JPH05218767A - Variable gain amplifier circuit - Google Patents

Abstract

PURPOSE:To form the variable gain amplifier circuit with a few circuit components and low power consumption by keeping a DC component of an output voltage constant without addition of a differential amplifier circuit or the like. CONSTITUTION:A control voltage Va1 is applied to bases of transistors(TRs) Q3, Q6 and a control voltage Va2 is applied to bases of TRs Q4, Q5. Even when a gain of the variable gain amplifier circuit is fluctuated by changing the voltages Va1, Va2, a DC component of a current flowing to load resistors R1, R2 is kept constant by a constant voltage source V2 of TRs Q7, Q8 having bases biased in common. Thus, the DC component of the output voltage is kept constant without being affected of even a change in a control voltage (Va2-Va1). As a result, number of components of the circuit is less and the power consumption is small and no fluctuation in the DC component of the output due to gain fluctuation is caused.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable gain amplifier circuit, and more particularly to a variable gain amplifier circuit used in an automatic gain control circuit.

[0002]

2. Description of the Related Art A conventional variable gain amplifier circuit is shown in FIG.
A variable gain amplifier circuit as shown in (1) is known. As shown in FIG. 2, in a conventional variable gain amplifier circuit, a bi-differential type circuit composed of transistors Q9 to Q14, load resistors R3 and R4, and a constant current source I2 is generally used.

Next, the operation of the conventional variable gain amplifier circuit shown in FIG. 2 configured as described above will be described. The input signal vi2 is superimposed on the bias voltage V3 and is input to the differential amplifier circuit composed of the transistors Q9 and Q10. The collector current i10 in the transistor Q10 is expressed by the following mathematical formula 1.

[0004]

## EQU1 ## i10 = I2 / (1 + exp (vi2 / VT)) where VT is the thermal voltage of the transistor.

Next, the collector current i14 in the transistor Q14 is obtained. The base potential of the transistor Q14 is V
If b1 and the base potential of the transistor Q13 are Vb2, the collector current i14 in the transistor Q14 is expressed by the following mathematical formula 2.

[0006]

## EQU2 ## i14 = i10 / (1 + exp ((Vb2-Vb1) / VT)) Substituting this equation 2 into the equation 1, the collector current i14 in the transistor Q14 is expressed by the following equation 3.

[0007]

I14 = {I2 / (1 + exp ((Vb2-Vb1) / VT))} / (1 + exp (vi2 / VT)) When the input signal vi2 is sufficiently smaller than VT, Formula 1 is It can be approximated by Equation 4.

[0008]

## EQU00004 ## i10 = gm.times.vi2 + I2 / 2 where gm is the mutual conductance of the transistors Q9 and Q10.

Therefore, the output voltage Vo of the variable gain amplifier circuit shown in FIG. 2 is expressed by the following equation 5.

[0010]

Vo = RL × i14 = {RL / (1 + exp ((Vb2-Vb1) / VT))} × (gm × vi2 + I2 / 2) where RL is a load resistance for the output of the variable gain amplifier circuit. is there. Further, Equation 5 is expressed by Equation 6 below.

[0011]

## EQU6 ## Vo = RL × gm × vi2 / (1 + exp ((Vb2-Vb1) / VT)) + RL × Io / 2 (1 + exp ((Vb2-Vb1) / VT)) where Io is shown in FIG. It is the output current of the variable gain amplifier circuit shown. In Equation 6, the AC component vo of the output voltage Vo
Is expressed by Equation 7 below.

[0012]

Vo = RL × gm × vi2 / (1 + exp ((Vb2-Vb1) / VT)) Therefore, the gain A of the variable gain amplifier circuit shown in FIG. 2 is expressed by the following formula 8.

[0013]

## EQU8 ## A = RL × gm / (1 + exp ((Vb2-Vb1) / VT)) From this equation 8, the gain A of the variable gain amplifier circuit shown in FIG.
Can be controlled by changing (Vb2-Vb1).

The DC component Vo 'of the output voltage Vo is expressed by the following equation 9.

[0015]

Vo ′ = RL × (Io / 2) / (1 + exp ((Vb2-Vb1) / VT))

From the equation (9), the DC component Vo 'of the output voltage Vo
It can be seen that also changes with (Vb2-Vb1). FIG. 3 is a circuit diagram showing another example of a conventional variable gain amplifier circuit that is conceived in order to prevent the above-described change of the DC component Vo ′. The variable gain amplifier circuit shown in FIG. 3 is obtained by adding two differential amplifier circuits having a common load to the variable gain amplifier circuit to the variable gain amplifier circuit of the bi-differential type shown in FIG.

Next, the operation of the conventional variable gain amplifier circuit shown in FIG. 3 configured as described above will be described. The input signal Vi3 is superimposed on the bias voltage V4 and is input to a differential amplifier circuit including transistors Q15 and Q16 and a constant current source I3. Here, the collector current i16 in the transistor Q16 is expressed by the following mathematical formula 10.

[0018]

I16 = I3 / (1 + exp (vi3 / VT)) Next, the collector current i20 in the transistor Q20 is obtained. Assuming that the base potential of the transistor Q19 is Vc2 and the base potential of the transistor Q20 is Vc1, the collector current i
20 is represented by the following mathematical formula 11.

[0019]

[Equation 11] i20 = i16 / (1 + exp ((Vc2-Vc1) / VT))
The collector current i20 at 20 is expressed by Equation 12 below.

[0020]

I20 = {I3 / (1 + exp ((Vc2-Vc1) / VT))} / (1 + exp (vi3 / VT)) Further, when the input signal Vi3 is sufficiently smaller than VT, Formula 10 is as follows. It can be approximated by Expression 13.

[0021]

I16 = gm × vi3 + I3 / 2 where gm is the transconductance of the transistors Q15 and Q16.

On the other hand, the differential amplifier circuit composed of the transistors Q22 and Q21 and the constant current source I5 has the same structure as described above.
By inputting Vc1 and Vc2, the transistor Q
The collector current i22 at 22 is expressed by the following formula 14.

[0023]

I22 = I5 / (1 + exp ((Vc1-Vc2) / VT)) The differential amplifier circuit composed of the transistors Q22 and Q21 and the variable gain amplifier circuit are connected to the load resistors R5 and R6 in common. Therefore, the current i6 flowing through the load resistor R6 is the sum of the collector current i20 of the transistor Q20 and the collector current i22 of the transistor Q22.

Therefore, the output voltage Vo of the variable gain amplifier circuit shown in FIG. 3 is expressed by the following equation 15.

[0025]

## EQU15 ## Vo = (i22 + i20) RL = {(gm × vi3 + I3 / 2) / (1 + exp ((Vc2-Vc1) / VT)) + I5 / (1 + exp ((Vc1-Vc2) / VT))} × RL = {RL × gm × vi3 / (1 + exp ((Vc2-Vc1) / VT))} + {(I3 / 2) / (1 + exp ((Vc2-Vc1) / VT)) + I5 / (1 + exp ((Vc1-Vc2)) / VT))} × RL where I2 / 2 = I5, the output voltage Vo of the variable gain amplifier circuit shown in FIG.

[0026]

Vo = RL × gm × vi3 / (1 + exp ((Vc2-Vc1) / VT)) + (I3 / 2) × RL In addition, the AC component vo of the output voltage Vo in Expression 15 is
It is expressed by the following mathematical formula 17.

[0027]

Vo = RL × gm × vi3 / (1 + exp ((Vc2-Vc1) / VT)) Therefore, the gain A of the variable gain amplifier circuit shown in FIG. 3 is expressed by the following formula 18.

[0028]

A = RL × gm / (1 + exp ((Vc2-Vc1) / VT)) From this formula 18, the gain A of the variable gain amplifier circuit shown in FIG. 3 can be obtained by changing (Vc2-Vc1). You can see that it can be controlled.

The DC component Vo 'of the output voltage Vo of the variable gain amplifier circuit shown in FIG.

Vo ′ = RL × (I3 / 2) From this equation 19, the influence of the DC component Vo ′ of the output voltage Vo of the variable gain amplifier circuit shown in FIG. 3 even if (Vc2-Vc1) changes It can be seen that a constant voltage is maintained without being affected.

[0030]

However, in the above-mentioned conventional variable gain amplifier circuit, the DC component Vo 'of the output in the variable gain amplifier circuit shown in FIG. It changes with the change of (Vb2-Vb1). When the conventional variable gain amplifier circuit described above is used as an automatic gain control circuit, the DC component Vo 'of the output of the variable gain amplifier circuit is at the maximum gain, that is, (Vc2-Vc1) <<-VT. When,
It is expressed by the following mathematical formula 20.

[0031]

## EQU20 ## Vo'≈RL × (Io / 2) Also, the DC component Vo 'of the output of the variable gain amplifier circuit.
At the minimum gain, that is, (Vc2-Vc1) >> VT, is expressed by the following equation 21.

[0032]

[Equation 21] Vo'≈0

Therefore, it is necessary to expect that the DC component Vo 'of the output of the above-mentioned conventional variable gain amplifier circuit varies in the range of 0 to RL x (Io / 2). This poses a problem that, when the circuit of the next stage is a circuit which must directly input the DC voltage signal, the setting of the bias voltage in the circuit is greatly restricted.

To solve this problem, as shown in FIG. 3, a circuit in which two sets of differential amplifier circuits are added to the variable gain amplifier circuit shown in FIG. 2 has been considered. However, this conventional circuit needs to flow the same current as the current value of the constant current source in the variable gain amplifying circuit to the differential amplifying circuit, which causes a problem that power consumption increases and a circuit for configuring the circuit. There is a problem that many elements are required.

The present invention has been made in view of the above problems, and it requires only a small number of elements to form a circuit, consumes less power, and changes in the DC component of the output due to changes in the gain. It is an object of the present invention to provide a variable gain amplifier circuit that does not occur.

[0036]

A variable gain amplifier circuit according to the present invention includes a first differential amplifier circuit having first and second transistors symmetrically connected to each other, and the first differential amplifier circuit. A second differential amplifier circuit having third and fourth transistors connected symmetrically to the first output section of the circuit, and a second output section of the first differential amplifier circuit. Fifth and sixth connected and further symmetrically connected
A third differential amplifier circuit having a transistor, an emitter commonly connected to the first output section of the second and third differential amplifier circuits, a commonly biased base, and first and second A seventh and an eighth transistor each having a collector connected to both load resistors, and the bases of the third and fifth transistors are connected together to provide a first gain in the circuit. A voltage to be controlled is inputted, and respective bases of the fourth and sixth transistors are connected to each other so that a voltage for controlling the second gain in the circuit is inputted.

[0037]

In the variable gain amplifier circuit according to the present invention, the voltage applied to the bases of the third, fifth and fourth and sixth transistors is changed to change the gain of the variable gain amplifier circuit. Also, the DC component of the current flowing through the load resistance is maintained at a constant value by the action of the constant voltage source of the seventh and eighth transistors each having a common biased base. Therefore, the DC component of the output voltage can maintain a constant value even if the gain is changed. Furthermore, the variable gain amplifier circuit according to the present invention can maintain the DC component of the output voltage at a constant value without adding a differential amplifier circuit or the like.
A variable gain amplifier circuit with a small number of elements constituting the circuit and low power consumption can be obtained.

[0038]

Embodiments of the present invention will now be described with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing a variable gain amplifier circuit according to an embodiment of the present invention. As shown in FIG. 1, in the variable gain amplifier circuit according to the present embodiment, the transistor Q
The input signal vi1 is input with the bias voltage V1 superimposed on the differential amplifier circuit composed of 1, Q2 and the constant current source I1. The common emitters of the transistors Q3 and Q4 are connected to the collector of the transistor Q1 and are connected to the transistors Q5 and Q4.
The common emitter of 6 is connected to the collector of transistor Q2. The bases of the transistors Q3 and Q6 are connected in common and the control voltage Va1 of the variable gain amplifier circuit according to the present embodiment is applied, and the bases of the transistors Q4 and Q5 are connected in common and the variable gain according to the present embodiment is connected. The control voltage Va2 of the amplifier circuit is applied. The collectors of the transistors Q3 and Q6 are load resistors R of the variable gain amplifier circuit according to this embodiment.
The collectors of the transistors Q4 and Q5 are respectively connected to 1 and R2, and are further commonly connected to the commonly connected emitters of the transistors Q7 and Q8. A constant voltage source V2 is commonly applied to the bases of the transistors Q7 and Q8, and the collectors of the transistors Q7 and Q8 are connected to load resistors R1 and R2, respectively.

Next, the operation of the variable gain amplifier circuit according to this embodiment having the above-described structure will be described. The collector currents i1 and i2 of the transistors Q1 and Q2 are expressed by the following formulas 22 and 23, respectively.

[0041]

[Equation 22] i1 = I1 / (1 + exp (-vi1 / VT))

[0042]

I2 = I1 / (1 + exp (vi1 / VT)) where VT is the thermal voltage of the transistor.
Next, collector currents i3, i4, i5, i6 of the transistors Q3, Q4, Q5, Q6 are expressed by the following formulas 24 to 27, respectively.

[0043]

[Expression 24] i3 = i1 / (1 + exp ((Va2-Va1) / VT))

[0044]

## EQU25 ## i4 = i1 / (1 + exp ((Va1-Va2) / VT))

[0045]

[Equation 26] i5 = i2 / (1 + exp ((Va1-Va2) / VT))

[0046]

I6 = i2 / (1 + exp ((Va2-Va1) / VT)) Substituting the expressions 22 and 23 into the expressions 24 to 27, the collector currents i3, i4, i5 and i6 are respectively expressed by the following expression 28 ~ Represented by Equation 31.

[0047]

I3 = {I1 / (1 + exp ((Va2-Va1) / VT))} / (1 + exp (-vi1 / VT))

[0048]

I4 = {I1 / (1 + exp ((Va1-Va2) / VT))} / (1 + exp (-vi1 / VT))

[0049]

I5 = {I1 / (1 + exp ((Va1-Va2) / VT))} / (1 + exp (vi1 / VT))

[0050]

I6 = {I1 / (1 + exp ((Va2-Va1) / VT))} / (1 + exp (vi1 / VT)) Further, when the input signal Vi1 is sufficiently smaller than VT, the equations 22 and 23 are Can be approximated by the following formulas 32 and 33, respectively.

[0051]

[Equation 32] i1 = -gm × vi1 + I1 / 2

[0052]

I2 = gm × vi1 + I1 / 2 where gm is the mutual conductance of the transistors Q1 and Q2. The collector currents i7 and i8 of the transistors Q7 and Q8 are represented by the following formula 34.

[0053]

I7 = i8 = (i4 + i5) / 2 = {{I1 / (1 + exp ((Va1-Va2) / VT))} / (1 + exp (-vi1 / VT)) + {I1 / (1 + exp ((Va1) −Va2) / VT))} / (1 + exp (vi1 / VT))} / 2 = (I1 / 2) / (1 + exp ((Va1-Va2) / VT))

Therefore, the output voltages Vo1 and Vo2 of the variable gain amplifier circuit according to this embodiment are expressed by the following equations 35 and 36, respectively, because the currents flowing through the load resistors R1 and R2 are i3 + i7 and i6 + i8, respectively.

[0055]

Vo1 = {(gm × vi1 + I1 / 2) / (1 + exp ((Va2-Va1) / VT)) + (I1 / 2) / (1 + exp ((Va1-Va2) / VT))} × R1

[0056]

Vo2 = {(-gm * vi1 + I1 / 2) / (1 + exp ((Va2-Va1) / VT)) + (I1 / 2) / (1 + exp ((Va1-Va2) / VT))} * R2 When Formula 35 and Formula 36 are arranged, the following Formula 37 is obtained.
And Equation 38.

[0057]

Vo1 = I1 × R1 / 2 + vi1 × RL × gm / (1 + exp ((Va2-Va1) / VT))

[0058]

Vo2 = I1 * R1 / 2 + (-vi1) * RL * gm) / (1 + exp ((Va2-Va1) / VT)) Therefore, the gain A of the variable gain amplifying circuit according to the present embodiment is
It is expressed by the following mathematical formula 39.

[0059]

A = RL × gm / (1 + exp ((Va2-Va1) / VT)) From this equation 39, the gain A of the variable gain amplifier circuit according to the present embodiment changes the control voltage (Va2-Va1). It can be seen that the control can be performed by

As described above, the DC component of the output voltage of the variable gain amplifier circuit according to this embodiment is not affected by the change of the control voltage (Va2-Va1) and can be maintained at a constant voltage. Therefore, the DC component of the output voltage of the variable gain amplifier circuit according to the present embodiment can maintain a constant voltage even if the gain is changed.

[0061]

As described above, according to the variable gain amplifier circuit of the present invention, the DC component of the current flowing through the load resistance maintains a constant value even if the gain is changed, so that the DC component of the output voltage is A constant value can be maintained even if the gain is changed. Further, the variable gain amplifier circuit according to the present invention can maintain the direct current component of the output voltage at a constant value without adding a differential amplifier circuit or the like, so that the number of elements constituting the circuit can be small,
In addition, a variable gain amplifier circuit with low power consumption can be obtained. Therefore, when the variable gain amplifier circuit according to the present invention is used for the automatic gain control circuit, even if the circuit at the next stage has to directly input the DC voltage signal,
The bias voltage in the circuit can be easily set.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a variable gain amplifier circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional variable gain amplifier circuit.

FIG. 3 is a circuit diagram showing another example of a conventional variable gain amplifier circuit.

[Explanation of symbols]

 I1; constant current source Q1, Q2, Q3, Q4, Q5, Q6; transistor R1, R2; resistance V1, V2; bias voltage

Claims (1)

[Claims] 1. A first differential amplifier circuit having first and second transistors symmetrically connected to each other, and further symmetrically connected to a first output section of the first differential amplifier circuit. Second with third and fourth transistors connected
And a third differential amplifier circuit having fifth and sixth transistors connected symmetrically to the second output section of the first differential amplifier circuit, and A second differential amplifier circuit having an emitter commonly connected to the first output section, a commonly biased base, and a collector connected to both the first and second load resistors; 7 and an 8th transistor,
The bases of the third and fifth transistors are connected to each other to input a voltage for controlling the first gain in the circuit, and the bases of the fourth and sixth transistors are connected to each other to form a book. A variable gain amplifier circuit, wherein a voltage for controlling a second gain in the circuit is input.