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Question about this Op-amp configuration

David Zhang16358
Expert 1860 points
Other Parts Discussed in Thread: OPA211, OPA277

I used two single supplied Op-amp to reverse signal polarity.

I have used application note : SLOA030A as reference, based on the calculation the output should be reversed signal with the right amplitude.

But the result is different since the amplitude is wrong, what are the possible reasons?

Many thanks.

  • 0
    Guru 21975 points

    David,

    It would help us to better understand what you want this circuit to do. Please describe the signal voltage VG2. Is it an AC signal with a DC offset? What is the desired output of the second op amp?  Is the output of the first op amp used for something other than driving the second op amp?

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Expert 1860 points

    Thank you so much for your quick reply.

    VG2 is a TTL square wave form and VS is a variable DC offset.

    The output of the first stage op-amp will be VG2+VS but the polarity of this signal is reversed, so I used the second stage to reverse the polarity of the output from the first stage.

    Based on SLOA030A, the output of  the first stage is (15 – (VG2+VS)) and the output of the second stage is (15 – (15 – (VG2 + VS)) = VG2 + VS, but the simulation result is wrong.

    Please check the attached simulation file.

    Many thanks.

    4628.reverse polarity - Copy.rar

  • 0 in reply to David Zhang16358
    Guru 21975 points

    David,

    I believe that the outputs of both amplifiers are not as you would expect because they do not have a rail-to-rail output capability. You expect to see a 10V to 15V output on the first op amp. With a 15V positive power supply, its output can only swing to approximately 13.5V. Likewise, with limited input from the previous stage and limited output swing to ground, the second stage does not swing all the way to ground.

    If in your simulations, you increase the positive supply voltages to 18V and connect a -3V supply to the negative supply terminals, you will see the op amp outputs without output voltage swing limitations. I'm still not sure that this is the output you want to achieve.

    Note that VS10 and R6 do nothing in your present circuit. They could be eliminated without changing the circuit operation. This can be seen because the non-inverting input is at 7.5V. With 7.5V applied to R6, no current flows in this resistor.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Expert 1860 points

    In the real circuit, I have implemented OPA211( which is rail to rail Op-amp) but it doesn't have model in TINA so I used OPA277 instead in TINA simulation.

    The result is still not right.

    Let me double check and keep you in touch.

    Many thanks.