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 feedback resistor R2.
Build an op amp with three discrete transistors figure 1Resistor 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.