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Can I determine what Q point is used by examining bias resistors and voltage? (FET)

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Terminator3

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For example, we repairing some FET-based oscillator.
Biasing resistors used: 30 Ohm on drain and 400 Ohm on source. Supply voltage is 2v, so i guess that I=2v/430Ohm=0.0046=0.5mA
Then i measure Vgs, for example it is -1v. Then I=1/400=0.0025A. So the Q point is -1V / 0.0025A, and DC load up to 0.5mA... I know the numbers are wrong, but how about idea?
 

Terminator3 said:
Biasing resistors used: 30 Ohm on drain and 400 Ohm on source. Supply voltage is 2v, so i guess that I=2v/430Ohm=0.0046=0.5mA
Click to expand...

Yes, it's a guess - but wrong. But where comes this guess from? Do you know the working principle of a FET?
 

Backwards from choosing bias resistors. I try to figure it out... What i should measure, for example in lna fet, to get its q? In terms of u,i,r...
 

Terminator3 said:
Backwards from choosing bias resistors. I try to figure it out... What i should measure, for example in lna fet, to get its q? In terms of u,i,r...
Click to expand...

The FET must be regarded as a voltage controlled current source - controlled by VGS. The voltage VGS is determined by the gate potential and the source potential (voltage drop across RS).
The control function can be derived from the FET theory.
 

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