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Operation of op-amp comparator
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15 10 5 Volts 0 5 10 15 0 01 02 03 04 05 06 Time, s 07 08 09 1 vout (t) vin (t)
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15 10 5 Volts 0 5 10 15 0 01 02 03 04 05 06 Time, s 07 08 09 1 vout (t) vin (t)
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Figure 1537 Input and output of noninverting comparator
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Figure 1538 Input and output of inverting comparator
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where Vsat is the saturation voltage for the op-amp (somewhat lower than the supply voltage, as discussed in 12) Typical values of supply voltages for practical op-amps are 5 V to 24 V A simple modi cation of the comparator circuit just described consists of connecting a xed reference voltage to one of the input terminals; the effect of the reference voltage is to raise or lower the voltage level at which the comparator will switch from one extreme to the other Example 1512 describes one such circuit
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EXAMPLE 1512 Comparator with Offset
Problem
Sketch the input and output waveforms of the comparator with offset shown in Figure 1539
+ _ Vref
Solution
Known Quantities: Input voltage, voltage offset Find: Output voltage, vout (t) Schematics, Diagrams, Circuits, and Given Data: vin (t) = sin( t); Vref = 06 V Analysis: We rst compute the differential voltage across the inputs of the op-amp:
vin + ~
+ _
vout
Figure 1539 Comparator with offset
= vin Vref Then, using equation 1529, we determine the switching conditions for the comparator:
+ vin > Vref vout = Vsat vin < Vref vout = Vsat
Thus, the comparator will switch whenever the sinusoidal voltage rises above or falls below the reference voltage Figure 1540 depicts the appearance of the comparator output voltage Note that comparator output waveform is no longer a symmetrical square wave
Comments: Since it is not practical to use an additional external reference voltage
source, one usually employs a potentiometer tied between the supply voltages to achieve
15
Electronic Instrumentation and Measurements
15 10 5 Volts 0 5 10 15 0 01 02 03 04 05 06 Time, s 07 08 09 1 vin (t) vout (t) Vref
Figure 1540 Waveforms of comparator with offset
any value of Vref between the supply voltages by means of a resistive voltage divider This circuit will be explored later in this chapter
Another useful interpretation of the op-amp comparator can be obtained by considering its input-output transfer characteristic Figure 1541 displays a plot of vout versus vin for a noninverting zero-reference (no offset) comparator This circuit is often called a zero-crossing comparator, because the output voltage goes through a transition (Vsat to Vsat , or vice versa) whenever the input voltage crosses the horizontal axis You should be able to verify that Figure 1542 displays the transfer characteristic for a comparator of the inverting type with a nonzero reference voltage
+ vin + ~ _ vout Vref vin + ~ vout Vsat
+ _ vout
VS
vout Vsat
vin Vsat Vsat
Vref
Figure 1541 Transfer characteristic of zero-crossing comparator
Figure 1542 Transfer characteristic of inverting comparator with offset
Very often, in converting an analog signal to a binary representation, one would like to use voltage levels other than Vsat Commonly used voltage levels in
Part II
Electronics
this type of switching circuit are 0 V and 5 V, respectively This modi ed voltage transfer characteristic can be obtained by connecting a Zener diode between the output of the op-amp and the noninverting input, in the con guration sometimes called a level or Zener clamp The circuit shown in Figure 1543 is based on the fact that a reversed-biased Zener diode will hold a constant voltage, VZ , as was shown in 8 When the diode is forward-biased, on the other hand, the output voltage becomes the negative of the offset voltage, V An additional advantage of the level clamp is that it reduces the switching time Input and output waveforms for a Zener-clamped comparator are shown in Figure 1544, for the case of a sinusoidal vin (t) of peak amplitude 1 V and Zener voltage equal to 5 V
6 4 2 Volts 0 2 4 6 0 01 02 03 04 05 06 Time, s 07 08 09 1 vin (t) vout (t)
+ vin _
vout
VS vout Vsat VZ Voff
Vsat
Figure 1543 Level-clamped comparator
Figure 1544 Zener-clamped comparator waveforms
Although the Zener-clamped circuit illustrates a speci c issue of interest in the design of comparator circuits, namely, the need to establish desired reference output voltages other than the supply saturation voltages, this type of circuit is rarely employed in practice Special-purpose integrated circuit (IC) packages are available that are designed speci cally to serve as comparators These can typically accept relatively large inputs and have provision for selecting the desired reference voltage levels (or, sometimes, are internally clamped to a speci ed voltage range) A representative product is the LM311, which provides an open-collector output, as shown in Figure 1545 The open-collector output allows the user to connect the output transistor to any supply voltage of choice by means of an external pull-up resistor, thus completing the output circuit The actual value of the resistor is not critical, since the transistor is operated in the saturation mode; values between a few hundred and a few thousand ohms are typical In the remainder of the chapter it will be assumed, unless otherwise noted, that the comparator output voltage will switch between 0 V and 5 V Data sheets for integrated-circuit comparators may be found in the accompanying CD-ROM The Schmitt Trigger One of the typical applications of the op-amp comparator is in detecting when an input voltage exceeds a present threshold level The desired threshold is then represented by a DC reference, Vref , connected to the noninverting input, and the input voltage source is connected to the inverting input, as in Figure 1542 Under ideal conditions, for noise-free signals, and with an in nite slew rate for the opamp, the operation of such a circuit would be as depicted in Figure 1546 In practice, the presence of noise and the nite slew rate of practical op-amps will require special attention
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