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# What is Amplifier Inverting / Inverter? ()

The following circuit amplifies a voltage by a factor (R1 / R2).  What is important in this circuit is that it amplifies by almost  exactly (-R1 /R2) so that the gain of the circuit can be controlled precisely by controlling the resistor values precisely.

Inverting amplifier

Inverts and amplifies a voltage (multiplies by a negative constant)

$V_\mathrm{out} = -V_\mathrm{in} ( R_\mathrm{f} / R_\mathrm{in} ) \!\$
• Zin = Rin (because V is a virtual ground)
• A third resistor, of value $R_\mathrm{f} \| R_\mathrm{in} = R_\mathrm{f} R_\mathrm{in} / (R_\mathrm{f} + R_\mathrm{in})$, added between the non-inverting input and ground, while not necessary, minimizes errors due to input bias currents.