Question

What are the causes of bias instability in a transistor?

Answer 1

By instability I suppose you mean “imprecision in the design of the bias”. The main cause for that situation is the very large variability of the current gain of the transistor, called β or hFE; if, however, it is chosen an adequate bias network (such as the ‘H’ with 4 resistors) then we can have a relatively good bias stability, by exchanging that property with a decrease in the gain of the transistor amplifier.

Below is an example of ‘H’ biasing of an npn transistor. By choosing appropriate values for the resistors, we can achieve a good bias stability. In the end it is needed quite a fair amount of knowledge on electronics and feedback stabilization theory to fully appreciate this technique…