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

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SUMMARY

The discussion centers on the differing methods for determining input and output impedances in FET amplifiers. Specifically, it establishes that when calculating output impedance (Zo), the input is short-circuited (Vi=0), while the input impedance (Zi) is not calculated by short-circuiting the output (Vo=0). Additionally, to find the voltage gain (Av), the output side is open-circuited (Io=0). This approach highlights the unique characteristics of FETs, particularly their behavior as current sources.

PREREQUISITES
  • Understanding of FET amplifier configurations
  • Knowledge of impedance measurement techniques
  • Familiarity with circuit analysis principles
  • Basic concepts of voltage and current sources
NEXT STEPS
  • Study the characteristics of FETs as current sources
  • Learn about the small-signal model of FET amplifiers
  • Research methods for measuring input and output impedances in circuits
  • Explore the implications of impedance in amplifier design
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Electrical engineers, circuit designers, and students studying amplifier design and FET characteristics will benefit from this discussion.

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
 
Engineering news on Phys.org
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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