Bipolar Junction Transistor Configurations

[Jerose]

Hello , I am taking up Electronics Engineering and I am having a hard time understanding Bipolar Junction Transistor Configurations, I just want to know :
"HOW CAN I DETERMINE IF THE BIASING CONFIGURATION IS STABLE OR UNSTABLE." thank you!

 

[LvW]

Your question cannot be answered without knowing which circuit you are speaking of.

 

[(*steve*)]

Whilst LvW's answer is accurate, a general answer would be that if the bias point is sensitive to variations in the characteristics of the transistor, then the biasing is likely to be unstable.

An example would be biasing a common emitter stage using a high value resistor from Vcc to the base. To achieve a particular bias point the resistor has to be carefully chosen. A different transistor (or the same transistor at a different temperature) may result in drastic changes to the operating point. This is unstable.

 

[hevans1944]

... "HOW CAN I DETERMINE IF THE BIASING CONFIGURATION IS STABLE OR UNSTABLE." thank you!

Well, experience helps, but if you want to do it right you have to set up a Monte Carlo simulation and vary all the parameters that affect the bias of the BJT. Look at the results and determine what drives the circuit away from "expected" behavior. Don't forget to allow for differences in the "gain" of the BJT as function of temperature, collector current, and moon phase.