The inverting amplifier can be thought of as some type of signal conditioner that turns the input signal up-side down or inverts the signal. An easy way to build one is with an IC called an op-amp, short for operational amplifier. Op-amps have some seemingly peculiar characteristics which we will mention later. Fig. 1 shows a simple schematic of a basic op-amp inverting amplifier.

The power supply connections are not shown in Fig. 1 for the sake of simplicity. Some basic facts about the op-amp are as follows (some of the pecularities):

1. The negative input of the op-amp (the – terminal of A) is a virtual ground or zero potential relative to Vref (virtually ground if Vref is ground) and is sometimes referred to as the “summing point” as we shall later. This means that the current from V in through the resistor R1 is

I in = V in / R1

and

2.
I_f = -V out / R2

where I_f is the “feedback current” through R2. The feedback current has to be the negative of the input current so that the sum is zero and the current into the negative terminal of A is zero by definition. The negative sign indicates the inversion of the signal.

By doing a little algebra we can combine the above two equations and arrive at the basic equation for gain of the inverting amplifier:

G = (Vout / Vin) = - R2 / R1

again the negative sign in the gain equation (sometimes referred to as the “transfer function”) indicates the inversion.