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Why does the small signal voltage gain collapse as we increase the AC input voltage?
Voltage gain = Av = (output voltage) / (input voltage). Both small signal (AC).
When you draw the small signal circuit using T-Model, you can find that
Av= (gm * Rd) / (1 + gm Rs)
gm being the the trans-conductance.
Don't need an answer that says "Because the input voltage is in the denominator"
Since as we increase the input voltage, the output voltage will increase also, but not in the same ratio.
You can see the circuit here:
http://en.wikipedia.org/wiki/Common_source
Voltage gain = Av = (output voltage) / (input voltage). Both small signal (AC).
When you draw the small signal circuit using T-Model, you can find that
Av= (gm * Rd) / (1 + gm Rs)
gm being the the trans-conductance.
Don't need an answer that says "Because the input voltage is in the denominator"
Since as we increase the input voltage, the output voltage will increase also, but not in the same ratio.
You can see the circuit here:
http://en.wikipedia.org/wiki/Common_source