Expert: cleggsan - 11/7/2003

Question
I am having trouble with the FET amplifiers in the best way and what parameters to use in finding the voltage amplification and the drain efficiency. An example is VDS=18v, VGS=4v,ID=3mA,and the signal power delivered to the drain circuit by the FET is 21.6 milliwatts, how and what would you use to determine the operating efficiency?

Answer
I would like to see the entire circuit and the resistance values.  But we can do it.  Here is the quickest way:

If the ouput from the drain circuit is 21.6 mw (I am assuming this is class A operation and it is an audio frequency amp as opposed to rf) then the operating efficiency would be the ratio of output to power input from the power supply.

If the supply voltage is 18 and current 4ma, the input power is 3ma x 18v = 54mw.  So, the power operating ratio is 21.6mw/54mw = 0.40 or 40%.

If the assumptions I made are not correct, then you will have to give me more information.

Let me know if you need further explanations.

Cleggsan