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I'm having some difficulty understanding how Razavi determined \$V_{\text{in, eq}}\$. My understanding was that the small-signal model allows us to model the PMOS device as a constant current source (\$g_mv_{gs}\$) with a resistor in parallel (\$r_o\$) and then he transformed that from a Norton Equivalent to Thevenin Equivalent?

How is \$V_{\text{in, eq}}\$ derived?

Null
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AlfroJang80
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1 Answers1

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That's not what is normally called a "common-gate" amplifier. That is a source follower feeding into a common gate amplifier.

Vin,eq is the output voltage of the source follower. Treat M1 as a source follower and calculate its output voltage.

Kevin White
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  • oh, you're right! I missed that one of those was p-channel. – Hearth Feb 22 '21 at 03:35
  • Also known as a complementary differential pair, as in the linear* range, the input term is (Vb - Vin + 2Vgs(th)). (*Constant current region; often, "saturation" for FETs, in contrast to the triode region, often, confusingly, called "linear".) – Tim Williams Aug 14 '23 at 20:08