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OPA333A-EP: Unpredictable gain with OPA333 differential amp circuit

Goutam Ghosh
Prodigy 20 points
Part Number: OPA333A-EP
Other Parts Discussed in Thread: OPA333

Tool/software:

Hi,

I have experimented with two of the below circuits for diff op-amp. I have simulated the circuits and also I have the hardware ready for circuit 2. While testing the circuit 2, I have observed that the gain of the circuit is not as theoretical value (10) and is very unpredictable. So, I have simulated the circuit -2 and indeed in the simulations the gain is not 10 and is much lesser than 10. However, when I change the feedback resistor position (refer to circuit 1) and simulated the circuit 1, I get the desired gain. Can you please help me understand why the circuit has different gains for OPA333 or any kind of non-ideal op-amp but has same gain in ideal op-amp.

Looking for your response.

  • 0
    TI__Guru 67971 points

    Goutam,

    First, none of the circuits has gain of 10 - it's either 11 or 21. However, the way you drew your circuit is very confusing; thus, please make sure that in order to see correct differential output you look at the proper output nodes - see below.

    Second, in order to determine differential gain in difference amplifier you must calculate gain using following equation: 

    Gain = 1 + [R_feedback_total / R_input_total] - see below.

    Diif Amp.TSC