I am trying to build a servo which is needed to complete a project I am working on. This simple servo has an input potentiometer and the positive output voltage from this is to be compared to the positive output of a motor driven potentiometer to give an error voltage to eventually drive the motor.(via an H bridge with pwm via a TL494). I thought this feedback signal production would be trivial, but because Vpot1 may be higher or lower than Vpot2 depending on required motor direction, subtractor or differential op amp circuits do not work well - due to their outputs changing polarity. A relay crossover switch feeding a subtractor op amp operating as required would work I think, but is not very elegant.

How can I solve this?

  • 1
    \$\begingroup\$ Maybe add a control diagram and show schematics of the op-amp circuits you tried, and specifically why they don’t work. \$\endgroup\$
    – John D
    Commented Oct 1, 2023 at 23:28

1 Answer 1


If I read between the lines correctly this may be the solution to your problem.

This circuit uses a common subtractor but with the bottom end of R4 biased up to some voltage above 0 V.

Control circuit

When Vfeedback is equal to Vcontrol the error voltage, Verror is equal to Vbias. So you would set Vbias to a constant voltage and then when the servo is driven to the required position Verror becomes equal to Vbias. Obviously, in reality, Vfeedback can't become equal to Vcontrol (because this would drive the error signal to zero) except when Vcontrol is equal to Vbias. There must always be some steady state error associated with proportional only control but you can reduce the steady state error by increasing the forward gain.

The buffers are required so that the subtractor doesn't load the pots.

You will perhaps need to adjust the gain of the subtractor and the gain is equal to R2/R1 which should be set equal to R4/R3.


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