Transistors: Operating point

[santosh_iitm2002]

I was recently designing a transistor circuit for a oscillator of 4 mhz. I did not know at what point should the transistor be biased. please help .In general for any circuit how do we decide the operating point (Q-point).

 

[Optikon]

I was recently designing a transistor circuit for a oscillator of 4 mhz. I did not know at what point should the transistor be biased. please help .In general for any circuit how do we decide the operating point (Q-point).

Look at the curves. Decide what point based on the Gain you need at frequency of interest. While your shouldn't design your circuit around any particular parameter of your transistor, you may be interested in keeping the typical gain above some value, so design for a q-point that does it.

Think about this, chances are there are _many_ q-points that will work in your circuit (I could argue infinitely many) so, set one with some reasoning and think about reasons you might not want it to be what you set it up to be. For example, say I collector (DC) = 1 mA, look at the curves and decide "Is 1mA bad for some reason?"


Other reasons:

1) Sometimes, "higher" q-point = faster operation.
2) Sometimes, "lower" q-point = low heat = better efficiency