Page 311 - Op Amps Design, Applications, and Troubleshooting

CHAPTER SEVEN

Signal Processing Circuits

Most of the circuits presented in this chapter serve to condition analog signals for
subsequent input to another circuit Many of them could be categorized as wave-
shaping or conditioning circuits. We will, for example, examine circuits that can
rectify low amplitude signals, limit the maximum excursion of signals, and
change the DC level of waveforms. Many of the circuits are quite simple in terms
of component count, but they play important roles in overall systems design.

7.1 THE IDEAL DIODE

Several of these circuits behave as though they had perfect or ideal diodes. Figure
7.1 contrasts the forward-biased characteristics of a perfect diode with that of a
typical silicon diode.
Figure 7.1 (a) shows a simple series circuit driven by an AC source. The out-
put is taken across the diode. Basic circuit theory tells us that when the diode is
reverse-biased, it acts as a very high impedance (i.e., essentially an open circuit).
In the case of the circuit in Figure 7.1 (a), we can expect to see nearly the full input
voltage across the diode during times of reverse bias.
When the diode in Figure 7.1(a) is forward-biased, we expect it to act as a
short circuit (or at least a very low impedance). In this case, there would be very lit-
tle voltage across it. Figure 7.1(b) shows the output waveforms that we can expect.
We see that the ideal diode has no voltage across it when it is forward-biased,
which is in contrast with the silicon-diode waveform that has a 0.7-volt drop dur-
ing forward-biased times. For purposes of this chapter, we will limit our compar-
isons to the forward-biased performance of the diodes. Therefore, Figure 7.1(b)
shows similar waveforms for both ideal and silicon diodes during reverse bias.
With regard to effect on circuit operation, is the 0.7-volt drop across the
forward-biased silicon diode important? Does it adversely affect the circuit's per-
formance? Well, many times we ignore the 0.7-volt drop when analyzing or even
designing circuits and get along quite well. But consider the circuit shown in Fig-

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