JP3232856B2 - Analog filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an analog filter which is essential for signal processing such as waveform equalization. In both the fields of communication and information equipment, the speed of signals to be processed continues to increase, and there are still places where digital processing cannot cope. Therefore, it is no exaggeration to say that how well signal processing can be performed in analog is a decisive factor in device performance. The present invention provides a highly versatile analog filter that can cope with various forms of analog signal processing.

[0002]

2. Description of the Related Art A conventional analog filter is a passive filter using discrete components or a CMOS.
Switched capacitor (SC) filters that use technology have been the mainstream. The former is suitable for speeding up, but requires a mounting space because it has coils and the like, and is expensive and almost impossible to perform variable adjustment. The latter can be integrated into an IC and is widely used, but generally has a slow processing speed.
Also, because it requires a high-speed sampling clock,
Consideration should be given to the technology of separating from other analog circuits (to prevent noise).

Further, as shown in FIG. 12, there is a BIQUAD (biquad) circuit constituted by an integrator 17 having a loss, a negative-phase integrator 18, and a negative-phase amplifier 19 and realizing a biquadratic transfer function. Especially BIQUA using voltage / current conversion means
The D circuit is a BIQUAD using a switched capacitor.
It is much faster than circuits and is widely used in the field of magnetic recording, but is only for specific applications.

[0004]

As described above, the analog filter using the conventional BIQUAD circuit is only for a specific application.

Accordingly, an object of the present invention is to provide a highly versatile analog filter which can cope with various forms of analog signal processing.

[0006]

The above problem is solved by the circuit configuration shown in FIG. That is, in FIG.
(Claim 1) A first input voltage representing an input signal has a second input voltage.
A first voltage output by inverting and adding the phase of an input voltage and amplifying the added value with a voltage gain that can be varied from the outside
Amplifying means (corresponding to the first voltage gain variable means in FIG. 1) ;
The output voltage of the first voltage amplifying means is converted into a current, integrated and output, and the first voltage is variable from the outside .
(The first voltage / current conversion rate variable type integration shown in FIG. 1)
Means) and the second input to the output of the first integrating means .
Added by inverting the phase of the force voltage and the second voltage increase for amplifying and outputting a voltage gain that can be variable <br/> varying the added value from the outside
Width means (corresponding to the second voltage gain varying means in FIG. 1), and the output voltage of the second voltage amplifying means is converted into a current, integrated and output , the output is branched and the second input Voltage
First, a second integrating means (FIG. 4) capable of varying the current from outside
( Corresponding to the second voltage / current conversion rate variable type integrating means), and a BIQUAD (biquad) circuit comprising:
The input voltage is branched and input, and the voltage can be varied externally.
(K) to amplify and output positive and negative signals.
3 voltage amplifying means (corresponding to the third voltage gain varying means in FIG. 1).
A) and the third voltage amplification by an external switching signal.
Selecting and outputting one of the outputs of the means;
Switch means for adding to the output of the means.
You.

(Claim 2) The first and second integrals
Means, by the capacitor and resistors and an operational amplifier
Constitute.

[0008]

(Claim 3 ) The BI according to claim 1
Two analog filters each composed of a QUAD circuit, a third voltage amplifying unit , and a switch unit are configured to be operated in parallel.

[0010]

In FIG. 1, first voltage amplifying means ( the first voltage amplifying means in FIG. 1)
The input voltage to the first voltage gain varying means is Vin, and the first, second, and third voltage amplifying means (the first and second voltage amplifying means in FIG. 1).
The gains of the second and third voltage gain variable means are respectively G ₁ , G ₂ , K, and the first and second integrating means (FIG. 1).
For the first and second variable voltage / current conversion rate integrating means,
A), the voltage / current conversion rates are gm ₁ and gm ₂ , the integration capacity of the first and second integration means is C, and the second integration is
Assuming that the output voltage of the means is Vout, the output of the second integrating means is

[0011]

(Equation 1)

## EQU1 ## When Vin and Vout are combined,

[0013]

(Equation 2)

## EQU1 ## Therefore, the transfer function T (S) is

[0015]

(Equation 3)

Is required. Here, S = jω, ω = 2
πf, f are frequencies. When K in the above numerator polynomial is +, it is a band rejection type, when-is a high-frequency emphasis type, and when K = 0, it is a low-pass type. As a result, it is possible to cope with various forms of analog signal processing.

Further, the first and second integrating means and the first, second and third voltage amplifying means are used to externally supply power.
By arbitrarily varying the voltage / current conversion rate and the voltage gain by varying the flow, the transfer function of the filter can be freely created, and a wider adjustment range than before can be secured.

[0018]

FIG. 2 is a block diagram of a BIQUAD circuit according to an embodiment of the present invention. In the figure, 2-1 2-2 4-1 4-2
, 6-1 and 6-2 are voltage amplifiers, 3-1, 3-2 7-1 and 7-2 are voltage / current converters, 1 and 8 are buffers, and 5-1 and 5-2 are switches. One BIQUAD circuit 9 is constituted by a circuit composed of the voltage amplifiers 2-1 and 6-1, the voltage / current converters 3-1 and 7-1, and the capacitor C, and the voltage amplifiers 2-2 and 6- Two
Another BIQUAD circuit 10 is constituted by a circuit comprising the voltage / current converters 3-2 and 7-2 and the capacitor C. The two BIQUAD circuits 9 and 10 operate differentially.

First, the transfer function of the BIQUAD circuit is obtained. The positive input voltage of the voltage amplifier 2-1 is Vin, the output voltage of the voltage / current converter 7-1 is Vout, the gain of the voltage amplifier 2-1 is G _a , and the conductance of the voltage / current converter 3-1 is g.
m _a , the gain of the voltage amplifier 6-1 is G _b , the voltage / current converter
The conductance of the 7-1 and the gm _b,

[0020]

(Equation 4)

## EQU1 ## Summarizing by Vin and Vout respectively

[0022]

(Equation 5)

Therefore, the transfer function T (S) is given by the following equation (2).

[0024]

(Equation 6)

Is required. Here, S = jω,
ω = 2πf, f is the frequency. From the equation (3), the parameters ω _o (resonance angular frequency) and Q (selectivity) of the active filter are respectively given by the following equations.

[0026]

(Equation 7)

In the equation (3), when the sign of K in the numerator polynomial is +, the band rejection type, when-, the high-frequency emphasis type, and K =
When it is 0, it becomes a low-pass type, and switches 5-1 and 5-
It is determined by the switching state of 2. The frequency characteristics of T (jω) in the case of the band rejection type and the high frequency emphasis type are shown in FIGS. 3A and 3B, respectively. In the figure, ω _n , ω
_b is the resonance angular frequency, Q _b represents the selectivity in the case of high-tone-emphasized.

Firstly, in order to obtain the required Q value, a ratio of gm, determines the ratio of G, by combining the absolute value of the Thereafter omega _o, can be arbitrarily manipulate the transfer function. BIKU
There are two types of AD circuits, (1) and (2) shown in FIG. 4, where the S / N (signal / noise ratio) of each type is determined.
To consider. For each type of circuit, assuming that the value of the capacitor is negligible within the passband,
The voltage / current converter can be considered as a high gain A voltage amplifier, and each type of BIQUAD circuit can be represented by a model shown in FIG.

[0029] In FIG. 5, for the type (1), the noise n _a to the input of the voltage / current converter, assuming that n _b is mixed, obtaining the S / N at the output Vout.

[0030]

(Equation 8)

When Va and Vb are erased and Vout is summarized,

[0032]

(Equation 9)

## EQU1 ## Since A ≧ 1, Vout is given by the following equation.

[0034]

[Equation 10]

Therefore, S at the output of type (1)
/ N is as follows.

[0036]

[Equation 11]

The type (2) BIQUAD circuit can be obtained in the same manner, and the following equation is established.

[0038]

(Equation 12)

When Va and Vb are erased and Vout is summarized,

[0040]

(Equation 13)

## EQU1 ## Since A ≧ 1, Vout is given by the following equation.

[0042]

[Equation 14]

Therefore, S at the output of type (2)
/ N is as follows.

[0044]

(Equation 15)

As can be seen by comparing Equations (11) and (17), in the case of Type (2), the noise component n _b mixed in the input at the subsequent stage is the voltage gain component of the voltage / current converter. Only being suppressed. From this, it can be said that the noise characteristic of the BIQUAD circuit is better in the type (2) than in the type (1). In the embodiment of the present invention shown in FIG. 2 described above, the above results are used.

FIG. 6 shows the voltage / current converter 3-1 (3-2) of FIG.
FIG. 8 is a configuration diagram of an embodiment of a BIQUAD circuit when operational amplifiers 11 and 12 are used instead of 7-1 (7-2). In this case, the only variable parameters are the gains (G _a , G _b ) of the voltage amplifiers 2 and 6, and the circuit is also the S (N) numerator of T (S).
Is inferior to the case of FIG. 2 because of the use of an operational amplifier because of the use of an operational amplifier.
The be replaced with switched capacitor using the CMOS, the same effect can be expected as to vary the gm _a, gm _b in FIG.

FIG. 7 is a circuit diagram of the whole analog filter according to the embodiment of the present invention. As shown in the figure, by connecting a plurality of BIQUAD circuits shown in FIG. 2 in cascade, the cutoff characteristic of the filter can be made steep, and the number can be arbitrarily selected according to the application. In addition, energy saving can be achieved by turning off the power supply to the unused BIQUAD circuit.

FIG. 8 is a configuration diagram of a K value setting circuit in the embodiment. The K value setting circuit includes an analog differential amplifier 14 for determining a reference value of K, a circuit 15 including two differential amplifiers for determining a variable portion of K, and one of the two differential amplifiers. And a switch circuit 16 for selecting one.

The voltage gain G1 of the preceding-stage differential amplifier 14 is given by the following _equation , where the internal resistance of the diode D as a load is _reref.

[0050]

(Equation 16)

The gain G2 of the differential amplifier of the subsequent circuit 15 is
_Assuming that the internal resistance of the emitter-coupled transistor is _revar ,

[0052]

[Equation 17]

Is as follows. From equations (18) and (19), the overall gain G, ie, | K |

[0054]

(Equation 18)

Is as follows. Where k is Boltzmann's constant, T
Is the absolute temperature, q is the electron charge, I _Kref is the reference current, I
_Kvar is a variable current. The sign of K can be selected by switching SW.

FIG. 9 is a configuration diagram of a voltage gain variable circuit in the embodiment. The circuit configuration of FIG. 9 is basically the same as the circuit configuration of FIG. 8 described above, but FIG. 9 does not include the switch circuit 16 of FIG. As described above, the gain G of the circuit of FIG. 9 is given by the following equation.

[0057]

(Equation 19)

Here, I _Gref is a reference current, and I _Gvar is a variable current. FIG. 10 and FIG. 11 are configuration diagrams of the variable voltage / current conversion circuit in the embodiment. Although FIG. 10 shows the case of a bipolar transistor, the circuit arrangement of FIG even a circuit configuration basically the same as in FIG. 9 described above, the conductance gm of the voltage / current conversion circuit, the current flowing through the I _{RE F} terminal 2I
_Iref, and the current flowing through the I _VAR terminal and 3I _Ivar,

[0059]

(Equation 20)

Is obtained. As described above, the transfer function of the expression (3) can be freely operated. FIG. 11 shows a CMOS
Although the case of a transistor is shown, there is one control current terminal, and gm cannot be linearly changed with respect to the control current I _{VARA (B)} . However, the non-linearity of gm can be compensated for by using together with a variable gain voltage amplifier as in the present invention. (I _{VARA (B)} is fixed and the variable parameter is only G.)

[0061]

As described above, according to the present invention, various forms of analog signal processing can be handled by switching the output of the third voltage gain variable means by the switch means. In addition, the first and second voltages /
Freely creating a transfer function of a filter by arbitrarily varying a voltage / current conversion rate and a voltage gain by using a current conversion rate variable integration means and first, second, and third voltage gain variable means. And a wider adjustment range than before can be secured. At the same time, the relative variation of the capacitance value of the capacitor can be corrected by the above means.

[Brief description of the drawings]

FIG. 1 is a principle diagram of the present invention,

FIG. 2 is a configuration diagram of a BIQUAD circuit according to an embodiment of the present invention;

FIG. 3 is a frequency characteristic diagram of T (jω) in the embodiment;

FIG. 4 is a diagram showing two types of BIQUAD circuits of an example;

FIG. 5 is a diagram showing passband models of each type of the BIQUAD circuit of FIG. 4;

FIG. 6 is a configuration diagram of a BIQUAD circuit according to another embodiment of the present invention;

FIG. 7 is a circuit configuration diagram of an entire analog filter according to an embodiment of the present invention;

FIG. 8 is a configuration diagram of a K value setting circuit in the embodiment,

FIG. 9 is a configuration diagram of a voltage gain variable circuit according to the embodiment;

FIG. 10 is a configuration diagram (part 1) of a variable voltage / current conversion circuit in an embodiment;

FIG. 11 is a configuration diagram (part 2) of a variable voltage / current converter circuit according to the embodiment;

FIG. 12 is a configuration diagram of a conventional BIQUAD circuit.

[Explanation of symbols]

 200 is first voltage gain variable means, 300 is first voltage / current conversion rate variable integration means, 400 is third voltage gain variable means, 500 is switch means, 600 is second voltage gain variable means, Reference numeral 700 denotes a second voltage / current conversion rate variable type integrating means.

Claims (3)

(57) [Claims] A first input voltage representing an input signal;
A first voltage output by inverting and adding the phase of an input voltage and amplifying the added value with a voltage gain that can be varied from the outside
Amplifying means, together with integrating the outputs after converting the output voltage into a current of the first voltage amplifier means, first can vary the electric current from the outside 1
And integrating means, the output of the first integrating means the phase of the second input voltage anti of
A second voltage amplifying means for amplifying and outputting the added value with a voltage gain which can be varied from outside , converting the output voltage of the second voltage amplifying means into a current, integrating and outputting the current , If the output is branched to be the second input voltage
In both cases, a BIQUAD (biquad) circuit comprising a second integrating means capable of varying the current from the outside , and a branch input of the first input voltage, which can be varied from the outside.
Amplify with positive voltage gain (K) to produce positive and negative signals
The third voltage amplifying means, and the output of the third voltage amplifying means by an external switching signal.
One of the forces is selected and output, and the output of the second integrating means is output.
An analog filter , comprising: switch means for adding to a force . 2. The analog filter according to claim 1, wherein said first and second integration means each comprise a capacitor, a resistor, and an operational amplifier. 3. The BIQUAD circuit according to claim 1, wherein:
And third voltage amplifying means, and switch means.
Operating two analog filters in parallel
An analog filter.