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Non-Inverting Amplifiers

We have said that the operational amplifiers in an analog computer are always connected so that they invert the output signal even as they add input signals together. This is necessary, so that the summing junction remains at a virtual ground, thus preventing various input resistors from affecting other input signals. But what if we have only a single input signal? Can we apply it to an op amp in such a way as to obtain a controllable gain without signal inversion?

A non-inverting amplifier circuit.

The circuit to the right shows a non-inverting op amp circuit. In this circuit, the input signal is effectively used as the reference voltage at the "+" input to the differential amplifier, while the "-" input is indirectly referenced to ground. In order to keep the two input voltages to the amplifier the same, the amplifier must set Vout to whatever voltage is required to make the feedback voltage to the "-" input match the input voltage to the "+" input.

Since Rf and Rin form a voltage divider, the feedback voltage will be VoutRin/(Rf + Rin). The gain of this circuit, then, calculated as Vout/Vin, is (Rf + Rin)/Rin, or (Rf/Rin) + 1. Resistor Rz has no effect on the gain of the circuit. However, to balance out variations caused by the small input current to the amplifier, Rz should be made equal to the parallel combination of Rf and Rin.

As we said at the top of this page, it is not generally a good idea to apply multiple input signals and resistors to the non-inverting amplifier. That doesn't mean it can't be done; there are some special-purpose circuits that use the interactions quite effectively. Indeed, a notable application of multiple inputs to a non-inverting amplifier is the R-2R ladder network used in some digital to analog conversion circuits. But unless you're prepared to do some very careful analysis and circuit testing, you should avoid multiple inputs to this circuit.

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