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Op Amp Circuit design

Deepen Solanki
Prodigy 30 points

Hello. I need to design a circuit with the following considerations.

Input varies from 0-12V

For input between 0-10V, output should vary linearly between 0-5V

For input more than 10V, output should get saturated at 5V

I was thinking of using a general purpose amplifier with -0.5 gain and then a unity gain buffer with -1 gain. (Vcc=5V)

Will this work ? Also, what is the best Op Amp choice for this circuit design ?

Thanks :)

  • 0
    TI__Guru* 93105 points

    Hi Deepen,

    If you are using a single positive supply, the inverting input stage will not be able to swing below zero volts at the output which it will attempt to do with a positive input voltage. A dual supply would of course allow that.

    A problem might be encountered with the second stage because the output would be driven into saturation. When an op amp is pushed into non-linear operation the loop control can fall apart and the results may be different than expected. It is better to keep the op amp in its linear input and output operating ranges.

    My thought is to set up a single, non-inverting amplifier stage with a gain of +0.5 V/V. A resistive divider having equal resistors would be placed at the non-inverting input and the amplifier would be connected as a unity-gain buffer. Then, place a 5.1 V, or 5.6 V, zener diode having a sharp knee across the divider resistor that goes from the non-inverting input to ground. This isn't very elegant, but it avoids driving the op amp into saturation.

    If you need more precise behavior the circuit is going to be more complex.

    Regards, Thomas

    PA  - Linear Applications Engineering