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Operational Amplifier
Electronic engineers created a packaged circuit called the operational amplifier, or op-amp. It performs many basic amplification tasks in a format with 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 ̶0;black box,̶1; and need to know only its general behavior, not the details of its internal parts. Designers can use either the inverting or non-inverting inputs, or both together, depending on their goals.
Inverting Input
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 remove signals selectively. The inverting input also accepts feedback from the amplifier̵7;s output. With no feedback, an op-amp has infinite gain, or amplification potential, so any positive signal drives the output to the amplifier̵7;s positive supply voltage. This useful process also produces severe distortion. Feeding back a portion of the signal to the inverting input reduces gain to a reasonable figure and allows for accurate signal reproduction.
Non-Inverting Input
Whereas the inverting input produces a ̶0;mirror image̶1; of the voltage at the output, the non-inverting input produces an amplified copy at the output. The non-inverting input enables a designer to reproduce signals 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 typically does not accept feedback, which would increase a gain factor that is already infinite.
Adding and Subtracting
Among an op-amp̵7;s many tricks, it can combine signals. An audio engineer uses a multichannel console to mix the microphone signals from vocalists and instruments. At the heart of the mixer, an op-amp adds the signal from each of the mixer̵7;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. When multiple sources arrive at the inverting input, they add together before they invert. 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.