The inverting input remains disconnected from the feedback loop, and is free to receive an external voltage. In its simplest form, we could connect a straight piece of wire from output to noninverting input and see what happens: Unlike negative feedback, where the output voltage is “fed back” to the inverting (-) input, with positive feedback the output voltage is somehow routed back to the noninverting (+) input. How Does Positive Feedback Work in an Op-Amp?Īnother type of feedback, namely positive feedback, also finds application in op-amp circuits. Whether the output is directly fed back to the inverting (-) input or coupled through a set of components, the effect is the same: the extremely high differential voltage gain of the op-amp will be “tamed” and the circuit will respond according to the dictates of the feedback “loop” connecting output to inverting input. Rather, the output voltage drives only as high or as low as needed to balance the two inputs’ voltages:
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With negative feedback (the output voltage “fed back” somehow to the inverting input), the circuit tends to prevent itself from driving the output to full saturation. In an op-amp circuit with no feedback, there is no corrective mechanism, and the output voltage will saturate with the tiniest amount of differential voltage applied between the inputs. The basic principle of negative feedback is that the output tends to drive in a direction that creates a condition of equilibrium (balance). Negative feedback makes all these circuits stable and self-correcting. It is what allows us to create all these practical circuits, being able to precisely set gains, rates, and other significant parameters with just a few changes of resistor values.
As we’ve seen, negative feedback is an incredibly useful principle when applied to operational amplifiers.
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