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I have a question about the small signal model of MOSFET. As you know there is a voltage-controlled current source in the model. I've seen somewhere in books that we can replace it with a resistor with r=1/gm and the cause they provide for that, was since the voltage across current source is VGS and the current is gmVgs we can replace it with v/i = vgs/gmvgs =1/gm resistor.

How we say the voltage across current source is vgs since only 1 pin of it is common with VGS and another one is connected to the drain? The voltage of the current source is a little ambiguous for me.

Small Signal Model Of MOSFET

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    \$\begingroup\$ Take a look here electronics.stackexchange.com/questions/536586/… any additional question? \$\endgroup\$
    – G36
    Commented Apr 11 at 17:00
  • \$\begingroup\$ Thanks for that, but in my mentioned question, I meant Pi equivalent circuit as figure shows, and I think that answer couldn't answer my question directly. \$\endgroup\$
    – Sina M
    Commented Apr 11 at 17:48
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    \$\begingroup\$ You cannot replace a dependent current source with a 1/gm resistor in a normal situation. You can do it only when you connect the MOS gate directly with a drain terminal (diode-connected MOS). Then you will end up with Vgs = Vds and Req = (1/gm) || ro \$\endgroup\$
    – G36
    Commented Apr 11 at 18:05
  • \$\begingroup\$ Aha, that is a great point , I got my answer now. Thank you! \$\endgroup\$
    – Sina M
    Commented Apr 11 at 18:27

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