I'm designing a 3 stage amplifier, being the first stage a common source MOSFET amplifier (using a 2N7000 MOSFET). I have made some basic calculations to calculate the resistor values. It must have a 1.25M Ohm input resistance, and a maximum drain current of 10 mA

I have calculated some values and simulated the common source amplifier. The gain seems to be fine but the output waveform seems more like a square wave rather than a sine wave.

enter image description here

Does anyone have an idea on what might be happening? How can I solve this?

I used the characteristics of the MOSFET that I obtained through a DCA Pro:

V(GS(on))=2.390 V at i_D=5 mA at i_G=5 μA

V(GS(off))=1.678 V at i_D=4.7 μA

gm=27.7 mAV at i_D=3 mA to 5 mA

enter image description here

  • \$\begingroup\$ Isn't it obvious? That "square wave" is nearly 9Vpk-pk from a 9V supply. Reduce the input level, reduce the gain or increase the supply. \$\endgroup\$
    – user16324
    May 4, 2020 at 22:31
  • \$\begingroup\$ @BrianDrummond, I've tried changing the resistor values to reduce the gain (Rd/Rs), but it still behaves like that. Thanks for your reply! \$\endgroup\$ May 5, 2020 at 0:18
  • \$\begingroup\$ Increase the resistor in series with C3. (The one hiding behind the probe PR3) \$\endgroup\$
    – user16324
    May 5, 2020 at 10:01

1 Answer 1


350mV is far too big. Try <=50 mVp

Vo=Vin/gm*Rc= 350/27*4.7k = 60V means you get a square-ish wave.


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