The following circuit amplifies a voltage by a factor (R_{1} / R_{2}). What is important in this circuit is that it amplifies by almost exactly (R_{1 }/R_{2}) 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)


 Z_{in} = R_{in} (because V _{−} is a virtual ground)
 A third resistor, of value
, added between the noninverting input and ground, while not necessary, minimizes errors due to input bias currents.
