I was trying to simulate a two stage op-amp I had just learned about. I am going step by step i.e. as you can see from the schematic I have started with ideal current sources.

I know that this op-amp will require a dc offset of vdd/2, so I tried giving it an input of 100mv 1kHz sine wave with a 0.9v DC offset (Vdd is 1.9v for these transistors). And I setup the op amp as a unity gain buffer, but the output waveform was heavily attenuated instead of being same as the op-amp.

Can somebody please tell me what I am doing wrong here?

enter image description here

  • \$\begingroup\$ I believe the power supply should be dual i.e. (±Vdd). \$\endgroup\$
    – Franc
    Commented Jul 1 at 11:54
  • 1
    \$\begingroup\$ Did you simulate the opamp as-is or put it in a circuit with feedback? I have not analyzed your circuit in detail, but if it is a classic opamp, the open loop gain would be extreme without the feedback and a clipped output if what would be expected. \$\endgroup\$ Commented Jul 1 at 14:32
  • 2
    \$\begingroup\$ I setup the op amp as a unity gain buffer Your schematic doesn't show it. Please show the actual schematic you simulated. It also seems that the current sources may be set much too strong for the geometries you used. What process is this? You may want to start with discrete mosfets to get something that works and that you could prototype on a breadboard. \$\endgroup\$ Commented Jul 1 at 15:19
  • 1
    \$\begingroup\$ In addition to the previous comments, if you do hook it up as a unity-gain buffer, it's likely to be unstable since I don't see any form of compensation. A Miller capacitor on the second stage is probably necessary for unity-gain stability. \$\endgroup\$
    – John D
    Commented Jul 1 at 15:52
  • \$\begingroup\$ \$V_{DD}\$ is 1.9 V   What is the significance of 1.8 in the transistor designations pfet_01.8 & nfet_01.8? \$\endgroup\$
    – greybeard
    Commented Jul 1 at 19:16

1 Answer 1


You could start by simulating it using discrete devices and ensuring that it works that way:


simulate this circuit – Schematic created using CircuitLab

V1 is a 0.8Vpp sine wave with 2.5V DC offset:

enter image description here


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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