The job of an electronic amplifier is to boost the current or voltage of an incoming signal. Amplifiers come in inverting and noninverting types. A non-inverting amplifier's output follows the input exactly. An inverting amplifier shifts the input 180 degrees, so positive voltages become negative and vice versa. An operational amplifier contains both types in a single convenient package.
Electronic engineers created a packaged circuit called the operational amplifier, or op-amp. It performs many basic amplification tasks in a format having only three connections: an inverting input, a non-inverting input and an output. Electronic designers treat the op-amp integrated circuit, or IC, as a "black box," needing to know only its general behaviour, not the details of its internal parts. Designers can use either the inverting or non-inverting inputs, or both together, depending on their design goals.
The inverting input reverses the sign of the input voltage, so a positive voltage at the input appears as a negative one at the output. For some applications, the sign of the output voltage does not matter, so the engineer may choose to use the inverting input if it simplifies a circuit design. For other uses, such as cancelling a positive voltage with a negative one, the inverting input allows a circuit to selectively remove signals.
The inverting input also accepts feedback from the amplifier's output. With no feedback, an op-amp has infinite gain, so any positive signal drives the output to the amplifier's positive supply voltage. This is useful but produces severe distortion. Feeding back a portion of the signal to the inverting input reduces gain to a reasonable figure, allowing accurate signal reproduction.
Whereas the inverting input produces a "mirror image" of the voltage at the output, the non-inverting input produces a copy, though amplified, at the output. A designer uses the non-inverting input for signals that must be reproduced as closely as possible. A direct current, or DC, signal, for example, is more sensitive to a reversed sign than an audio signal, so a designer will likely choose the non-inverting input for DC. Unlike the inverting input, the non-inverting input does not normally accept feedback, as it would only increase a gain factor that is already infinite.
Adding and Subtracting
One of an op-amp's many tricks is its ability to combine signals. An audio engineer, for example, uses a multichannel console to mix the microphone signals from vocalists and instruments. At the heart of the mixer is an op-amp that adds the signal from each of the mixer's microphone inputs to produce the song with all its parts properly balanced.
The op-amp can add signals from either of its two inputs. Multiple sources that arrive at the inverting input are first added together, then inverted. Those that arrive at the non-inverting input simply add together. The op-amp then subtracts the total of the inverted inputs from the total of the non-inverting inputs. The combinations of different inputs give the designer flexibility in creating circuits.