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How to compensate a Difference Amplifier?

xyz xyz
Intellectual 1000 points

Other Parts Discussed in Thread: OPA659, VCA821

Hello

I want to build a difference amplier with discrete parts. I want to compensate the amplifier. I found two strategies to compensate it. which one is better? with one Cc or with two Cc?

why did they use two Cc to compensate it? essentially a difference Amp is a non-inverting Amp with two inputs at the non-inverting inputs (in+ and ref) and a third input at the inverting input (in-).

especially in the instrumentation Amp the difference Amp is configured like below (consider I have an unity gain buffer from the output that named ref):

so if we want to compensate it's not enough to put a capacitor in parallel with R1 in the above pink pic?

Regards

  • 0
    TI__Genius 15975 points

    Hi,

    Is there a reason you want to compensate the difference amplifier? In Figure 1, the two compensation capacitors (Cc) are present for treating the differential signals symmetrically. If you do the math for calculating the transfer function of the difference amplifier for Figure 1, you end with the below equation which is only valid if both the differential paths are symmetric. This basically ensures the difference gain at both the low and high frequencies is the same for each of the single-ended inputs that need to act in difference.

    Vout = [(Rf||1/sCc)/(Rg||1/sCg)]x(Vin+ - Vin-). 

    The Figure 2 involves variable gain circuit (inverting gain or -1) incorporated inside the difference amplifier in feedback loop, and the compensation capacitor (C2) is primarily to stabilize this variable gain circuit. I believe this circuit could work at low frequencies, but would have to simulate with OPA659 for higher frequencies. If using the Figure 2 for difference amplifier application, then you might be better off using the VCA821 which is a high impedance input buffer difference amplifier for diff in to single-ended conversion.

    Best Regards,

    Rohit