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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?

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    \$\begingroup\$ Would adding a resistor from Vin to ground be okay? Anything you put there will provide an input resistance. \$\endgroup\$
    – jonk
    Commented Nov 22, 2022 at 4:22
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    \$\begingroup\$ @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. \$\endgroup\$
    – Hani908
    Commented Nov 22, 2022 at 4:34
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    \$\begingroup\$ How is this not a duplicate? After 13 years. \$\endgroup\$ Commented Nov 23, 2022 at 0:27

2 Answers 2

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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.

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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!

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