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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.

Edit:

Mosfet operating points for VIN_A = VIN_B = 0.5V

enter image description here

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  • \$\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

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