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Unity Gain Buffer

A practical op-amp connected in a unity gain configuration will have a very high input resistance (mega-ohms or higher). How can I add an external resistor to modify the input resistance to the kilo-ohm range (while keeping the output resistance low)? Can I connect a resistor between the inverting and non-inverting terminal?

ocrdu
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Hani908
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    Would adding a resistor from Vin to ground be okay? Anything you put there will provide an input resistance. – jonk Nov 22 '22 at 04:22
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    @jonk, would a resistor from Vin to ground appear in parallel with the op-amp's high input resistance? If that is the case, then the resistor will approximately set the input resistance. – Hani908 Nov 22 '22 at 04:34
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    How is this not a duplicate? After 13 years. – Peter Mortensen Nov 23 '22 at 00:27

2 Answers2

9

Here you go:

schematic

simulate this circuit – Schematic created using CircuitLab

The input resistance is now 10kΩ, and the output resistance isn't affected at all!

Simon Fitch
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7

A resistor from In+ to In- is not ideal to lower input impedance. It increases noise and has a capacitor-like impedance due to the feedback that constantly eliminates the voltage across this resistor. Its DC impedance will be very high.

To get what you want, add a resistor from In+ to a fixed voltage node, instead. E.g. to Gnd or to one of the opamp supplies.

tobalt
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