Build an op amp with three Discrete Transistors

You can use three discrete transistors to build an operational amplifier with an open-loop gain greater than 1 million (Figure 1). You bias the output at approximately one-half the supply voltage using the combined voltage drops across zener diode D₁, the emitter-base voltage of input transistor Q₁, and the 1V drop across 1-MΩ feed-back resistor R₂.

Figure 1.	This ac-coupled inverting op amp has an open-loop gain of 1 million. R1 and R2 set a closed-loop gain of −10.

Resistor R₃ and capacitor C₁ form a compensation network that prevents the circuit from oscillating. 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.

You can configure this op amp as an active filter or as an oscillator. It drives a load of 1 kΩ. The square-wave response is good at 10 kHz, and the output reduces by 3 dB at 50 kHz. Set the 50-Hz low-frequency response with the values of the input and the output capacitors. You can raise the high-frequency response by using faster transistors and doing careful layout. Link