1
\$\begingroup\$

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

\$\endgroup\$
3
  • \$\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\$ May 4 '20 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 '20 at 0:18
  • \$\begingroup\$ Increase the resistor in series with C3. (The one hiding behind the probe PR3) \$\endgroup\$ May 5 '20 at 10:01
2
\$\begingroup\$

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

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

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.