I constructed a single stage differential amp and used it to make a subtractor. The input amplitudes are 2 mV and 3 mV, however the output is about 1.4 mV. Can anyone tell me why?

circuit wave

  • \$\begingroup\$ Your open loop gain is crappy so how can expect a true and accurate subtraction with just negative feedback from M4? \$\endgroup\$ – Andy aka Nov 4 '15 at 22:25
  • \$\begingroup\$ Hi Andy, thanks for your reply, I just begin to take some analog circuit courses recently so I can not understand what you said, why you say my gain is crappy? Are you saying my differential amp is not correct? \$\endgroup\$ – xiaohan ni Nov 4 '15 at 23:22
  • \$\begingroup\$ BTW, the gain is about 36dB for single input. \$\endgroup\$ – xiaohan ni Nov 4 '15 at 23:23
  • \$\begingroup\$ Most op-amps have dc gains around 1 million to achieve dc accuracy - your gain is 63.1 plus, input offset errors will account for any other errors. \$\endgroup\$ – Andy aka Nov 5 '15 at 9:10

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