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Inverting Amplifier

<^< Input Offset Voltage | Course Index | Worksheet: Inverting Amplifier >^>

A basic inverting operational amplifier uses just three external resistors: R_IN which defines the input resistance, R_F which defines the closed-loop voltage gain, and a third resistor described below. Thus:

Input resistance = R_IN

and it can be shown that:

Voltage gain, A_V = R_F/R_IN

Since the input signal is taken to the inverting '-' input of the operational amplifier, the output is said to be 180° out of phase with the input. In other words, the amplifier inverts the signal.

The third resistor, R₁, is inserted in the non-inverting input in order to improve symmetry. Its value is chosen so that it is equal to the parallel combination of the input resistance, R_IN, and the feedback resistance R_F. In practice, R_F is usually very much larger than R_IN and thus the resistance value is often made the same as that used for R_IN.

Note that the value of the resistor has no effect on the gain of the stage it is simply there to ensure that both inputs "see" the same external resistance.

<^< Input Offset Voltage | Course index | Worksheet: Inverting Amplifier >^>