Input Impedance of a MOSFET circuit

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SUMMARY

The discussion centers on calculating the input impedance of a MOSFET circuit as presented in Razavi's "Design of Analog CMOS Integrated Circuits" (Chapter 6). The initial approach involved considering the series combination of capacitors C1 and C2 while neglecting Cgs. The participant realized the need to apply Thevenin's Theorem by zeroing independent sources in the small-signal model, leading to a straightforward application of Kirchhoff's Current Law (KCL) for the final calculation.

PREREQUISITES
  • Understanding of MOSFET circuit design principles
  • Familiarity with small-signal models in analog circuits
  • Knowledge of Thevenin's Theorem and its application
  • Proficiency in applying Kirchhoff's Current Law (KCL)
NEXT STEPS
  • Study the input impedance calculation techniques for MOSFET circuits
  • Learn about small-signal analysis in analog electronics
  • Explore Thevenin's Theorem applications in circuit analysis
  • Review Kirchhoff's Current Law (KCL) in the context of complex circuits
USEFUL FOR

Electrical engineering students, circuit designers, and professionals working on analog integrated circuits who seek to deepen their understanding of MOSFET input impedance calculations.

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Homework Statement


hdiN9.png

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(From Razavi's Design of Analog CMOS Integrated Circuits - Chapter 6)

Homework Equations

The Attempt at a Solution



Not too sure about calculating the input impedance of this. Initially I thought that it would be the series combination of C1 and C2, since we are neglecting Cgs, so
43a4bf8673.png


But after I actually did it on paper, I got something like this,
422d797d3e.png


Usually, I would try and use superposition for this but I don't see how that would work since Vx would dissapear. Honestly, I have no idea what to do now?
 

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EDIT: I just realized my mistake. When calculating input/output resistances, all independent sources have to be zeroed just like in Thevenin's Theorem, so that current source there shouldn't be there in my small-signal model.
From that it's just one KCL and job done.
 

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