JP2538708B2 - Variable gain amplifier - Google Patents

Description

The present invention relates to a variable gain amplifier, and more 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 concrete circuit example thereof is shown in FIG.

In FIG. 2, this circuit is a current output type differential circuit A ₁ , A
₂ , the resistors R ₁ and R ₂ , the input terminal d, the output terminal b, the constant power source voltages V ₁ and V ₂ , the control voltage source V _c , the constant current source I _0, and the control circuit 10. Included and configured. In FIG. 3, transistors Q _{1 to} Q ₁₅ , resistors R ₁ and R ₂ , constant current source I ₀ , constant voltage sources V ₁ and V ₂ ,
The control voltage source V _c is shown. Where transistor Q ₁
~ Q ₅ and the constant current sources I ₀ , I ₃ constitute the current output type differential circuit A _{1 of} Fig. 2, and the transistors Q _{8 to} Q ₂₅ and the constant current source I ₀ are
The current output type differential circuit A 2 shown in FIG. ₂ is constructed. Further, the transistors Q ₆ , Q ₇ , the constant current source I ₀ , the constant voltage source V ₂ , and the control voltage source C _c constitute the control circuit 10. However, the transistor Q ₁ is included as a buffer in order to reduce an error due to the pace current of the transistors Q ₄ and Q ₈ .

Next, the operation of the conventional circuit will be briefly described with reference to FIG. Here, it is assumed that the input impedance of the differential circuits A ₁ and A ₂ is sufficiently high.

Now, when the input signal v _i is applied to the input terminal d, the output terminal v _o at the output terminal b is obtained by the following equation.

First, when the input / output characteristics of the differential circuit A ₁ are obtained, the following equation is obtained.

_{v C = v I + i ·} R 1 ...... i = -g m1 · v c ...... it said, the equation, the following equation is obtained.

v _c = v _i / (1 + g _m1 · R ₁ ) ... Next, the output current i _o of the differential circuit A ₂ is given by the following equation.

i _o = g _m2 · v _c Therefore, v _c = g _m2 · v _c · R _d ...... From the above equation, the voltage gain of the conventional circuit is obtained as the following equation.

Where v _i ; input signal voltage at point d, v _C ; output signal voltage at point _c , v _c ; output signal voltage at point b, i; AC output current of differential circuit A ₁ , i _o ; AC output current of differential circuit A ₂ , g _m1 ; Differential circuit A ₁
Transconductance of, g _m2 ; _{Transconductance of} differential circuit A ₂ .

Here, the voltage gain A _v is controlled by controlling the mutual conductances g _m1 and g _m2 of the differential circuits A ₁ and A ₂ .

Next, control of the mutual conductances g _m1 and g _m2 will be described.

The mutual conductances g _m1 and g _m2 of the differential circuits A ₁ and A ₂ are obtained by the following equation.

Here, the mutual conductance g _m is controlled by the currents I ₁ and I ₂ in FIG. Therefore, the following equation is obtained.

Further, from the above equation, the following equation is obtained.

From the above equation, the following equation is obtained.

Here, I ₁ ; collector current of transistor Q ₆ , I ₂ ; collector current of transistor Q ₇ , V ₂ ; constant voltage source, V _c ; control voltage source.

Here, when the condition of g _m1 · R ₁ >> ₁ is satisfied, the following equation is obtained from the above equation and the above equation.

As described above, in the case of the conventional voltage-controlled amplifier, the control voltage V can be set only when the usage condition is always g _m1 · R ₁ >> _1.
The voltage gain A _V exponentially control can be with respect to _C.

[Problems to be Solved by the Invention]

For conventional variable gain amplifier as described above, simply the voltage gain of the amplifier is not determined by the ratio of the transconductance, especially when the voltage gain becomes larger, i.e., the value of [g _m1 · R _1] becomes smaller, [g _m1・ R ₁ + 1 ≒ g _m1・ R ₁ ], the error component becomes dominant and the control characteristic becomes
There is a drawback that it deviates from the set exponential function characteristic.

The object of the present invention is to solve the above-mentioned drawbacks and to provide control characteristics
Another object of the present invention is to provide a variable gain amplifier adapted to meet a set exponential function characteristic.

[Means for solving the problem]

The configuration of the present invention includes the first and second resistors and the current output type first resistor.
1, a second differential circuit, a voltage output type adder, and gain control means, wherein one end of the first resistor is connected to the output and inverting input of the first differential circuit and the second differential circuit. Connected to the non-inverting input of the differential circuit, and connecting the non-inverting input of the first differential circuit and the inverting input of the second differential circuit to a constant potential source,
A parallel connection differential amplifier in which a second resistor is connected to the output of the second differential circuit is provided, and the gain is provided so as to complementarily control the transconductance of the first and second differential circuits. In the variable gain amplifier circuit provided with the control means, the first
Two-input voltage output type in which the input of is connected to the input terminal, the output is connected to the other end of the first resistor, and the second input is connected to the output of the first differential circuit. A feature is that an adder is provided.

〔Example〕

 Next, the present invention will be described 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 according to the present embodiment includes current output type differential circuits A ₁ and A ₂ , a voltage output type adder A ₃ , resistors R ₁ and R ₂ , an input terminal a, and an output. Terminal b
, Control circuit 10, constant voltage V ₁ , V ₂ , and variable control voltage source V
_{It has c} and.

In FIG. 1, elements having the same symbols and numbers as in FIG. 2 indicate the same elements.

In this embodiment, similar to the conventional circuit of FIG. ₂ , it is composed of differential circuits A ₁ and A ₂ , resistors R ₁ and R _2, etc., but the point that an adder A ₃ is added to the input circuit Different from conventional. The input impedance of this adder A ₃ is sufficiently high. In addition, the control of mutual conductance is similar to the conventional circuit.
10. Control voltage source V _C , constant voltage source X ₂ , constant current source I ₀ .

Next, the operation of the variable gain amplifier of this embodiment shown in FIG. 1 will be described. However, the method of controlling the voltage gain is the same as that of the conventional circuit, and therefore will be omitted.

When the input signal voltage v _i is applied to the input terminal a, the output signal voltage v _o at the output terminal b is obtained by the following equation.

 First, the voltage at point c is obtained by the following equation.

v _c = (v _i + v _c ) + iR ₁ ... i = -g _m1 · v _c ... From the above equation, the following equation is obtained.

v _c = v _i / g _m1 · R _z ...... Then, the output current of the differential circuit A ₂ is i _o = g _{m2v c} , so the following equation is obtained.

v ₀ = g _m2 v _c · R ₂ ... From the above equation, the voltage gain of this embodiment is as follows.

Here, from the same control method as the conventional circuit,
From the above equation, the following equation is obtained.

In this way, it is possible to obtain a voltage-controlled amplifier in which the voltage gain A _V changes exponentially with respect to the control voltage at all times.

In the present embodiment, the output of the second differential circuit is connected to the load resistance to take out the voltage, and the first and second differential circuits A ₁ , A
_It controls the mutual conductance of ₂ and changes the voltage gain.

〔The invention's effect〕

As described above, according to the present invention, by including the adder in the input circuit, the voltage gain change with respect to the control voltage is
It has the effect that it can always be controlled with the required exponential function.

[Brief description of drawings]

FIG. 1 is a block diagram of a variable gain amplifier according to an embodiment of the present invention, FIG. 2 is a block diagram of a conventional variable gain amplifier, and FIG.
The figure is a specific circuit diagram of the conventional circuit of FIG. A ₁ , A ₂ ...... Differential circuit, A ₃ ...... Adder, R ₁ , R ₂ ...... Resistance, a, d…
… Input terminal, b …… Output terminal, Q _{1 to} Q ₁₅ … Transistor, I
₀ , I ₃ …… Constant current circuit, V ₁ , V ₂ …… Constant voltage circuit, V _C …… Control voltage source.

Claims (1)

(57) [Claims] 1. A first and a second resistance, and a first and a second of a current output type.
Differential circuit, a voltage output type adder, and gain control means, and one end of the first resistor is connected to the output and inverting input of the first differential circuit and the second differential circuit. Of the first differential circuit and the inverting input of the second differential circuit are connected to a constant potential source, and are connected to the output of the second differential circuit. A variable gain amplifier circuit comprising a parallel-connected differential amplifier formed by connecting a second resistor, and the gain control means so as to complementarily control the mutual conductance of the first and second differential circuits. , A first input connected to the input terminal, an output connected to the other end of the first resistor, and a second input connected to the output of the first differential circuit. A variable gain amplifier comprising a voltage output type adder.