Improved differential amplifier circuit

Improved differential amplifier circuit

In the differential amplifier circuit, increasing the resistance of the emitter resistor Re can improve the common mode rejection ratio. However, it is not easy to make a large resistance resistor in an integrated circuit; using a large resistor Re requires a high voltage stabilized power supply, which is not suitable. If the quiescent current of the transistor emitter is 0.5mA, the current in Re is 1mA. When Re is 10kΩ, the value of the power supply VEE is 10.7V. Under the same quiescent operating current, if Re＝100kΩ, the value of VEE is about 100V.



In order to use a lower power supply voltage and a large equivalent resistance Re, a constant current source circuit can be used to replace Re. When the transistor works in the amplification area, its collector current is almost only determined by the base current and has nothing to do with the tube voltage drop. When the base current is a constant DC current, the collector current is a constant current. Therefore, using the working point stabilization circuit to replace Re, the circuit shown in the upper right figure is obtained.

The constant current source circuit not only sets a suitable static operating current for the differential amplifier circuit under a low power supply voltage, but also greatly enhances the common-mode negative feedback effect, making the circuit more capable of suppressing common-mode signals.

As shown in the upper right figure, the constant current source circuit can be replaced by a constant current source. In the actual circuit, a potentiometer with a very small resistance is often added between the emitters of the two tubes, see Rw in the figure below.

Zero potentiometer Rw: By adjusting the position of the sliding end of the potentiometer, the circuit can be set to uI1＝uI2＝0, uO＝0, and Rw is called a zero potentiometer.



In order to obtain a differential amplifier circuit with high input resistance, a field effect tube can be used to replace the transistor, as shown in the upper right figure. This circuit is particularly suitable for the input stage of a direct-coupled multi-stage amplifier circuit. Under normal circumstances, its input resistance can be considered infinite. Its application is the same as that of a transistor differential amplifier circuit.