What is Negative feedback

Chapter 8.4 Negative feedback

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 368CHAPTER 8. OPERATIONAL AMPLIFIERS• A differential amplifieris one amplifying the voltage differencebetween two signal inputs.In such a circuit, one input tends to drive the output voltage to the same polarity of theinput signal, while the other input does just the opposite. Consequently, the first input iscalled the noninverting(+) input and the second is called the inverting(-) input.• An operational amplifier(or op-ampfor short) is a differential amplifier with an extremelyhigh voltage gain (AV = 200,000 or more). Its name hails from its original use in analogcomputer circuitry (performing mathematical operations).• Op-amps typically have very high input impedances and fairly low output impedances.• Sometimes op-amps are used as signal comparators, operating in full cutoff or saturationmode depending on which input (inverting or noninverting) has the greatest voltage.Comparators are useful in detecting ”greater-than” signal conditions (comparing one tothe other).• One comparator application is called the pulse-width modulator, and is made by compar-ing a sine-wave AC signal against a DC reference voltage. As the DC reference voltageis adjusted, the square-wave output of the comparator changes its duty cycle (positiveversus negative times). Thus, the DC reference voltage controls, or modulatesthe pulsewidth of the output voltage.8.4Negative feedbackIf we connect the output of an op-amp to its inverting input and apply a voltage signal tothe noninverting input, we find that the output voltage of the op-amp closely follows thatinput voltage (I’ve neglected to draw in the power supply, +V/-V wires, and ground symbol forsimplicity):−+VinVoutAs Vin increases, Vout will increase in accordance with the differential gain. However, asVout increases, that output voltage is fed back to the inverting input, thereby acting to decreasethe voltage differential between inputs, which acts to bring the output down. What will happenfor any given voltage input is that the op-amp will output a voltage very nearly equal to Vin,but just low enough so that there’s enough voltage difference left between Vin and the (-) inputto be amplified to generate the output voltage.The circuit will quickly reach a point of stability (known as equilibriumin physics), wherethe output voltage is just the right amount to maintain the right amount of differential, whichin turn produces the right amount of output voltage. Taking the op-amp’s output voltage andcoupling it to the inverting input is a technique known as negative feedback, and it is the keyto having a self-stabilizing system (this is true not only of op-amps, but of any dynamic systemin general). This stability gives the op-amp the capacity to work in its linear (active) mode, as