BJT class A biasing stability comparison

This circuit shows three common methods of biasing a BJT class A common-emitter amplifier, the first is a basic constant current bias that is extremely unstable without emitter degeneration, the second is somewhat more stable by utilising feedback and the third is the most stable by using a constant base voltage and emitter degeneration resistor, it's worth noting that without emitter degeneration the gain is highly non-linear so the first and second example are best suited for switching applications. Each example has the exact same quiescent current and voltage, try vary the forward beta of each transistor and observe the effect on the bias point (5V @ 1mA).

published 12 years ago