I'm working on an 5T OTA with the following schematic:

enter image description here

Here's the simulation circuit:

enter image description here

I'm using a VDD = 2.5V since the transistors are rated for that in the model library I'm using.

My issue is that no matter what VIN_A or VIN_B values I use, the VOUT is always just VDD!

enter image description here

enter image description here

enter image description here (Both VIN_A and VIN_B are 0.5V)

5T OTA is supposed to amplify the differential voltage, or VIN_A - VIN_B.

I just get VOUT = VDD without fail and am stupified.


Mosfet operating points for VIN_A = VIN_B = 0.5V

enter image description here

  • \$\begingroup\$ For first 2 cases, VA > VB always. So, output saturating to VDD is not surprising. For the last case, VA=VB but still output is high means that the amp has a systematic offset or the DC operating point is not set properly. If you set VA=VB=0.5V DC voltage, are all the amp devices in saturation? \$\endgroup\$
    – sai
    Commented Mar 25, 2023 at 5:50
  • \$\begingroup\$ @sai Yes, they are in saturation. I edited post to add figure with mosfet operating points. How can the DC operating point be fixed? \$\endgroup\$
    – MFerguson
    Commented Mar 25, 2023 at 7:17
  • \$\begingroup\$ the biasing looks ok (since you are using an ideal tail current source). I guess you have changed the tail current from 20uA to 2uA. With this, you can now run a DC simulation fixing VA at 0.5V and sweep VB from 0.45V to 0.55V and calculate the gain by plotting VOUT versus VA-VB. Alternatively, if you want to do the transient simulation, fix VA at 0.5V and give a sine wave with an offset value of 0.5V on VB. \$\endgroup\$
    – sai
    Commented Mar 25, 2023 at 7:29


Your Answer

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

Browse other questions tagged or ask your own question.