Electronics Components: Feedback in Closed Loop Amplifiers - dummies

Electronics Components: Feedback in Closed Loop Amplifiers

By Doug Lowe

If you’re interested in understanding how the feedback circuit works, remember this rule of electronic op amps: If the input voltage is anything other than zero, the op amp will be saturated, and the output voltage will be the maximum allowed.

The purpose of the feedback circuit is to return some of the output voltage to the inverting input, which results in the input-voltage difference’s being driven toward zero. As the voltage approaches zero, the op amp’s gain begins to drop to a useful range.

Suppose that the input voltage difference in an inverting op amp circuit is +0.5 V. This difference results in the op amp’s becoming saturated, so –8 V appears at the output.

A portion of the negative voltage that depends on the voltage divider created by R1 and R2 is returned to the V– input, which has the effect of reducing the input voltage. That results in a smaller voltage difference, but not small enough to prevent the op amp from still being saturated.

Remember, however, that the feedback loop is just that: a loop. As more and more of the saturated output voltage gets fed back through the loop, the input voltage difference gets closer to zero. When it gets oh-so-close to zero, the output voltage drops to a range between zero and the maximum voltage.

The best feature of the closed loop amplifier circuit is that the two resistors, which are outside the op-amp IC, give you precise control of the amount of gain that the circuit will ultimately have. All you have to do to get any gain you want (within the limits of the op amp) is choose the right resistor values.