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Homework Help: Equivalent Models

  1. May 14, 2010 #1

    When it comes to the hybrid-pi model and the T-model for the AC equivalents of an FET, what's the exact difference?

    For example I know that in a BJT circuit, the hybrid-pi includes the resistance [tex] r_\pi [/tex] in the base while the t-model includes the resistance [tex] r_e = \frac{r_\pi}{1+ \beta} [/tex] in the emitter instead.

    In the case of an FET equivalent model, all I can notice is that in the hybrid-pi model, the gate is not connected to the source-drain junction, instead, it's represented as an open circuit given that the gate current is theoretically 0 and thus the FET can be said to have an infinite input impedance.

    Also, for some reason, in the T-model there's a resistance included in the source which is equal to [tex] \frac{1}{g_m} [/tex], what does this represent? and why is it omitted in the hybrid-pi model?

    I would really appreciate any help, I really need to know this because I have tests soon enough!

    Thank you in advance.
  2. jcsd
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