ch 13 electronics

Chapter 13Chapter 13Op-Amp CircuitsOp-Amp Circuits

ObjectivesObjectives

Analyze the operation of several basic comparator circuits

Analyze the operation of several types of summing amplifiers

Troubleshoot basic op-amp circuits

Analyze the operation of integrators and differentiators

IntroductionIntroduction

Op-amps are used in many different applications. We discuss the operation of three fundamental applications. Keep in mind that the basic operation and characteristics of the op-amps do not change—the only thing that changes is how we use them.

Comparators - Comparators - Zero-level Detection Just as the name implies, when an op-amp is used as a comparator it is comparing one signal to another. In this particular example of a zero-level detector the op-amp is in open-loop configuration. The incoming signal drives the op-amp into saturation, producing a square-wave output. Remember that the op-amp is driven into saturation easily when in open-loop configuration.

Comparators - Comparators - Nonzero-level Detection

With nonzero-level detection the voltage divider or zener diode sets the reference voltage at which the op-amp turns goes to the maximum voltage level.

Comparators - Comparators - Nonzero-level DetectionFigure 13–3 Figure 13–3

Thomas L. Floyd Thomas L. Floyd Electronic Devices, 7eElectronic Devices, 7e

FigureFigure 13–4 13–4

ComparatorsComparators

Remember that the comparator is configured in open-loop, making the gain very high. This is open-loop configuration. This makes the comparator very susceptible to unwanted signals (noise) that could cause the output to arbitrarily switch states.

ComparatorsComparatorsThe effects of the noise can be reduced with feedback for hysteresis. This sets the trigger upper and lower trigger points, set by the voltage divider, such that it is unaffected by the minor variations caused by noise. This type of op-amp configuration is sometimes called a Schmitt trigger.

Figure 13–8 Operation of a comparator with hysteresis.Figure 13–8 Operation of a comparator with hysteresis.

Figure 13–9Figure 13–9

Figure 13–10 Comparator with output bounding. Figure 13–10 Comparator with output bounding.

Figure 13–11Figure 13–11 Operation of a bounded Operation of a bounded comparator. comparator.

ComparatorsComparatorsIf the level of the pulse must be less than the output of a saturated op-amp, a zener-diode can be used to limit the output to a particular voltage. This is called output bounding. Either positive, negative, or both halves of the output signal can be bounded by use of one or two zener diodes respectively.

Figure 13–16 A simultaneous (flash) analog-to-digital converter (ADC) Figure 13–16 A simultaneous (flash) analog-to-digital converter (ADC)

using op-amps as comparators.using op-amps as comparators.

Figure 13–17 Sampling of values on analog waveform for conversion to Figure 13–17 Sampling of values on analog waveform for conversion to

digital.digital.

Figure 13–18 Resulting digital outputs for sampled values in Figure 13–17. Figure 13–18 Resulting digital outputs for sampled values in Figure 13–17.

DD0 is the least significant digit.0 is the least significant digit.

Summing AmplifiersSumming Amplifiers

The summing amplifier basically has two or more inputs and the output voltage is the negative sum of the inputs. With the unity gain summing amplifier the output can be determined simply by the addition of each of the input voltages. Rf is the same each of the input resistors for a summing amplifier with unity gain.

For gain to be greater than unity, Rf must be greater than the input resistances. The gain is calculated as normal with any one of the input resistances. The sum of the inputs and the gain would be the output.

Figure 13–22Figure 13–22 EX. 13-5 EX. 13-5

Figure 13–23Figure 13–23 EX 13-6 EX 13-6

Another type of summing amplifier, the averaging amplifier, can produce an average voltage of the inputs setting the ratio of Rf to input resistance to the reciprocal of the inputs.

Rf/R = 1/n

Figure 13–24Figure 13–24 EX 13-7 EX 13-7

The scaling adder is a summing adder with input given a weight by choice of each value of input resistance. The Rf to input resistance ratio would determine what the voltage output would be with a signal present at each input. This type of adder could be used for digital-to-analog conversion.

VOUT = - (Rf/R1 VIN1 + Rf/R2 VIN2 +….+ Rf /Rn VINn )

Figure 13–26Figure 13–26 A scaling adder as a four-digit A scaling adder as a four-digit digital-to-analog converter (DAC). digital-to-analog converter (DAC).

Figure 13–29Figure 13–29 An An RR/2/2RR ladder DAC. ladder DAC.

Figure 13–30 Figure 13–30 Analysis of the Analysis of the RR/2/2RR

ladder DAC.ladder DAC.

Integrators And DifferentiatorsIntegrators And Differentiators

The basic integrator is easily identified by the capacitor in the feedback loop. A constant input voltage yields a ramp output. Note the input resistor and the capacitor form a RC circuit. The slope of the ramp is determined by the RC time constant. The integrator can be used to change a square wave input into a triangular wave output. The rate of change can be determined by the formula below.

Vout/Vin = -Vin/RiC

Figure 13–33Figure 13–33 A linear ramp voltage is A linear ramp voltage is produced across C by the constant charging produced across C by the constant charging current. current.

Integrators And DifferentiatorsIntegrators And DifferentiatorsThe differentiator does the opposite of the integrator in that it takes a sloping input and provides an output that is proportional to the rate of change of the input. Note the capacitor is in the input circuit. The output voltage can be determined by the formula below. A triangular input would yield a square wave output.

Vout = -(Vc/t)RfC

Figure 13–37Figure 13–37 A differentiator with a ramp A differentiator with a ramp input. input.

TroubleshootingTroubleshooting

Op-amps are very reliable. However, internal failures do occur perhaps as a result of an external failure. A stuck output in either positive or negative maximum voltage is one type of internal failure.

Bounded comparator op-amps could fail as a result of faulty zener(s). In this example with one open zener the comparator operates unbounded. What would the effects be with a shorted diode?

With a summing type of amplifier an open input resistor would cause the output to be less than normal.

SummarySummary

The op-amp comparator’s output changes state when the input voltage exceeds the reference voltage. Interference from noise can be lowered by hysteresis. Bounding limits the output of comparator.

The summing amplifier’s output is the sum of the inputs.

An averaging amplifier yields an output that is the average of all the inputs.

SummarySummary

The scaling adder has inputs of differing weight with each contributing more or less to the input.

Integrators change a constant voltage input to a sloped output.

Differentiators change a sloping input into a step voltage proportional to the rate of change.

ch 13 electronics

upper saddle

summing amplifier

input resistance

reference

voltage divider

output voltage

level detection

sloping input

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