I'm curious what exactly happens in a noninverting amp config where the gain setting resistor (the "bottom" half resistance of the divided neg. feedback) goes HIGHER than the feedback resistor? If the opamp is unity gain stable will it just become closer to a unity gain buffer, or will strange problems arise as the resistor becomes further larger than the feedback resistor? Could it just be removed from the circuit, with the feedback resistor still present?
What happens in a noninverting amp when...
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pstamler
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[quote author="Ethan"]I'm curious what exactly happens in a noninverting amp config where the gain setting resistor (the "bottom" half resistance of the divided neg. feedback) goes HIGHER than the feedback resistor? If the opamp is unity gain stable will it just become closer to a unity gain buffer, or will strange problems arise as the resistor becomes further larger than the feedback resistor? Could it just be removed from the circuit, with the feedback resistor still present?[/quote]
It'll just get closer to being a unity-gain buffer. If, for example, the gain-set resistor is 20k and the feedback resistor is 10k, then the gain will be the usual (Rf/Rin + 1), or 10k/20k + 1, or 1.5x.
Sure, you can remove that resistor completely while leaving the feedback resistor in place. With op-amps having significant input bias current, using the same resistor for the - terminal to the output as is present from the + terminal to ground (assuming there's a coupling cap to the left of the + terminal) decreases problems with offset.
Peace,
Paul
It'll just get closer to being a unity-gain buffer. If, for example, the gain-set resistor is 20k and the feedback resistor is 10k, then the gain will be the usual (Rf/Rin + 1), or 10k/20k + 1, or 1.5x.
Sure, you can remove that resistor completely while leaving the feedback resistor in place. With op-amps having significant input bias current, using the same resistor for the - terminal to the output as is present from the + terminal to ground (assuming there's a coupling cap to the left of the + terminal) decreases problems with offset.
Peace,
Paul
