# is this small signal equivalent circuit of mosfet amplifier correct?

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 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.

• 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. Apr 18 '20 at 13:15
• I edited the post. I forgot to include RS. Also gave a link to someone who analyzed it. Apr 18 '20 at 14:30

Yes,

your circuit is correct. r0 is also placed correctly.

• 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) Apr 18 '20 at 1:28
• @EEstudent Rin = Vg/Ig in your circuit we have Ig= Vg/(R1|R2) thus Rin = R1||R2
– G36
Apr 18 '20 at 7:34