I am trying to do a small signal analysis for a dynamic microphone preamplifier circuit. The original circuit is using BJT but I did not have much experiences with BJT, so I switched all BJT to MOSFET.

Then I end up with the following circuit enter image description here

I am trying to do a small signal analysis of the circuit but I am not sure if I am doing it right.

enter image description here

Then I combined resistors/capacitors for simplicity.

enter image description here

Here's my question

  • Is my small signal analysis correct?
  • If it is correct, is input resistance = Z1, and output resistance=C5 + Rload?
  • How do I find open circuit voltage gain? I know it's Vo/Vi, but I am not quite sure what to do next. Will it be easier to obtain from simulation?
  • 3
    \$\begingroup\$ I wouldn't even consider the idea of an analog discrete MOSFET pre-amplifier (at least, not without careful consideration of some kind of DC NFB to find the quiescent operating point -- which I've never tried to do before -- so perhaps that's my own limitation.) Discrete JFET? Yes. Discrete BJT? Yes. But discrete MOSFET? No. \$\endgroup\$
    – jonk
    Commented Dec 2, 2020 at 21:30
  • 1
    \$\begingroup\$ 1) don't "blindly" replace BJTs with MOSFETs just because you lack experience with BJTs, the circuit will end up not working. 2) M1 will be off because its Vgs is too low, it is not biased properly to conduct the signal. Why is M1 there anyway? 3) M2 will not work, there is no proper DC path so the DC voltage at the gate of M2 is undefined, M2 will not work properly because of that. 4) M3 doesn't appear to do anything, it will be off as well as its gate is pulled to Vcc by R7. It is pointless to do so small signal analysis on a circuit that cannot work. \$\endgroup\$ Commented Dec 2, 2020 at 21:58
  • \$\begingroup\$ umm.. I am doing this for a project and we didn't really go over BJT in class so I thought switching to MOSFET will be a good idea... \$\endgroup\$
    – lsi
    Commented Dec 2, 2020 at 22:16
  • \$\begingroup\$ The polarity of C6 is backwards. Why is C3 in series with C4? some connection dots are missing on the schematic and there are too many wires crossing over other wires. \$\endgroup\$
    – Audioguru
    Commented Dec 3, 2020 at 1:05
  • \$\begingroup\$ @lsi that is a problem with many universities, the BJT is lightly covered and the JFET has practically disappeared from the course curricula. Like jonk mentioned, discrete BJTs are commonly used because of their lower noise voltage, lower than JFETs or MOSFETs, you can also use JFETs which have higher noise voltage but lower noise current, it all depends on your source resistance. \$\endgroup\$
    – S.s.
    Commented Dec 3, 2020 at 1:06

1 Answer 1


Agreed with jonk and bimpelrekkie. Look at BJT's and if you want to do analysis on simpler terms think of the BJTs as a two port network. Open the collector,base, emitter and short the collector, base and emitter and perform KCL and KVL. Gain is just difference in output over difference in input. For reference look at small signal analysis of Class AB amplifiers and maybe it will lead you to the answer you are searching for.


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