Why Do We Treat Input and Output Impedances Differently in FET Amplifiers?

AI Thread Summary
In FET amplifiers, the output impedance (Zo) is determined by short-circuiting the input (setting Vi=0) to isolate the output side, while the input impedance (Zi) is not found by short-circuiting the output (Vo=0) because it would alter the operating conditions. To find the gain (Av), the output side is open-circuited (Io=0) to ensure that the input signal is not affected by the output load. The FET's collector-emitter configuration acts as a current source, influencing how impedances are measured. Understanding these methods is crucial for accurately analyzing FET amplifier performance.
MHR-Love
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Hi guys,
can anyone tell me the reason why we set Vi=0 (short circuit the input side) when we want to find the output impedance Zo but we don't do the same by setting Vo=0 when we find the input impedance Zi?
and why do we let Io=0 (open circuit the output side) to find the gain Av?

thank you
 
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Think of what is happening in the FET from the point of view of impedance. The collector-emitter side can be viewed as a current source. What are the implications of this?
 

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