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We are given an assigment about mosfet common source amplifiers and we are given this circuit. I drew its small signal equivalent circuit but I am so unsure about placement of ro(transistor resistor). Can you tell me if it is correct or not and help me if it is not correct

enter image description here

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This is correct small signal model of a common source MOSFET amplifier. You’ve placed Ro correctly across drain and source terminals. Also your input impedance would simply be R1||R2 while your output impedance is RD||(1+ gmRs)Ro. This is a little more complex to derive but can be done with circuit analysis.

See this link for Analysis: I/O Resistance of common source MOSFET with source degeneration

Note: I used Microelectronic circuits 6th sedra/Smith so they approximate output resistance as RD||(1+ gmRs)Ro.

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  • \$\begingroup\$ should not output impedance be seen after RD(right to drain word in circuit)? I am asking to make sure because I also need to derive it. Thanks a lot and can you give some tips to calculate it using node analysis etc. \$\endgroup\$
    – EEstudent
    Apr 18 '20 at 13:15
  • \$\begingroup\$ I edited the post. I forgot to include RS. Also gave a link to someone who analyzed it. \$\endgroup\$
    – Leoman12
    Apr 18 '20 at 14:30
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Yes,

your circuit is correct. r0 is also placed correctly.

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  • \$\begingroup\$ thank you first of all. if it is not too much, can you tell me how to find input resistance? I look it up in my book but it did not explain it. I believe input resistance is taken from node G in the circuit below(small signal equivalent circuit) \$\endgroup\$
    – EEstudent
    Apr 18 '20 at 1:28
  • \$\begingroup\$ @EEstudent Rin = Vg/Ig in your circuit we have Ig= Vg/(R1|R2) thus Rin = R1||R2 \$\endgroup\$
    – G36
    Apr 18 '20 at 7:34

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