Active load differential amplifier circuit diagram

In order to improve the gain of the integrated operational amplifier differential input stage, its collector load resistor Rc can also be replaced by a pair of mirror constant current sources, as shown in the figure for an active collector load differential amplifier.

Figure Active Collector Load Differential Amplifier

VT1 and VT2 are a pair of differential amplifier tubes, and VT3 and VT4 form a mirror constant current source. Their collector potentials can all float, so Ic3 and Ic4 can change, but always remain equal. There is often a VT4 collector output, and rCE4 serves as the load of the differential amplifier. Since rCE4 is very high, the gain of the differential amplifier is also very high. In order to make the current on both sides of the differential amplifier more consistent, an improved mirror constant current source is often used as its load.

The introduction of active loads greatly improves the voltage gain at all levels, which is an important symbol of the second generation of integrated operational amplifiers.