JP3063124B2 - Amplifier circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier circuit using an operational amplifier, and more particularly to an IC amplifier circuit improved to obtain a high CMRR.

(Prior Art) An example of a simplified amplifier circuit using an operational amplifier will be described with reference to FIG.

The amplifier circuit has a non-inverting input terminal of the operational amplifier OP _2,
Input terminal T via inverting input terminal via resistors R ₁ and R ₂ respectively
_A, connect the T _B, formed by connecting a feedback resistor R ₃ between the output terminal and the non-inverting input terminal. This circuit amplifies the difference voltage between the two inputs with a constant amplification factor.

(Problems to be Solved by the Invention) In an IC amplifier circuit using such an operational amplifier, in order to obtain a high CMRR (common mode voltage rejection ratio), a resistor
It requires very high resistor relative precision as R ₁ to R _4.

That is, in the amplifier circuit shown in FIG.
_Assuming that the voltages of T _A and T _B are V _INA and V _INB , respectively, and the voltage of the output terminal T _OUT is V _OUT , V _OUT = V _INA · R ₄ · (R ₁ + R ₂ ) / (R ₃ + R ₄ ) • R ₁ −V _INB · R ₂ / R ₁ Note that R _{1 to} R ₄ represent the resistance values of the resistors R _{1 to} R ₄ , respectively.

Here, considering the conditions of R ₁ = R ₃ and R ₂ = R ₄ , V _OUT = (V _INA −V _INB ) R ₂ / R ₁ , and when V _INA = V _INB
V _OUT = 0.

However, in the general IC manufacturing stage, it is not sufficient to satisfy the above-mentioned conditions, and there is a problem that a device that has gone through a special manufacturing stage to increase the relative accuracy is very expensive. .

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an IC amplifier circuit that can obtain a high CMRR without being affected by the relative accuracy of a resistor.

(Means for Solving the Problems) An amplifier circuit according to the present invention includes a voltage-current conversion circuit that converts a difference voltage between two inputs into a current signal, and a current-voltage that converts the current signal into a voltage signal and outputs the voltage signal. And a voltage-current conversion circuit, wherein the voltage-current conversion circuit comprises a differential circuit.
Voltage changes at both ends of a first resistor connected between the emitters of the two transistors are taken out as current changes by current mirror circuits respectively connected to both ends of the first resistor. The mirror circuit includes a first transistor having a collector connected to one end of the first resistor and a collector connected to a base of one of two transistors constituting a differential circuit different from the differential circuit. A first current mirror circuit including a connected second transistor; a third transistor having a collector connected to the other end of the first resistor; and two transistors forming the another differential circuit And a second current mirror circuit including a fourth transistor having a collector connected to the base of the other transistor of the second transistor. When a current flows through the collectors of two transistors in one of the first and second current mirror circuits due to the voltage change, the current is transmitted to the current-voltage conversion circuit through the another differential circuit. The current-to-voltage conversion circuit is characterized in that the current-to-voltage conversion circuit comprises a negative-feedback amplifier circuit using an operational amplifier to which a feedback resistor is connected.

(Embodiment) An embodiment of the present invention will be described with reference to FIG.

This circuit consists of a bias circuit 1 having a bias terminal V _bi.
3, the input switching circuit 14, a first input circuit 11 having a first input terminal T _A, the second input circuit 12 having a second input terminal T _B,
A voltage-current conversion circuit 15 for converting a difference voltage between the first and second input terminals T _A and T _B into a current signal, and a current which converts the current signal into a voltage signal and outputs from the output terminal T _out A voltage conversion circuit 16;

Each of the first and second input circuits 11 and 12 has transistors Q ₅ and Q ₇ for NPN input and transistors Q ₆ and Q ₈ for PNP input, respectively. Operating states of these transistors is switched by changing the connection of the input switching terminal T _S. For example As for the first input circuit 11, connecting the input switching terminal T _S to the ground GND line (low level), the transistor through the diode D ₁ by the input switching circuit 14 first current mirror circuit 14-1 in Q ₆ turns on and PNP
Input by the input stage becomes possible. On the other hand, input switching terminal
When the T _S to the high level, the transistor Q ₅ is turned on, it is possible to input by NPN input stage by the input switching circuit 14 in the second current mirror circuit 14-2 through the diode D _1. This is the same for the second input circuit 12.

In the voltage-current conversion circuit 15, the first and second input terminals T _A , T
A difference voltage between _B is generated across the resistor _RA through the transistors Q ₁ and Q ₂ . For example, the collector current I of the transistor Q ₁₁ by differential voltage developed across the resistor R _A flows, even current I flows in one of the collectors of the transistors Q ₁₂ which constitute a current mirror circuit. Current I due to the differential voltage, the current through the differential circuit of transistors Q _3, Q ₄ - is transmitted to the voltage conversion circuit 16. This is a transistor
The same applies to the current mirror circuit according to Q _13, Q _14.

Current - The voltage conversion circuit 16, a voltage by the operational amplifier OP ₁ feedback type of connecting the feedback resistor R _B - is converted and output current signal from the current converter circuit 15 into a voltage signal. here,
The potential of one end of the feedback resistor R _B is a potential the same potential applied from the terminal T _C. Therefore, the output terminal T _out
The potential applied from T _C output a pseudo-zero potential is obtained. This is input to the terminal T _C terminal T _A, T _B potential and the power supply V
By giving a stable potential that does not _contribute to _CC , it means that an extremely high CMRR can be obtained.

Further, the amplification factor A _V as the amplifier circuit (the resistance value of the resistor _{R A) A V = K ·} ( resistor R _B resistors) /,: represented by (K current transfer function), the resistance The matching accuracy between the devices R _A and R _B basically does not affect the CMRR. In addition, in particular, I _C (collector current) −V of each transistor in the voltage-current exchange circuit 15
_CE - using I _C constant properties in (collector-emitter voltage) characteristic, moreover the cascode connection structure in key locations, by constructing symmetrical circuit in the current mirror circuit, a voltage-dependent current transfer function K It is possible to make the shape less likely.

As a result, in this circuit, the power voltage V _OUT is represented by V _OUT = A _V · (V _INA −V _INB ) = K · (V _INA −V _INB ) · R _B / R _A , and V _INA = V _At the time of _INB , the output voltage V _OUT can be made extremely close to zero.

According to the present invention As has been described (Effect of the Invention), the two differential voltage input generated across the resistor R _A by they were taken out in the form of a current signal, a resistor R _A, it is possible to provide an amplifier circuit capable of obtaining a high CMRR without being influenced by the relative accuracy of the R _B.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention, and FIG. 2 is a conventional IC.
Diagram showing an example of an amplifier circuit for use. In the figure, 11: first input circuit, 12: second input circuit, 13: bias circuit, 14: input switching circuit, 15: voltage-current conversion circuit, 16: current-voltage conversion circuit.

Claims (1)

(57) [Claims] A voltage-current conversion circuit that converts a voltage difference between two inputs into a current signal; and a current-voltage conversion circuit that converts the current signal into a voltage signal and outputs the voltage signal. The circuit detects the respective voltage changes at both ends of the first resistor connected between the emitters of the two transistors constituting the differential circuit by using current mirror circuits respectively connected at both ends of the first resistor. The current mirror circuit comprises a first transistor having a collector connected to one end of the first resistor, and two differential circuits different from the differential circuit. A first current mirror circuit including a second transistor having a collector connected to a base of one of the transistors; and a collector connected to the other end of the first resistor. 2 constituting the said other of the differential circuit and continue the third transistor
And a fourth transistor having a collector connected to the base of the other transistor.
And a current mirror circuit of one of the first and second current mirror circuits.
When a current flows through the collector of one of the transistors due to the voltage change, the current is transmitted to the current-to-voltage conversion circuit through the another differential circuit, and the current-to-voltage conversion circuit connects a feedback resistor. An amplifier circuit comprising a negative feedback type amplifier circuit formed by an operational amplifier.