I tried to make a differential op-amp, but I get around 65k ohm of output impedance.

I read online that op-amps should have ~50ohm to about maximum ~500ohm output impedance, but however I change parameters in this schematic I cannot get it under like 50k ohm and still keep ~45 voltage amplification.

The only good thing about this differential op-amp is that is has CMRR of like 65dB so I got that right, but I cannot get a good output impedance while keeping good voltage amplification.

Is it possible to use this schematic without changing it too much and still get low output impedance?

enter image description here

  • 5
    \$\begingroup\$ Techniclly this is not an opamp, it's an OTA. OTAs are used extensively in cmos circuits since usually they don't need to drive resistive loads, so the high output impedance is no big deal. \$\endgroup\$
    – Mike
    Jan 8, 2022 at 18:27
  • \$\begingroup\$ @Mike So even if I had like 1.3 mega ohm output impedance (which I got when I started this 'project'), it would still be considered ok in cmos circuits? \$\endgroup\$
    – Jon Nezbit
    Jan 8, 2022 at 19:35
  • \$\begingroup\$ yes even multi-mega ohm output impedance is ok and useful in integrated cmos circuits. The loads these OTAs usually drive are between 5 fF to few pF max. \$\endgroup\$
    – Mike
    Jan 9, 2022 at 4:07

1 Answer 1


This is only the input stage of a usual opamp (the differential pair) with some constant current biasing circuit around. actual opamps will have at least another stage as output stage. To achieve low output impedance throughout, it should be a push pull stage.

In practice, there is also usually at least one intermediate stage to provide sufficient open loop voltage amplification.

  • \$\begingroup\$ So, in conclusion, this schematic doesn't rly need to have low output impedance like Mike said? I am yet to learn what to do with this output ? (like add push pull stage, etc. etc.) \$\endgroup\$
    – Jon Nezbit
    Jan 10, 2022 at 15:35
  • \$\begingroup\$ @JonNezbit if your only load is more MOSFET gates, then high output impedance is fine. But if you need high frequency, you might need to add more stages. If you want to connect actual low impedances like feedback resistors of a few kiloohm, you definitely should add another stage. \$\endgroup\$
    – tobalt
    Jan 10, 2022 at 17:49

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