Three discrete transistors to build an op amp

With three discrete transistors, an op amp with an open-loop gain greater than 1 million can be used (Figure 1). The output is biased at approximately half of the supply voltage by combining the Zener diode D1, the base-emitter voltage of input transistor Q1, and the 1V voltage drop across the 1MΩ feedback resistor R2.

Resistor R3 and capacitor C1 form a compensation network that prevents circuit oscillations. The values in the figure still provide a good square wave response. The ratio of R2 to R1 determines the inverting gain, which is -10 in this example.

Figure 1. This AC-coupled inverting op amp has an open-loop gain of 1 million. The closed loop gain set by R1 and R2 is -10

This op amp can be configured as an active filter or oscillator. It drives a 1 kΩ load. It has a good square wave response at 10 kHz and a 3 dB drop in output at 50 kHz. A 50 Hz low frequency response can be set with the input and output capacitor values. Faster transistors and careful layout can improve high frequency response.