, the input signal i_in of the filter of Fig. 8a — or a signal proportional to i_in — can be used as the input signal Ij_n of the envelope detector of Fig. 12. The collector current of one of the output transistors Q₁₂₈ (illustrated in Figs. 12 and 13) can then be used as the bias input I_bias of the filter of Fig. 8a. A matching collector current approximately equal to I ias and produced by, e.g., a different one of the output transistors Q₁₂₈ can similarly be used to bias an auxiliary circuit such as the circuit of Fig. 8b.

[0088] An envelope detector such as the one illustrated in Fig. 12 can also be used, in conjunction with a current mirror, to provide a bias current (I_2p/l3_P)Ibias into the output transistor Q_4p of the filter of Fig. 8a. In addition, the envelope detector and current mirror can be used to provide a bias current ( l_bi_as into the output transistor Q_4n of the filter

of Fig. 8b.

[0089] An example of such a configuration is illustrated in Fig. 16, in which the envelope detector 1200 of Figτl-2-pullsxurrent from a diode-connected PNP -transistor Q_ml, the - base and collector of which are connected to the respective bases of current-mirror transistors Q_m2 and Q^. The emitters of Q_ml; Q_m2; and Q^ are connected to a voltage

source VHIO-

[0090] Transistor Q_mι has a cross-sectional area A_xl . Transistor Q^ has an approximately equal cross-sectional area, and therefore produces approximately the same current,

I_bi_as-ccl₁₂₄, as is pulled through Q_ml. However, transistor Qπ-3 is designed to have a cross-

sectional area (I_2p I_3p)A_xl. Therefore, because the collector current of a bipolar transistor is generally proportional to the area of the transistor, Qnύ produces a current (I_2p/I_3p)I_bi_as which can be used to bias transistor Q_4p of the filter of Fig. 8a. In a preferred embodiment, an additional transistor having an area (l_2n/l3_n)A_xl can be included in the current mirror of Fig. 16. The resulting current, (I2n/l3n)lbias, can be used to bias transistor Q_4n of the filter of Fig. 8b. Moreover, it is desirable to provide yet another transistor Q_π^' having an area of approximately A_xl, in order to provide bias current for Q_ln of the filter of Fig. 8b.

[0091 ] In accordance with an additional embodiment of the invention, the input (i. e. , compressing) stage of a companding filter can be eliminated, leaving only the frequency- dependent components and the expanding stage. An example of such a technique is illustrated by Figs. 20a and 20b. Fig. 20a is a block diagram of an exemplary log-domain circuit having an input circuit 2002 and an output circuit 2004. The input circuit receives an input current iin₂o and performs a logarithmic operation on the input current ij_n2o, thus

— - generating a logarithmically-compressed voltage Nι_og20-Jhe output circuit 2004 filters and exponentiates the compressed voltage Vι_og2o, thus generating an output current i_out2o- [0092] In accordance with the invention, the input circuit 2002 can be eliminated, leaving only the output circuit 2004, as illustrated in Fig. 20b. The output circuit 2004 acts as a combination of an exponentiator and a low-pass filter. An input voltage V_ιn2o can be applied directly to the input of the circuit 2004 which then generates a filtered, exponentiated output current i_out₂0 based upon the input voltage V^o. Such a circuit can be useful for applications requiring an exponential filter. For example, the filter- exponentiator 2004 of Fig. 20 can, optionally, be used to replace the filter 1104 and the exponentiator 1102 of the circuit of Fig. 11.

[0093] It has been demonstrated by the foregoing discussion that the design of syllabic companding log-domain filters can be greatly simplified by eliminating the compensation circuit in accordance with the invention. As discussed in detail above, the approach of the invention has numerous advantages over conventional methods involving syllabic companding and instantaneous companding log-domain filters.

[0094] Although the present invention has been described in connection with specific exemplary embodiments, it should be understood that various changes, substitutions and alterations can be made to the disclosed embodiments without departing from the spiit and scope of the invention as set forth in the appended claims.

Claims

CLAIMS WHAT TS CLAIMED IS: 1. An apparatus for processing a signal, comprising: a selected one of a class-AB circuit and a class-B circuit, the selected one having at least one input and at least one bias, the at least one input being adapted to receive at least one input signal, and the selected one being configured to process the at least one input signal to thereby generate at least one output signal related to the at least one input signal by an input-output characteristic having a crossover region which introduces distortion; and an amplitude detector configured to perform the operations of: receiving the at least one input signal, detecting at least one amplitude of the at least one input signal, and dynamically adjusting the at least one bias in accordance with the at least one amplitude, wherein the at least one bias controls a level of the at least one output signal such that the at least one output signal avoids the crossover region.

2. An apparatus as recited in claim 1, wherein the selected one comprises an amplifier.

3. An apparatus as recited in claim 1, wherein the selected one comprises a filter.

4. An apparatus as recited in claim 3, wherein the filter is internally non-linear.

5. An apparatus as recited in claim 3, wherein the filter comprises a companding

filter.

6. An apparatus as recited in claim 3, wherein the filter comprises a log-domain filter.

7. An apparatus as recited in claim 1, wherein the amplitude detector comprises a filtered rectifier.

8. An apparatus as recited in claim 1, wherein the amplitude detector comprises a filter-exponentiator configured to low-pass-filter and exponentiate a detected signal comprising the at least one input signal, thereby generating a filtered-exponentiated signal, wherein an output signal of the amplitude detector comprises the filtered-exponentiated signal, and wherein the detected signal comprises the output signal of the amplitude detector.

9. An apparatus for processing a signal, comprising a filter having at least one input and at least one bias, wherein the at least one input comprises: a first input for receiving a first input signal; and a second input for receiving a second input signal, wherein the filter is configured to perform the steps of: applying a first filtering operation to the first input signal, thereby generating a first output signal which is communicated to at least one output of the filter, the first filtering operation having a first frequency characteristic in which low frequencies are

suppressed, and applying a second filtering operation to the second input signal, the second input signal controlling the at least one bias, the second filtering operation having a second frequency characteristic in which low frequencies are passed, and the second input signal being adjusted in accordance with an amplitude of the first input signal.

10. An apparatus as recited in claim 9, wherein the first frequency characteristic comprises at least one of a band-pass characteristic and a high-pass characteristic, the at least one bias being isolated from low-frequency components of the first input signal.

11. An apparatus for processing a signal, comprising a filter having: at least one input; and first and second biases, wherein the at least one input comprises first and second inputs, the first input being adapted to receive a first input signal and a first bias signal related to an amplitude of at least one of the first and second input signals, the first bias signal being for controlling the first bias, the second input being adapted to receive a second input signal and a second bias signal, the second bias signal being for controlling the second bias, the second bias signal being approximately equal to the first bias signal, and the filter being configured to filter a difference of the first and second input signals, thereby generating a filter output signal.

12. An apparatus as recited in claim 11, wherein the filter is internally non-linear.

13. An apparatus as recited in claim 11, further comprising an amplitude detector configured to perform the operations of: receiving the at least one of the first and second input signals; detecting the amplitude; and adjusting at least one of the first and second biases in accordance with the amplitude.

14. An apparatus as recited in claim 11, wherein the filter comprises a companding filter.

15. An apparatus as recited in claim 11, wherein the filter comprises a log-domain

filter.

16. An apparatus as recited in claim 11, wherein the filter is configured to apply, to a third input signal comprising at least one of the first and second input signals, a compression operation, a filtering operation, and an expansion operation.

17. An apparatus as recited in claim 16, wherein the filter comprises a compression section, comprising: a first transistor having a first signal-receiving terminal and first and second current-carrying terminals, the first current-carrying terminal being for receiving the third input signal, and the second current-carrying terminal being adapted to be connected to a first voltage source; and a second transistor having a second signal-receiving terminal and third and fourth current-carrying terminals, the second signal-receiving terminal being connected to the first current-carrying terminal, the third current-carrying terminal being connected to the first signal-receiving terminal, and the fourth current-carrying terminal being adapted to be connected to a second voltage source, wherein an output signal of the compression operation comprises a voltage at the second signal-receiving terminal.

18. An apparatus as recited in claim 17, wherein the filter further comprises an expansion section, comprising: a third transistor having a third signal-receiving terminal and fifth and sixth current-carrying terminals, the third signal-receiving terminal being for receiving the output signal of the compression operation; and a high-frequency shunt connected between the third signal-receiving terminal and the fifth current-carrying terminal, wherein an output signal of the filter comprises a signal generated at the sixth current-carrying terminal.

19. An apparatus as recited in claim 16, wherein the filter comprises an expansion

section, comprising: a transistor having a signal-receiving terminal and first and second current- carrying terminals, the signal-receiving terminal being for receiving an output signal of the compression operation; and a high-frequency shunt connected between the signal-receiving terminal and the first current-carrying terminal, wherein an output signal of the filter comprises a signal generated at the second current-carrying terminal.

20. An apparatus as recited in claim 16, wherein the filter comprises a compression section, comprising a first transistor having a first signal-receiving terminal and first and second current-carrying terminals, the first signal-receiving terminal being adapted to be connected to a first voltage source, the first current-carrying terminal being configured to receive the third input signal, and the second current-carrying terminal being adapted to be connected to a current source, wherein an output signal of the compression operation comprises a voltage at the second current-carrying terminal.

21. An apparatus as recited in claim 20, further comprising: a first node for receiving the output signal of the compression operation; a high-frequency shunt adapted to be connected between the first node and at least one of the first voltage source and a second voltage source; and an expansion section, comprising a second transistor having a second signal- receiving terminal and third and fourth current-carrying terminals, the second signal- receiving terminal being adapted to be connected to at least one of the first voltage source, the second voltage source, and a third voltage source, and the fourth current-carrying terminal being for receiving a signal from the first node, wherein an output signal of the filter comprises a signal generated at the third current-carrying terminal.

22. An apparatus as recited in claim 16, wherein the filter comprises an expansion section, comprising a transistor having a signal-receiving terminal and first and second current-carrying terminals, the signal-receiving terminal being adapted to be connected to a voltage source, and the second current-carrying terminal being for receiving an output signal of the compression operation, wherein an output signal of the filter comprises a signal generated at the first current-carrying terminal.

23. An apparatus as recited in claim 16, wherein the third input signal further comprises at least one of the first and second bias signals.

24. A combined filter, comprising: a first filter having: a first filter configuration, a first bias input for receiving a first bias, a first input for receiving a first input signal, and a first output for providing a first output signal; a second filter having: a second filter configuration, a second bias input for receiving a second bias, a second input for receiving a second input signal, and a second output for providing a second output signal, the second filter configuration matching the first filter configuration, the first bias and the second bias being adjusted in accordance with at least one amplitude of at least one of the first input signal and the second input signal, and the first bias and the second bias being adjusted to be approximately equal; and a combined filter output configured to provide a combined output signal comprising a difference of the first output signal and the second output signal.

25. A combined filter as recited in claim 24, wherein the second input signal is approximately equal to ah inverse of the first input signal.

26. A combined filter as recited in claim 24, wherein the first input signal comprises a third input signal and a first bias control signal for controlling the first bias, and wherein the second input signal comprises a second bias control signal for controlling the second bias, the second bias control signal being approximately equal to the first bias control signal.

27. A combined filter as recited in claim 26, wherein the second input signal further composes a fourth input signal approximately equal to an inverse of the third input signal.

28. A combined filter as recited in claim 24, further comprising a differencing block configured to perform the operations of: receiving the first output signal; receiving the second output signal; and generating a single-ended output signal comprising a difference of the first and second output signals.

29. An apparatus for processing a signal, comprising: a first transistor, comprising: a first signal-receiving terminal, a first current-carrying terminal adapted to be connected to a voltage source, and a second current-carrying terminal connected to the first signal-receving terminal; a second transistor, comprising: a second signal-receiving terminal connected to the first signal-receiving terminal, a third current-carrying terminal adapted to be connected to the voltage source,

and - a fourth current-carrying terminal; a first adjustable current source in communication with the second cuoent-carrying terminal and allowing a first bias current to flow through the second current-carrying terminal; a second adjustable cuoent source in communication with the fourth cuoent- carrying terminal and allowing a second bias cuoent to flow through the fourth cuoent- carrying terminal, the second bias cuoent being approximately equal to the first bias cuoent, and the first and second adjustable cuoent sources being adjusted in accordance with an amplitude of a first input signal coupled into at least one of the second current- carrying terminal and the fourth cuoent-carrying terminal; and an output connected to the fourth cuoent-carrying terminal.

30. An apparatus as recited in claim 29, further comprising: a third transistor, comprising: a fifth cuoent-carrying terminal connected to the second cuoent-caoying teoninal, a sixth cuoent-caoying teoninal connected to the first adjustable cuoent source, and a first input terminal for receiving a second input signal; and a fourth transistor, comprising: a seventh cuoent-caoying terminal connected to the fourth cuoent-carrying terminal, an eighth cuoent-carrying terminal connected to the second adjustable current source, and a second input terminal for receiving a third input signal, wherein the first input signal comprises a difference of the second and third input signals.

31. An apparatus as recited in claim 29, wherein the first adjustable cuoent source comprises a third transistor, the first transistor comprising: a fifth cuoent-caoying teoninal connected to a voltage source; a sixth cuoent-caoying teoninal connected to the second cuoent-caoying teoninal; and a first transistor input terminal, wherein the second adjustable cuoent source comprises a fourth transistor, the second transistor comprising: a seventh cuoent-caoying teoninal connected to the voltage source; an eighth cuoent-caoying teoninal connected to the fourth cuoent-carrying terminal; and a second transistor input terminal connected to the first transistor input terminal, the first transistor input terminal and the second tiansistor input terminal being adapted to receive a bias control voltage which is adjusted in accordance with the amplitude of the first input signal.

32. An apparatus for processing a signal, comprising: a dynamically biased signal-processing circuit having an input and an output; and a feedback path providing a feedback signal from the output to the input.

33. An apparatus as recited in claim 32, further comprising: a differencing block configured to perform the operations of: receiving the feedback signal, receiving an input signal, and generating a difference of the input signal and the feedback signal; and a gain stage configured to perform the operations of: receiving the difference, amplifying the difference to thereby generate an amplified signal, and providing the amplified signal to the input.

34. An apparatus as recited in claim 32, wherein the dynamically biased signal- processing circuit comprises a filter.

35. An apparatus as recited in claim 34, wherein the filter comprises a companding filter.

36. A signal-size detector, comprising: a differencing block configured to perform the operations of: receiving a first input signal, receiving a second input signal, and generating a difference signal comprising a difference of the first and second input signals; an exponentiator configured to exponentiate a signal comprising the difference signal, thereby generating an exponentiated signal, wherein an output signal of the detector comprises the exponentiated signal; and a filter configured to perform low-pass filtering of a signal comprising the difference signal, thereby generating a filtered signal, wherein the output signal further comprises the filtered signal, and wherein the second input signal comprises the output signal.

37. A signal-size detector as recited in claim 36, wherein the exponentiator and the low-pass filter are combined to form an exponentiator-filter, comprising: a transistor having an input terminal and first and second cuoent-caoying terminals, the first cuoent-caoying terminal being adapted to be connected to at least one voltage source, the second cuoent-caoying terminal being connected to an output of the exponentiator-filter, and the input terminal being connected to an input of the exponentiator-filter; and a high-frequency shunt adapted to be connected between the input terminal and the at least one voltage source, the high-frequency shunt comprising a capacitor.

38. A signal-size detector, comprising: first, second, third, fourth, and fifth nodes, wherein an input signal is received by the first node; a first transistor, comprising: a first signal-receiving terminal connected to the second node, a first cuoent-carrying terminal connected to the third node, and a second cuoent-caoying teoninal adapted to receive a first bias cuoent; a second transistor, comprising: a second signal-receiving terminal connected to the fourth node, a third cuoent-carrying terminal connected to the third node, and a fourth cuoent-caoying teoninal adapted to receive a second bias cuoent, the fourth cuoent-carrying terminal being connected to the fourth node; a high-frequency shunt connected between the fourth node and a first voltage node, the first voltage node being adapted to be connected to a first voltage source; a third transistor, comprising: a third signal-receiving terminal connected to the fourth node, a fifth cuoent-carrying terminal connected to the fifth node, and a sixth cuoent-caoying teoninal adapted to receive a third bias cuoent; and a fourth transistor, comprising: a fourth signal-receiving termmal adapted to be connected to a second voltage source, a seventh cuoent-caoying teoninal connected to the fifth node, and an eighth cuoent-caoying terminal connected to the first node.

39. A signal-size detector as recited in claim 38, further comprising: a voltage limiter connected between the first node and the fourth signal-receiving terminal; a fifth transistor, comprising: a fifth signal-receiving terminal connected to the fourth node, a ninth cuoent-caoying teoninal connected to a sixth node, and a tenth cuoent-caoying teoninal adapted to receive a fourth bias cuoent; a sixth transistor, comprising: a sixth signal-receiving terminal adapted to be connected to the second voltage source, an eleventh cuoent-caoying teoninal connected to the sixth node, and a twelfth cuoent-caoying teoninal; and a cuoent mioor, comprising: a controlling branch in communication with the twelfth cuoent-caoying teoninal, wherein the controlling branch is adapted to conduct a controlling cuoent, and a controlled branch adapted to conduct a controlled current, wherein the cuoent mioor is configured to control the controlled cuoent in accordance with the controlling current.

40. A method of processing a signal, comprising: receiving at least one input signal into a selected one of a class-AB circuit and a class-B circuit, the selected one having at least one bias; processing, by the selected one, the at least one input signal to thereby generate at least one output signal related to the at least one input signal by an input-output characteristic having a crossover region which introduces distortion; receiving, into an amplitude detector, the at least one input signal; detecting, by the amplitude detector, at least one amplitude of the at least one input signal; and dynamically adjusting, by the amplitude detector, the at least one bias in accordance with the at least one amplitude, wherein the at least one bias controls a level of the at least one output signal such that the at least one output signal avoids the crossover region.

41. A method as recited in claim 40, wherein the selected one comprises an amplifier.

42. A method as recited in claim 40, wherein the selected one comprises a filter.

43. A method as recited in claim 42, wherein the filter is internally non-linear.

44. A method as recited in claim 42, wherein the filter comprises a companding filter.

45. A method as recited in claim 42, wherein the filter comprises a log-domain filter.

46. A method as recited in claim 40, wherein the amplitude detector comprises a filtered rectifier.

47. A method of as recited in claim 39, further comprising: low-pass-filtering and exponentiating a detected signal comprising the at least one input signal, thereby generating a filtered-exponentiated signal; and generating a detector output signal comprising the filtered-exponentiated signal, wherein the detected signal comprises the detector output signal.

48. A method of processing a signal, comprising: receiving a first input signal into a filter having at least one bias; receiving a second input signal into the filter; using the filter to apply a first filtering operation to the first input signal, thereby generating a first output signal which is communicated to at least one output of the filter, the first filtering operation having a first frequency characteristic in which low frequencies are suppressed; using the filter to apply a second filtering operation to the second input signal, the second input signal controlling the at least one bias, the second filtering operation having a second frequency characteristic in which low frequencies are passed; and adjusting the second input signal in accordance with an amplitude of the first input signal.

49. A method as recited in claim 48, wherein the first frequency characteristic comprises at least one of a band-pass characteristic and a high-pass characteristic, the at least one bias being isolated from low-frequency components of the first input signal.

50. A method of processing a signal, comprising: receiving a first input signal into a filter having first and second biases; receiving a second input signal into the filter; receiving, into the filter, a first bias signal related to an amplitude of at least one of the first and second input signals, the first bias signal being for controlling the first

bias; receiving, into the filter, a second bias signal, the second bias signal being for contiolling the second bias, and the second bias signal being approximately equal to the first bias signal; and filtering a difference of the first and second input signals, thereby generating a filter output signal.

51. A method as recited in claim 50, wherein the filter is internally non-linear.

52. A method as recited in claim 50, further comprising: receiving the at least one of the first and second input signals into an amplitude detector; detecting, by the amplitude detector, the amplitude; and adjusting, by the amplitude detector, at least one of the first and second biases in accordance with the amplitude.

53. A method as recited in claim 50, wherein the filter comprises a companding

filter.

54. A method as recited in claim 50, wherein the filter comprises a log-domain

filter.

55. A method as recited in claim 50, further comprising: compressing, by the filter, a third input signal comprising the at least one of the first and second input signals; filtering the third signal; and expanding the third signal.

56. A method as recited in claim 55, wherein the compressing step is performed by a compression section, comprising: a first transistor having a first signal-receiving terminal and first and second cuoent-caoying terminals, the first cuoent-caoying teoninal being for receiving the third input signal, and the second cuoent-carrying terminal being adapted to receive a first voltage; and a second transistor having a second signal-receiving terminal and third and fourth cuoent-caoying teoninals, the second signal-receiving teoninal being connected to the first cuoent-caoying teoninal, the third cuoent-caoying teoninal being connected to the first signal-receiving terminal, and the fourth cuoent-caoying terminal being adapted to be connected to a second voltage source, wherein an output signal of the compressing step comprises a voltage at the second signal-receiving terminal.

57. A method as recited in claim 56, wherein the expanding step is performed by an expansion section, comprising: a third transistor having a third signal-receiving terminal and fifth and sixth cuoent- carrying terminals, the third signal-receiving terminal being for receiving the output signal of the compressing step; and a high-frequency shunt connected between the third signal-receiving terminal and the fifth cuoent-caoying teoninal, wherein an output signal of the filter comprises a signal generated at the sixth cuoent-carrying terminal.

58. A method as recited in claim 55, wherein the expanding step is performed by an expansion section, comprising: a transistor having a signal-receiving terminal and first and second cuoent-carrying terminals, the signal-receiving terminal being for receiving an output signal of the compressing step; and a high-frequency shunt connected between the signal-receiving terminal and the first cuoent-caoying teoninal, wherein an output signal of the filter comprises a signal generated at the second cuoent-caoying teoninal.

59. A method as recited in claim 55, wherein the compression step is performed by a compression section, comprising a first tiansistor having a first signal-receiving terminal and first and second cuoent-carrying terminals, the first signal-receiving terminal being adapted to be connected to a first voltage source, the first current-carrying terminal being configured to receive the third input signal, and the second cuoent-carrying terminal being adapted to be connected to a cuoent source, wherein an output signal of the compressing step comprises a voltage at the second cuoent-carrying terminal.

60. A method as recited in claim 59, wherein the expanding step is performed by an expansion section, comprising: a first node for receiving the output signal of the compressing step; a high-frequency shunt adapted to be connected between the first node and at least one of the first voltage source and a second voltage source; and a second transistor having a second signal-receiving terminal and third and fourth cuoent-caoying terminals, the second signal-receiving terminal being adapted to be connected to at least one of the first voltage source, the second voltage source, and a third voltage source, the fourth cuoent-carrying terminal being for receiving a signal from the first node, wherein an output signal of the filter comprises a signal generated at the third cuoent-carrying terminal.

61. A method as recited in claim 55, wherein the expanding step is performed by an expansion section, comprising a transistor having a signal-receiving terminal and first and second cuoent-caoying teoninals, the signal-receiving terminal being adapted to be connected to a voltage source, and the second cuoent-caoying teoninal being for receiving an output signal of the compressing step, wherein an output signal of the filter comprises a signal generated at the first cuoent-carrying terminal.

62. A method as recited in claim 55, wherein the third input signal further comprises at least one of the first and second bias signals.

63. A method of processing a signal, comprising:

receiving a first input signal into a first filter having a first filter configuration and a first bias; using the first filter to generate a first output signal; receiving a second input signal into a second filter having a second filter configuration and a second bias, the second filter configuration matching the first filter configuration, the first bias and the second bias being adjusted in accordance with at least one amplitude of at least one of the first input signal and the second input signal, and the first bias and the second bias being adjusted to be approximately equal; using the second filter to generate a second output signal; and providing a combined output signal comprising a difference of the first output signal and the second output signal.

64. A method as recited in claim 63, wherein the second input signal is approximately equal to an inverse of the first input signal.

65. A method as recited in claim 63, wherein the first input signal comprises a third input signal and a first bias control signal for controlling the first bias, and wherein the second input signal comprises a second bias control signal for controlling the second bias, the second bias control signal being approximately equal to the first bias control signal.

66. A method as recited in claim 65, wherein the second input signal further comprises a fourth input signal approximately equal to an inverse of the third input signal.

67. A method as recited in claim 63, further comprising: receiving the first output signal; receiving the second output signal; and generating a single-ended output signal comprising a difference of the first and second output signals.

68. A method of processing a signal, comprising: using a first transistor to control a cuoent in a second transistor, wherein the first transistor comprises: a first signal-receiving terminal, a first cuoent-caoying teoninal adapted to be connected to a voltage source, and a second cuoent-caoying terminal connected to the first signal-receiving terminal, and wherein the second transistor comprises: a second signal-receiving terminal connected to the first signal-receiving terminal, a third cuoent-caoying teoninal adapted to be connected to the voltage source,

and a fourth cuoent-carrying terminal; using a first adjustable cuoent source in communication with the second cuoent- carrying terminal to allow a first bias cuoent to flow through the second cuoent-caoying terminal; using a second adjustable cuoent source in communication with the fourth cuoent- caoying teoninal to allow a second bias cuoent to flow through the fourth cuoent- caoying teoninal, such that the second bias cuoent is approximately equal to the first bias cuoent; adjusting the first and second adjustable cuoent sources in accordance with an amplitude of a first input signal coupled into at least one of the second cuoent-caoying terminal and the fourth cuoent-caoying teoninal; and providing an output signal from an output connected to the fourth cuoent-caoying teoninal.

69. A method as recited in claim 68, further comprising controlling the output signal in accordance with the input signal, wherein the controlling step is performed using a circuit comprising: a third transistor, comprising: a fifth cuoent-carrying terminal connected to the second cuoent-caoying teoninal, a sixth cuoent-caoying teoninal connected to the first adjustable cuoent

source, and a first input terminal for receiving a second input signal; and a fourth transistor, comprising: a seventh cuoent-carrying terminal connected to the fourth cuoent-carrying terminal, an eighth cuoent-caoying terminal connected to the second adjustable cuoent source, and a second input terminal for receiving a third input signal, wherein the first input signal comprises a difference of the second and third input signals.

70. A method as recited in claim 68, wherein the first adjustable cuoent source comprises a third transistor, the first transistor comprising: a fifth cuoent-caoying teoninal connected to a voltage source; a sixth cuoent-caoying terminal connected to the second cuoent-carrying

terminal; and a first tiansistor input terminal, wherein the second adjustable cuoent source comprises a fourth transistor, the second transistor comprising: a seventh cuoent-caoying teoninal connected to the voltage source; an eighth cuoent-caoying teoninal connected to the fourth cuoent-carrying

terminal; and a second transistor input terminal connected to the first transistor input terminal, the first transistor input terminal and the second transistor input terminal being adapted to receive a bias control voltage which is adjusted in accordance with the amplitude of the first input signal.

71. A method of processing a signal, comprising: dynamically biasing a signal-processing circuit having an input and an output; and providing a feedback signal from the output to the input.

72. A method as recited in claim 71, further comprising: receiving the communication; receiving an input signal; generating a difference of the input signal and the feedback signal; amplifying the difference to thereby generate an amplified signal; and providing the amplified signal to the input.

73. A method as recited in claim 71, wherein the signal-processing circuit comprises a filter.

74. A method as recited in claim 73, wherein the filter comprises a companding filter.

75. A method of detecting signal size, comprising: receiving a first input signal; receiving a second input signal; generating a difference signal comprising a difference of the first and second input signals; exponentiating a signal comprising the difference signal, thereby generating an exponentiated signal; low-pass filtering a signal comprising the difference signal, thereby generating a filtered signal; and generating an output signal comprising the exponentiated signal and the filtered signal, wherein the second input signal comprises the output signal.

76. A method as recited in claim 75, wherein the steps of exponentiating and low- pass filtering are combined to form an exponentiating-fϊltering step, comprising: receiving the signal comprising the difference signal into an input terminal of a transistor having first and second cuoent-caoying teoninals, the first cuoent-caoying teoninal being adapted to be connected to at least one voltage source, and the second cuoent-caoying teoninal being connected to an output terminal; and suppressing high-frequency components of the signal comprising the difference signal, using a high-frequency shunt adapted to be connected between the input terminal and the at least one voltage source, the high-frequency shunt comprising a capacitor.

77. A method of detecting signal-size, comprising: receiving an input signal into a first node; driving a first bias cuoent through a first transistor, the first transistor comprising: a first signal-receiving terminal connected to a second node, a first cuoent-caoying teoninal connected to a third node, and a second cuoent-caoying teoninal through which the first bias cuoent is driven; driving a second bias cuoent through a second transistor, the second transistor comprising: a second signal-receiving terminal connected to a fourth node, a third cuoent-carrying terminal connected to the third node, and a fourth cuoent-caoying teoninal through which the second bias cuoent is driven, the fourth cuoent-carrying terminal being connected to the fourth node; using a high-frequency shunt to suppress high frequency components of the input signal, the high-frequency shunt being adapted to be connected between the fourth node and a first voltage source; driving a third bias cuoent through a third transistor, the third transistor comprising: a third signal-receiving terminal connected to the fourth node, a fifth cuoent-caoying teoninal connected to a fifth node, and a sixth cuoent-carrying termmal through which the third bias cuoent is driven;

and receiving an eoor signal from a fourth transistor, the fourth transistor comprising: a first signal-receiving terminal adapted to be connected to a second voltage source, a seventh cuoent-caoying terminal connected to the fifth node, and an eighth cuoent-caoying teoninal connected to the first node.

78. A method as recited in claim 77, further comprising: limiting a voltage between the first node and the second voltage source; driving a fourth bias cuoent through a fifth transistor, the fifth transistor comprising: a fifth signal-receiving terminal connected to the fourth node, a ninth cuoent-caoying teoninal connected to a sixth node, and a tenth cuoent-caoying terminal through which the fourth bias cuoent is driven; using a sixth transistor to drive a controlling cuoent through a controlling branch of a cuoent mioor, the sixth transistor comprising: a sixth signal-receiving terminal adapted to be connected to the second voltage source, an eleventh cuoent-carrying terminal connected to the sixth node, and a twelfth cuoent-caoying teoninal in communication with the contiolling branch; and using the cuoent mioor to control an output cuoent in accordance with the controlling cuoent, the output cuoent flowing through a controlled branch of the cuoent mioor.