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How to identify the circuit has negative feedback or positive feedback?

Hari T O0
Genius 3820 points

Dear Team,

In some opamp circuits feedback is connected to both the inverting and non-inverting terminals.Please see the attached image

In such circuits how to identify in which feedback opamp is operating.

Regards

Hari

  • 0
    TI__Guru**** 164476 points

    Hari,

    To simplify things make Rload set to 0, a short. Assuming R4=R2 and R3=R1, the resulting circuit is a normal difference amplifier.

    VOUT = (VIN+ - VIN-) * R4/R3 + 0V. This makes output current, VOUT/R5 = (VIN+ - VIN-) * R4/R3 * 1/R5

    Now suppose V[RLOAD] is 1V (Instead of 0V short). Now VOUT = (VIN+ - VIN-) * R4/R3 + 1V. Now output current is  (VOUT - 1V) / R5 = (VIN+ - VIN-) * (R4/R3 + 1V -1V) * 1/R5 which simplifies to IOUT = (VIN+ - VIN-) * R4/R3  * 1/R5. It is still a negative feedback difference amplifier. Notice that output current didn't change with V[RLOAD] and output current is set by input difference and R1 through R5.

    In summary it is negative feedback circuit where RLOAD voltage is another input to the difference amplifier. This external input is a function of IOUT and RLOAD resistance.   

     

  • 0 in reply to Ron Michallick
    Genius 3820 points

    Hi Ron,

    Thank you so much.

    May l know how can we generalize this method,

    I mean l have an arbitrary  opamp circuit in which feedback is connected to both the terminals.

    In that case how will l check whether the Opamp circuit is in negative feedback or positive feedback.

    Could you please suggest a general method.

    Regards

    Hari

  • 0 in reply to Hari T O0
    TI__Guru**** 164476 points

    Hari,

    Only hysteresis comparators, monostable latches, bistable latches, and oscillators (a-stable latches) are positive feedback circuits. 

    Any circuit that has a linear DC operating point is negative feedback. If the output settles (but not stuck high or low, VOH, VOL) when the input is constant then it is negative feedback.

  • 0 in reply to Ron Michallick
    Genius 3820 points

    Hi Ron,

    Thank you very much.

    May I know is it possible to check whether opamp is negative FB or positive FB by finding the feedback factor.

     If the feedback factor value is more at inverting I/P then opamp is in Negative feedback else positive feedback.

    Regards

    Hari

  • 0 in reply to Hari T O0
    Guru 140200 points

    Hi Hari,

    there's no simple answer.

    1. Usually, a feedback resistor from the output to the +input of OPAmp is positive feedback. I once have designed a PT100 linearisation using this methode.

    2. But if an inverter is inserted, then it is negative feedback, usually. Some DC servos are working like this.

    3. Usually, a feedback resistor from the output to the -input of OPAmp is negative feedback. This is how the most OPAmp circuits are working.

    4. But if an inverter is inserted, then it is positive feedback, usually.

    5. This "usually" above shall point out that no additional phase shift is assumed in the involved stages. But this cannot be taken for granted in all cases. I remember a situation where a complex discrete transistor output stage was driven by an OPamp and the feedback was connected to the +input of OPAmp. But nevertheless is was negative feedback!

    Kai