JPS6144409B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable impedance circuit that can electrically control impedance between two terminals,
It relates particularly to variable impedance circuits suitable for monolithic integrated circuit configurations.

The simplest way to implement a variable impedance circuit is to use the nonlinearity of diodes and FETs. However, these devices generally have a very narrow linear region, and are of little practical use, especially in monolithic integrated circuits, due to restrictions on elements and circuit configuration. For this reason, other variable impedance circuits have been considered, but all of them are ground plane variable impedance circuits that vary the impedance between an arbitrary terminal and a ground terminal.

When considering variable impedance circuits in relation to their uses, for example in AGC circuits and gain control circuits, there are no particular problems with grounded variable impedance circuits. However, when the transfer function of a circuit is controlled by a DC control voltage, the degree of freedom in circuit configuration is too small with a variable impedance circuit. Application examples of variable impedance circuits for controlling the transfer function of circuits include tone control circuits and active filters in audio equipment.

For variable impedance circuits used in such applications, floating types are often more convenient than grounded types. The reason is as follows.

FIG. 1 shows a conceptual diagram of a variable impedance circuit. In other words, the part R surrounded by the broken line is a variable impedance circuit, which includes two terminals 1 and 1 whose impedance changes.
2 and a terminal 3 to which a control signal for impedance control is applied. In the most general circuit, there is a restriction that one of the terminals 1 and 2 must be grounded, and the variable impedance circuit R acts as a variable impedance only when viewed from the terminal 1.

The first reason why the grounded variable impedance circuit is inconvenient as a means of controlling the transfer function is that the variable impedance circuit can only be used with one terminal grounded, which greatly limits the circuit configuration. Is Rukoto. The second reason is that, especially in the form of a monolithic integrated circuit, floating the variable impedance circuit makes it easier to reduce the number of pins for connecting external elements. This second reason will be specifically explained using a tone control circuit as an example.

Figure 2 shows the principle of the tone control circuit. For example, if a CR series circuit is prepared as shown in Figure 2a and a signal V ₀ is applied to it, C, R
Primary LPF output Vc and primary HPF output VR at each end
Therefore, by controlling the gains and time constants (C×R) of the three voltages Vo, Vc, and _VR , it is possible to adjust the sound quality in any low or high range. This second
In the case of Figure a, R can be configured with a grounded variable impedance circuit, but C is floating. Therefore, in the form of a monolithic integrated circuit, two pins are required for connecting C externally. FIG. 2b shows a configuration in which C is a ground plane. In this case, there is an advantage that C can be connected externally by a pin, but R requires a floating variable impedance circuit.

Variable impedance circuits that can be used in a floating state are advantageous over their grounded counterparts in a number of ways. However, with conventional variable impedance circuits, such as those that change the impedance between the drain and source by controlling the voltage between the gate and source of an FET, when used in a floating state, the gate potential changes depending on the signal voltage, so the drain - It had a fatal drawback that the impedance between the sources varied greatly depending on the signal voltage, and it could not be used in a floating state.

The present invention has been made in view of the above points, and
The purpose is to provide a variable impedance circuit that can be used in a floating state.

Another object of the present invention is to provide a variable impedance circuit with good linearity.

The present invention will be explained in detail below using examples.

FIG. 3 shows a first embodiment of the present invention, in which the variable impedance circuit includes a differential voltage-current converter (hereinafter referred to as a V/I converter) 10, and the V/I converter 10. A current amplifier 20 that amplifies the output current, a current divider circuit 30 that divides the output current of this current amplifier 20 at the same division ratio, and a V/I converter 1 that converts changes in the output current of this current divider circuit 30.
0, and is configured to change the impedance between the differential input terminal pair of the V/I converter 10 by controlling the division ratio of the current dividing circuit 30. has been done.

That is, the V/I converter 10 has a differential input terminal pair 11 and 12 and a differential output terminal that outputs a differential output current corresponding to the difference in the input voltage to the input terminal pair 11 and 12. It has a configuration in which the emitter electrodes of PNP transistors Q ₁₁ and Q ₁₂ are connected via a resistor R ₁ and connected to a power input terminal 15 via current sources 16 and 17. The base electrodes of Q ₁₁ and Q ₁₂ serve as a differential input terminal pair 11 and 12, and the collector electrodes serve as a differential output terminal pair 13 and 14. Note that the differential input terminal pair 11 and 12 are connected to first and second terminals 1 and 2, which are variable impedance terminals, and the power input terminal 15 is connected to the terminal 4 to which a positive power supply +V is applied.

Differential output terminal pair 13, 14 of V/I converter 10
are connected to the input terminal pair 21 and 22 of the current amplifier 20. The current amplifier 20 has a pair of input terminals 21 and 22.
The input current is amplified and output to the output terminal pair 23, 24. In this example, one end is connected to the input terminal pair 2.
1 and 22, and the other end was connected to terminal 5 to which the negative power supply -V is applied via resistors R ₂₁ and R ₂₂ .
The base electrode is connected to the PN junctions Q ₂₁ and Q ₂₂ and the input terminal pair 21 and 22, and the emitter electrode is connected to the resistor R ₁₃ ,
_R14 is connected to the terminal 5 to which -V is applied, and the collector electrode is connected to the output terminal pair 23, 24.
It consists of transistors Q ₂₃ and Q ₂₄ connected to.

A pair of output terminals 23 and 24 of the current amplifier 20 are connected to a pair of input terminals 31 and 32 of a current dividing circuit 30. The current dividing circuit 30 divides and outputs the input current to the input terminal pair 31, 32 to the output terminal pair 33, 34 at an arbitrary division ratio, and is composed of two emitter-coupled differential transistor pairs 35, 36. That is, the emitter-coupled differential transistor pair 35, 36
The emitter electrodes of each of the two transistors Q ₃₁ , Q ₃₂ and Q ₃₃ , Q ₃₄ forming the input terminal pair 31,
32, the collector electrodes of Q ₃₁ and Q ₃₄ are connected to terminal 4 to which +V is applied,
Furthermore, the collector electrodes of Q ₃₂ and Q ₃₅ are connected to output terminal pair 3.
3 and 34. A control voltage Vc is applied between the base electrodes of the emitter-coupled differential transistor pair 35, 36 via control terminals 3a, 3b, and the current division ratio is controlled by this control voltage Vc. It's getting old.

One terminal 33 of the pair of output terminals 33, 34 of the current dividing circuit 30 is connected to the input terminal 41 of the current mirror circuit 40, and the other terminal 34 is connected to the output terminal 42 of the current mirror circuit 40, and the V/ Differential input terminal pair 11, 1 of I converter 10
It is connected to the other terminal 12 of 2 to form a return path.

Next, the operation of the variable impedance circuit shown in FIG. 3 will be explained. The differential output current of the V/I converter 10 is approximately expressed by the following equation.

i ₃ = I ₁ - 1/R ₁ (V ₁ - V ₂ ) ...(1) i ₄ = I ₁ + 1/R ₁ (V ₁ - V ₂ ) ... (2) 1/R ₁ is K ₁ , (V ₁ −V ₂ ) is the voltage at the impedance end and Vimp, then i ₃ and i ₄ are obtained by the following equations.

i ₃ = I ₁ − _{K 1} Vimp ……(3) i ₄ = I ₁ + K ₁ Vimp ……(4) Generally, R ₂₁ = R ₂₂ , R ₂₅ = R ₂₄ , and K ₂ =
If R ₂₁ /R ₂₃ , the output current i ₅ of the current amplifier 20,
i ₆ is expressed as i ₅ = k ₂ i ₄ = K ₂ (I ₁ + K ₁ Vimp) ……(5) i ₆ = K ₂ i ₃ = K ₂ (I ₁ − K ₁ Vimp) ……(6) It can be done. Here, if the current division ratio of the current division circuit 30 is f (Vc), then i ₇ = f (Vc) · K ₂ (I ₁ + K ₁ Vimp) ... (7) i ₆ = f (Vc) · K ₂ ( I ₁ −K ₁ Vimp), where f(Vc) is f(Vc)=1/1+exp(8Vc/kT)...(8) where q is the elementary charge, k is Boltzmann's constant, and T is the absolute It's temperature. Therefore, the output current i ₂ is i ₂ = i ₇ − i ₈ = f(Vc)・Vimp (9), and the conductance changes depending on the control voltage Vc, and the conductance is f(Vc)・K ₁・Represented by K ₂ .

Therefore, the variable impedance circuit of FIG. 3 can be equivalently represented as a circuit in which a voltage follower is connected to one end of a variable impedance element as shown in FIG.

According to the above configuration, the impedance between terminals 1 and 2 is as clear from equation (9).
It is kept constant regardless of changes in individual ground voltages. Therefore, there is an advantage that both terminals 1 and 2 can be used in a state where they are both lifted from the ground potential, that is, in a floating state.

Further, since the impedance between the terminals 1 and 2 is varied by controlling the division ratio of the current dividing circuit 30, the impedance line formation between the terminals 1 and 2 is improved. That is, in general, in a differential voltage-current converter in which feedback is applied as shown in Fig. 3, it is possible to change the impedance between terminals 1 and 2 by changing the amount of feedback. As a specific means for this, in addition to the method of controlling the division ratio of the current dividing circuit as described above, there is also a method of controlling the gain of a current amplifier provided in the feedback loop. In this case, the current amplifier receives the input current through a PN junction pair, applies the voltage generated across the PN junction pair between the base electrodes of the emitter-coupled differential transistor pair, and extracts the output current from the collector electrode. One possibility is to control the common emitter current of a coupled differential transistor pair to change its gain. However, in such a current amplifier, the difference in current-voltage characteristics of each PN junction of a PN junction pair, or the difference in the base-emitter voltage vs. collector current characteristics of each transistor of an emitter-coupled differential transistor pair, is used as a current amplifier. There is a problem that appears as nonlinearity.

On the other hand, as shown in FIG.
By controlling the division ratio of 0 to change the impedance between terminals 1 and 2, the current amplifier 20 can be of a fixed gain, so instead of a differential type, a simple current reversal type with good linearity can be used. On the other hand, the current dividing circuit 30 also has small nonlinear elements. Therefore, the impedance line formation between terminals 1 and 2 is very good.

As described above, the variable impedance circuit according to the present invention can be used in a floating state and has good linearity, so it is particularly suitable for integrated circuits in audio tone control circuits and active filters that use the variable impedance circuit as a transfer function control element. This is extremely advantageous in that it is possible to improve the degree of freedom in circuit configuration and reduce the number of pins for connecting external elements such as capacitors.

FIG. 5 shows a second embodiment of the present invention, in which two sets of current dividing circuits 30a and 30b are provided,
Each of the input terminal pairs 31a, 32a and 31b,
32b is the output terminal pair 23, 24 of the current amplifier 20.
, and each output terminal pair 33a, 34a and 33b, 34b is connected to a current mirror circuit 40a,
Input/output terminals 41a, 42a and 41 of 40b
b, 42b, respectively, and one output terminal 34a, 34b is connected to the differential input terminal pair 11, 12 of the V/I converter 10, respectively, to form a feedback path. In this case, the division ratios of the current dividing circuits 30a and 30b are controlled by the same control voltage Vc.

According to the configuration of this embodiment, the conductance seen from terminal 1 and the conductance seen from terminal 2 both have finite values. Therefore, the variable impedance circuit of FIG. 5 can be equivalently represented as two variable impedance circuit elements connected in anti-parallel, each having a voltage follower connected to one end, as shown in FIG.

FIG. 7 shows a third embodiment of the present invention, and the difference from FIG. 5 is that two transistors are added to the current amplifier 20, and two output terminal pairs 23 are provided.
a, 23b and 23b, 24b are provided, and these are connected to input terminal pairs 31a, 32a and 31b, 32b of two sets of current dividing circuits 30a, 30b, and control voltages Vca, Vcb of current dividing circuits 30a, 30b are provided. It is designed so that it can be changed independently. In this way, Vca, Vcb
Therefore, the impedances seen from terminals 1 and 2 can be arbitrarily made different.

Note that the differential type V/I converter 10 in the present invention
is not limited to those shown in the first to third embodiments. For example, V/ shown in the first to third embodiments.
Although the I converter 10 is configured as a current source that outputs an output current, it is also possible to configure the I converter 10 as a current sink that sinks the output current. However, in general, it is advantageous to configure it in the form of a current source because there are fewer direct current restrictions and there is no need to consider special level shifting means.

As another example of the configuration of the differential voltage-current converter 10, as shown in FIG. 8, an emitter follower including two NPN transistors Q ₁₂ and Q ₁₄ and resistors R ₁₃ and R ₁₄ inserted on each emitter side are used. It is also possible to have a configuration consisting of. In this case, the differential output terminal pair 13,
Although the impedance of terminal 14 has a low value, as a result, the impedance of terminal 11(1)-1 with respect to differential current amplifier 20
Since it is sufficient to provide a differential output current proportional to the potential difference between the two (2), the impedance at this point does not matter.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the concept of a variable impedance circuit, FIGS. 2 a and b are diagrams showing examples of how the variable impedance circuit is used, FIG. 3 is a diagram showing the first embodiment of the present invention, and FIG. 5 is a circuit diagram showing the second embodiment of the present invention, FIG. 6 is an equivalent circuit diagram thereof, and FIG. 7 is a circuit diagram showing the third embodiment of the present invention. FIG. 8 is a circuit diagram showing another example of the configuration of the differential voltage-current converter used in the present invention. 10... Differential voltage-current converter, 20... Current amplifier, 30, 30a, 30b... Current dividing circuit, 40, 40a, 40b... Current mirror circuit.

Claims (1)

[Claims] 1. A differential voltage-current converter having a differential input terminal pair and a differential output terminal pair; a current amplifier that outputs the current that is output from the output terminal pair and outputs the amplified current from the output terminal pair, and two sets of emitters that divide the current output from the output terminal pair of this current amplifier with the same current division ratio and output it to different output terminals. a current divider circuit comprising a coupled differential transistor pair; an input terminal connected to one output terminal of the current divider circuit; and an output terminal connected to the other output terminal of the current divider circuit and one of the differential input terminal pair; A variable impedance circuit, comprising: a current mirror circuit connected thereto, wherein impedance between the differential input terminals changes by controlling a current division ratio of the current division circuit. 2. A differential voltage-current converter having a differential input terminal pair and a differential output terminal pair, and a current from the differential output terminal pair of this voltage-current converter is output to the input terminal pair, and an output terminal A current amplifier that outputs the amplified current from the pair of output terminals, and two pairs of emitter-coupled differential transistors that divide the current output from the pair of output terminals of this current amplifier by the same current division ratio and output it to different output terminals. a second current divider circuit consisting of two pairs of emitter-coupled differential transistors that divide the currents respectively output from the pair of output terminals of the current amplifier by the same current division ratio and output the divided currents to different output terminals; a current dividing circuit, an input terminal connected to one output terminal of the first current dividing circuit, and an output terminal connected to the other output terminal of the current dividing circuit and one of the differential input terminal pair. An input terminal is connected to one output terminal of the first current mirror circuit and the second current dividing circuit, and an output terminal is connected to the other output terminal of the current dividing circuit and the other of the differential input terminal pair. A variable impedance circuit comprising: a second current mirror circuit having a second current mirror circuit, wherein impedance between the pair of differential input terminals changes by controlling a current division ratio of the first and second current division circuits. 3 a differential voltage-current converter having a differential input terminal pair and a differential output terminal pair; a current from the differential output terminal pair of this voltage-current converter is output to the input terminal pair; and a current amplifier that outputs the amplified current from a second pair of output terminals; and 2 that divides the current output from the first pair of output terminals of this current amplifier by the same current division ratio and outputs the divided currents to different output terminals. a first current dividing circuit consisting of a pair of emitter-coupled differential transistors; and a second pair of output terminals of the current amplifier, each of which divides the current outputted from the second pair of output terminals at the same current division ratio and outputs the divided currents to different output terminals. a second current dividing circuit comprising a pair of emitter-coupled differential transistors; an input terminal is connected to one output terminal of the first current dividing circuit, and an output terminal is connected to the other output terminal of the current dividing circuit; a first current mirror circuit connected to one of the pair of differential input terminals, an input terminal connected to one output terminal of the second current divider circuit, and an output terminal connected to the other output of the current divider circuit; and a second current mirror circuit connected to the other of the differential input terminal pair, the impedance between the differential input terminal pair being controlled by controlling the current division ratio of the first and second current division circuits. A variable impedance circuit characterized by a change in impedance.