COMPUTING

Chapter 5 Operational AmplifiersThe circuit of Figure 5.77is often referred to as a half−wave rectifier with limited negative output. Inthe circuit of Figure 5.77, the output is limited by the Zener diode during the negative half−cyclesof the output voltage, and during the positive half−cycles of the output voltage is limited by theZener diode forward voltage drop .5.21 ComparatorsV FA comparator is a circuit that senses changes in a varying signal and produces an output when athreshold value is reached. As a comparator, an op amp is used without feedback, that is, the opamp is used in the open loop configuration. Figure 5.78 shows a differential input amplifier withoutfeedback used as a comparator.+V CCv outv ref > v inv in+−+v ref−23746+v out−– V CCv ref < v inV– CCFigure 5.78. A differential input op amp without feedback used as a comparatorv refAs shown in Figure 5.78, v out = +V CC if v ref > v in , and v out = – V CC if v ref < v in . The switchingtime from – V CC to +V CC is limited by the slew rate of the op amp. Comparators are used extensivelyin analog−to−digital conversion as we will see in a subsequent section.Op amp applications are limitless. It is beyond the scope of this text to describe all. It will sufficeto say that other applications include zero−crossing detectors also known as sine−wave to square−wave converters, sample and hold circuits, square−wave generators, triangular−wave generators, sawtooth−wavegenerators, Twin−T oscillators, Wien bridge oscillators, variable frequency signal generators,Schmitt trigger, and multivibrators. We will discuss the Wien bridge oscillator * , the digital−to−analog converter, and the analog−to−digital converter in the next sections, and the Schmitt triggerand multivibrators in Chapter 7.5.22 Wien Bridge OscillatorThe circuit shown in Figure 5.79 is known as Wien bridge oscillator. This circuit produces a sinusoidaloutput.* We will revisit the Wien bridge oscillator in Chapter 8.5−50Electronic Devices and Amplifier Circuits with MATLAB Computing, Second EditionCopyright © Orchard Publications