JPH0320086B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable gain differential amplifier circuit using a differential circuit.

Conventionally, variable gain amplifier circuits have often been constructed from individual components or hybrid integrated circuits, and only small-scale integrated circuits have been realized. For this reason, AC coupling (capacitors and transformers) was often used for coupling between amplifier circuits.

Recently, integrated circuit technology has advanced and it has become possible to integrate large-scale circuits, but capacitors and transformers are difficult to integrate, and conventional circuits are therefore unsuitable for large-scale integration.

SUMMARY OF THE INVENTION An object of the present invention is to provide a variable gain amplifier circuit that is suitable for large-scale monolithic integration and allows direct coupling between stages.

Another object of the present invention is to provide an amplifier circuit that can widen the variable gain range.

A further object of the present invention is to provide a variable gain amplifier circuit that can be applied to a differential amplifier circuit.

A differential amplifier circuit according to the present invention includes at least first, second, and third differential pairs including transistor pairs, first and second load resistors, a current source, and a negative feedback circuit; A signal voltage is applied to the input of the pair, an output of the negative feedback circuit is applied to the input of the second differential pair, a gain control signal is applied to the input of the third differential pair, and the output of the current source is applied to the input of the third differential pair. given to the dynamic pair, the third
giving one output of the differential pair to the first differential pair, giving the other output of the third differential pair to the second differential pair,
One output of the first differential pair and one output of the second differential pair are applied to the first load resistor, and the other output of the first differential pair and the other output of the second differential pair are applied to the first load resistor. It has a circuit configuration that provides an output to a second load resistor, and by changing the gain control signal, the signal voltage changes to the first load resistance.
and changing the gain transmitted to the second load resistor.

More specifically, the current of the current source is arbitrarily divided by the third differential pair controlled by the gain control signal V _c , and the two divided currents are divided into the first differential pair and the third differential pair controlled by the gain control signal V c. It is applied to the second differential pair, and the gains of both are variable reciprocally. The first differential pair amplifies the signal voltage, the second differential pair amplifies the negative feedback signal, and the two differential pairs share a load resistance to add both signals. Therefore, the overall gain A of this circuit is the gain of the first differential pair A ₁ ,
When the gain of the second differential pair is A ₂ and the gain of the negative feedback circuit is β, it is expressed by the following equation.

A=A ₁ /1+βA ₂ If the current in the first differential pair is increased and the current in the second differential pair is decreased, A ₁ increases, A ₂ decreases, and the overall gain A increases. On the other hand, if the current in the first differential pair is decreased and the current in the second differential pair is increased, A ₁ decreases, A ₂ increases, and the overall gain A decreases. Therefore, the total gain A is determined by the gain control signal V _c
can be controlled.

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows an embodiment of the circuit elements necessary to construct a circuit according to the invention. Figure 1 a and b
is an example of a differential pair, and a is an example configured with only two transistors, 101 and 102 are signal input terminals, 103 and 104 are output terminals, 10
5 is a current input terminal. Component b is an example of a differential pair in which the emitters of two transistors are coupled through resistors 109 and 110, and the input and output terminals are exactly the same as those for component a. Although both examples are constructed with NPN transistors, they can also be constructed with PNP transistors or other active elements. Figures 1c and d are examples of current sources. c is an example of the configuration of a current source consisting of a transistor 112, a resistor 113, and a diode 114, which is a well-known circuit (Reference: Translated by Nakazawa et al., Analog Integrated Circuit Modern Science Co., Ltd.), and d is a current source configured only with a resistor 115. Although the performance is inferior to that of case c, it is sufficiently usable for practical use. FIG. 1 is an example of a load resistor, which is simply constituted by a resistor 117. resistance 1
One end of 17 is connected to the power supply _Vcc . Figure 1 f
shows an example of a negative feedback circuit. 118 is the input end;
119 is an output terminal, a transistor 120 and a resistor 123 form an emitter follower, and a resistor 121 and a resistor 122 form an attenuation circuit.
One end of the resistor 122 is connected to the power supply V _R for the purpose of preventing direct current from flowing through the resistor 121. The gain β of this negative feedback circuit is determined by the values of the resistors 121 and 122 as follows, assuming that the gain of the emitter follower is 1.

β=R ₀ /R ₀ +R ₁ where R ₁ is the value of the resistor 121 and R ₀ is the value of the resistor 122.

FIG. 2 shows an embodiment of a circuit according to the invention.
First, second and third differential pairs 201, 202 and 203, first and second load resistors 204 and 205, current source 206, and negative feedback circuit 207 are included. The signal voltage V _i is input to the first differential pair 201 from terminals 208 and 209. gain control signal
V _c is from the terminals 210 and 211 to the third differential pair 20
3 is input. The output of the negative feedback circuit 207 is input to the second differential pair from terminals 216 and 217.
The output voltage of this circuit is taken out from the emitter follower of the negative feedback circuit 207 through terminals 212 and 213.

The output current I _p of the current source 206 is divided into two outputs 214 and 215 by the third differential pair 203 controlled by the gain control signal V _c . The current of output 214 is I ₁ and the current of output 215 is I ₂ (however, I ₁
+I ₂ =I ₀ ), then I ₁ =e ^VC/VT /1+e ^VC/VT I ₂ =1/1+e ^VC/VT . However, V _T is a thermal voltage and is approximately 26 m at room temperature.
V (references; supra). Gains A 1 and A ₂ of the first differential pair 201 and the second differential pair ₂₀₂ driven by these currents are expressed by the following equations, respectively.

A ₁ = R _c /2V _T /I ₁ +R _E1 A ₂ = R _c /2V _T /I ₁ +R _E2 However, R _c is the value of the first and second load resistance (assumed to be equal), and R _E1 is the value of the resistance on the emitter side of the first differential pair, and R _E2 is the value of the resistance on the emitter side of the second differential pair. Therefore, the total gain A is expressed as follows.

A=A ₁ /1+βA ₂ =R _C /2V _T /I ₁ +R _E1 /1+β
R _C /2V _T /I ₁ +R _E2 Since I ₁ and I ₂ can be arbitrarily controlled by the gain control signal V _c , the total gain A can be controlled.

As a specific design example, consider a case where maximum gain Amax=10 and minimum gain Amin=2 are required.
The maximum gain Amax is the output of one of the third differential pairs when the gain control signal V _c is maximum, that is, V _c = 0.5V.
If I ₁ is the maximum (I ₀ ) and the other output I ₂ is the minimum 〓, the following equation is obtained.

Amax=A ₁ =R _c /2V _T /I ₀ +R _E1 If I ₀ =2 mA and R _c =500Ω, then R _E1 =24Ω.

Next, the minimum gain Amin should be set when the gain control signal V _c is 0, that is, when the two outputs I ₁ and I ₂ of the third differential pair are equal (I ₁ = I ₂ = 1/2I ₀ ). Amin＝R _C /4V _T /I ₀ +R _E1 /1+βR _C /
If 4V _T /I ₀ + R _E2 β = 1/2, then R _E2 = 385Ω. By configuring a circuit with the above constants, the gain control voltage V _c can be
By changing from 0v to about +0.5v, the total gain A can be changed from 2 times to 10 times. In the above embodiment, the signal voltages applied to the inputs of the first, second, and third differential pairs, the output signal of the negative feedback circuit, and the gain control signal are applied as balanced signals, but they may be applied as unbalanced signals. is also possible. In this case, a signal may be applied to one input of each differential pair, and an appropriate reference bias voltage may be applied to the other input.

The variable gain differential amplifier circuit according to the present invention has the following features.

(1) Since it is a differential type, good S/N ratio and stable operation can be obtained.

(2) Since the gain is controlled by negative feedback, good frequency characteristics can be obtained.

(3) Since differential pairs are effectively used, the DC level of the output voltage does not change due to gain control. Therefore, multi-stage DC coupling is possible and suitable for high integration.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of circuit elements for constructing a circuit according to the invention, and FIG. 2 shows an embodiment of the circuit according to the invention. 201, 202, 203; differential pair, 204, 2
05; Load resistance, 206; Current source, 207; Negative feedback circuit.

Claims (1)

[Claims] 1 a first, second and third transistor pair comprising a pair of transistors;
a differential pair of first and second load resistors, a current source,
and a negative feedback circuit, the first
An input signal voltage is applied between the inputs of the second differential pair, but the output of the negative feedback circuit is not applied; an output of the negative feedback circuit is applied to the input of the second differential pair, but no input signal voltage is applied; Apply a gain control signal to the input of the differential pair,
The output of the current source is applied to a third differential pair, one output of the third differential pair is applied to the first differential pair, and the other output of the third differential pair is applied to the second differential pair. one output of the first differential pair and one output of the second differential pair are applied to the first load resistor, and the other output of the first differential pair and the second differential pair are applied to the first load resistor. It has a circuit configuration that supplies the other output of the pair to a second load resistor, and is characterized by changing the current distribution ratio between both outputs of the third differential pair by changing the gain control signal. variable gain differential amplifier circuit.