Page 321 - Op Amps Design, Applications, and Troubleshooting
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Ideal Biased Clipper  299


                    TABU 7.1

                                          Design Goal             Measured Value
                    Input voltage   250 millivolts-5 volts (peak)  250 millivolts-5 volts (peak)
                    Output voltage  375 millivolts-7.5 volts (peak)  368 millivolts-7.8 volts (peak)
                    Input impedance       >3.0 kilohms              20 kilohms
                    Input frequency      0-25 kilohertz           0-25 kilohertz


               7,6(a) and 7.6(b) show the positive and negative outputs, respectively, with a 250-
               millivolt peak input signal at a frequency of 1.0 kilohertz. Notice that the effect of
               the 0.7-volt forward voltage drop of diodes D^ and D 2 is nonexistent. Figures 7.6(c)
               and 7.6(d) show the circuit's response to a 25-kilohertz signal with maximum
               input voltage. Finally, the actual performance of the circuit is contrasted with the
               original design goals in Table 7.1.



        7.3    IDEAL BIASED CLIPPER


               You may recall from basic electronics theory that a biased clipper or limiter circuit
               has no effect on the input signal as long as it is less than the clipping or reference
               voltage. Under these conditions, the input and output waveforms are identical If,
               however, the input voltage exceeds the clipping level of the circuit, then the out-
               put waveform is clipped or limited at the clipping level. Figure 7.7 shows a basic
               biased shunt clipper and its associated waveforms. We can make both series and
               shunt, and both biased and unbiased, clippers with an op amp, just as we can with
               simple diode circuits. The difference, however, is that the op amp version elimi-
               nates the effect of the diode's forward voltage drop (0.7 volts). This is a very
               important consideration when processing low-amplitude signals.

        7.3.1 Operation
               Figure 7.8 shows the schematic diagram of an op amp version of the biased shunt-
               clipper circuit. The basic purpose is similar to the simple diode clipper shown in
               Figure 7.7, but because the effects of the diode's forward voltage drop have been















                     FIGURE 7.7 A simple biased shunt-clipper circuit. The output signal cannot go
                     above V REF+0.7 volts.
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