JPH04109705A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To control a voltage gain change with respect to a control voltage always along with a prescribed exponential function by providing a two-input voltage output type adder to an input circuit. CONSTITUTION:This amplifier consists of differential amplifier circuits A1, A2 and resistors R1, R2 or the like and the mutual conductance is implemented by a control circuit 10, a control voltage source Vc, a low voltage power supply V2 and a constant current source I0. That is, the output of the 2nd differential amplifier circuit A2 is connected to the load resistor R2 and a voltage is extracted and the mutual conductance of the 1st and 2nd differential amplifier circuits A1, A2 is controlled to vary the voltage gain. A two-input voltage output type adder A1 is provided to the amplifier, in which its 1st input is connected to an input terminal (a), a its output is connected to the other terminal of the resistor R1 and its 2nd input is connected to an output of the 1st differential amplifier circuit A1. Thus, the voltage controlled amplifier is realized, where its voltage gain Av varies along an exponential function with respect to the control voltage Vc at all times.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable gain amplifier, and particularly to a variable gain amplifier circuit whose voltage gain changes exponentially with respect to a control voltage.

[Conventional technology]

An example of a conventional variable gain amplifier circuit is shown in FIG. 2, and a specific example of the circuit is shown in FIG.

In FIG. 2, this circuit includes a current output type differential circuit A1.
A2, resistors R1, :R2, input terminal d, output terminal, constant power supply voltage V + , V 2, and control voltage source ■
. And constant current source■. and a control circuit 10. In FIG. 3, transistors Q1 to Q15. Resistor R,, R2, constant current source. , constant voltage source V, ,V2. A control voltage source Vc is shown. Here, transistor Q
1 to Q, and constant current source engineering. r I3 constitutes the current output type differential circuit A1 in FIG.
g and constant current source I are current output type differential circuit A in Fig. 2.
2. Also, transistors QB, Qy, constant current source ■. , constant voltage source v2. Control circuit 1 with control voltage source vo
Configure 0. However, transistor Q is transistor Q 4 .

It is inserted as a buffer to reduce the error caused by the base current of Q.

Next, the operation of the conventional circuit will be briefly explained with reference to FIG.

Here, it is assumed that the input impedance of the differential circuits A + and A 2 is sufficiently high.

Now, when the input signal V is applied to the input terminal d, the output signal V at the output terminal. is calculated using the following formula.

First, the input pressure characteristics of the differential circuit A1 are determined by the following equation.

vo=:v, +i −R+ ””'
・■2 Niggi Yu・Vc...
...■ From the above formulas ■ and ■, the following formula is obtained.

v, = v, / (1+gm+-R+) +・+
・Next, the output current io of the differential circuit A2 is given by the following formula: 1
. = g I2” VC Therefore, ■. = g Ka = vc-Rz ・・・・
...■ From the above formulas ■ and ■, the voltage gain of the conventional circuit is:
It is obtained as the following equation.

Here, v,: input signal voltage at point d, VC: output signal voltage at point C, output signal voltage at point V□tb, i: AC output current of differential circuit A1, 1°; differential AC output current rgml of circuit A2: mutual conductance r of differential circuit A1
g m2 'Transconductance of differential circuit A2.

Here, the voltage gain A is controlled by a differential circuit At.

A + mutual conductance g. This is done under the control of zgm2.

Next, the mutual conductance g. Control of i+gm2 will be explained.

Mutual conductance g m of differential circuit A I, A 2
l +gm2 is determined by the following formula.

Here, when the condition gffi+-R+>1 is satisfied,
From the above equation 0 and the above equation 0, the following equation is obtained.

Here, the mutual conductance g is controlled by the current I2 shown in FIG. Therefore, the following equation is obtained.

Moreover, from the above formulas (1) and (2), the following formula is obtained.

I2 g+12 11 gml From the above equations (1) and (2), the following equation is obtained.

・・・・・・■ gml g+ez =e ΔV□) ・・・・・・■ gml Here, Il: collector current of transistor Q6.

■2: Collector current of transistor Q, ■2: Constant voltage source, V+: Control voltage source.

In this way, in the case of conventional voltage-controlled amplifiers, the operating condition is always g. Only when the condition 1.R1>1 is satisfied, the voltage gain A7 can be controlled exponentially with respect to the control voltage (2).

[Problem to be solved by the invention]

In the case of the conventional variable gain amplifier mentioned above, the voltage gain of the amplifier is not simply determined by the ratio of mutual conductance, and especially when the voltage gain becomes large, that is, the value of [g, 1・R1] becomes small, and Cg =i ・R1+1; ga, ・R1]
When this approximation is no longer possible, the error component becomes dominant and the control characteristic deviates from the set exponential characteristic.

SUMMARY OF THE INVENTION An object of the present invention is to provide a variable gain amplifier in which the above-mentioned drawbacks are solved and the control characteristics follow a set exponential characteristic.

[Means to solve the problem]

The configuration of the present invention is as follows. a second resistor, and a current output type first resistor. a second differential circuit, a voltage output type adder, and a gain control means, one end of the first resistor is connected to the pressure and inverting input of the first differential circuit, and the second difference the non-inverting input of the first differential circuit and the inverting input of the second differential circuit are connected to a constant potential source; A variable gain comprising a parallel-connected differential amplifier having a second resistor connected to its output, and the gain control means configured to complementarily control the mutual contactance of the first and second differential circuits. In an amplifier circuit, a first input is connected to an input terminal, an output is connected to the other end of the first resistor, and a second input is connected to the output of the first differential circuit. It is characterized by the provision of a two-input voltage output type adder.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing a variable gain amplifier according to an embodiment of the present invention. In FIG. 1, the variable gain amplifier of this embodiment is a current output type differential circuit AI.

A2, voltage output type adder A3, resistors R,, R2,
It has an input terminal a, an output terminal S, a control circuit 10, constant voltages V, V2, and a variable control voltage source (2).

In FIG. 1, elements having the same symbols and numbers as in the second section indicate the same elements.

In this embodiment, similarly to the conventional circuit shown in FIG. 2, the differential circuit A1. It is composed of A+, resistors R+, R2, etc., but differs from the conventional one in that an adder A3 is added to the input circuit.

It is assumed that the input impedance of this adder A3 is sufficiently high. Further, mutual conductance is controlled by a control circuit 1o, a control voltage source V9. Constant voltage source V
2. Constant current power supply. It will be held at

Next, the operation of the variable gain amplifier of this embodiment shown in FIG. 1 will be explained. However, the method of controlling the voltage gain is the same as that of the conventional circuit, so a description thereof will be omitted.

When the input signal voltage v1 is applied to the input terminal a, the output signal voltage v0 at the output terminal is determined by the following equation.

First, the voltage at the 0 point is determined by the following equation.

vc= (v, +ve)+iR+ ・−・・
@1--g□1・ve・・・・・・
・0 above 0. From the formula o, the following formula is obtained.

V x ” ” l / g ml ' Rl
'... Since the output current of the zero-order differential circuit A2 is +10'' g m2 Vz, the following equation is obtained.

VO: gll, 2 ·v c-R2--@ From the above equation 0, 0, the voltage gain of this embodiment is expressed as follows.

Here, the following equation can be obtained from the above equation 0 and the above equation 0 using a control method similar to that of the conventional circuit.

In this way, it is possible to obtain a voltage controlled amplifier in which the voltage gain AV always changes exponentially with respect to the control voltage.

In this embodiment, the output of the second differential circuit is connected to a load resistor to take out the voltage, and the output of the second differential circuit A1.
The mutual conductance of A2 is controlled and the voltage gain is varied.

〔Effect of the invention〕

As described above, the present invention has the advantage that by including the adder in the input circuit, the change in voltage gain with respect to the control voltage can always be controlled using a required exponential function.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a variable gain amplifier according to an embodiment of the present invention, Figure 2 is a block diagram of a conventional variable gain amplifier, and Figure 3 is a block diagram of a conventional variable gain amplifier.
The figure is a specific circuit diagram of the conventional circuit shown in FIG. A I, A2...Differential circuit, A3...
...Adder, R1゜R2...Resistance, a, d...
...Input terminal, b...Output terminal, Q1~Q1
5...Transistor+IO+I!・・・・・・
Constant current circuit, V,, V2... Constant voltage circuit, v. ...Control voltage source. Agent Patent Attorney Oto Uchihara

Claims (1)

[Claims] The device includes first and second resistors, first and second current output type differential circuits, a voltage output type adder, and gain control means, and one end of the first resistor is connected to the first resistor. and an output and an inverting input of the differential circuit, and a non-inverting input of the second differential circuit, and a non-inverting input of the first differential circuit and an inverting input of the second differential circuit. is connected to a constant potential source and a second resistor is connected to the output of the second differential circuit to complement the mutual conductance of the first and second differential circuits. In the variable gain amplifier circuit provided with the gain control means so as to control the gain, the first input is connected to the input terminal, the output is connected to the other end of the first resistor, and the second input A variable gain amplifier comprising a two-input voltage output type adder connected to the output of the first differential circuit.