Operational amplifiers, or op-amps, produce an output voltage that is a function of the difference between voltages at their input terminals. Analogue computers use them as the equivalent of NOT gates, and they can also be used as inverting buffers in digital circuits.
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An op-amp has two inputs: one inverting, one non-inverting. When resistors are tied to the non-inverting input, the resulting output voltage has the same sign as the input voltage. When resistors are tied to the inverting input, the output voltage has the opposite sign as the input voltage.
An op-amp inverter uses two resistors: R1 before the inverting input, and Rf between the inverting input and the output. The equation that describes the relationship between the resistor values and the voltages is Vout / Vin = -Rf / R1. When the values of Rf and R1 are equal, Vout is the negative of Vin.
Comparison to Transistor Inverters
No real op-amp performs identically to the ideal op-amp. The internal circuitry of an op-amp is fairly complex, and NOT gates made from transistors can be much smaller. Large integrated circuits, such as CPUs, thus typically use transistor-based inverters in order to cram more logic circuits onto a small chip. However, op-amp inverters are widely used any time an analogue signal needs to be inverted.
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