Page 358 - Op Amps Design, Applications, and Troubleshooting

336 SIGNAL PROCESSING CIRCUITS

7.8 TROUBLESHOOTING TIPS FOR SIGNAL PROCESSING
CIRCUITS

Many of the troubleshooting procedures discussed with reference to basic ampli-
fiers (Chapter 2) are applicable to the signal processing circuits discussed in this
chapter. A few additional techniques, however, may help isolate problems more
quickly.
If the input connection to any of the circuits presented in Chapter 7 is con-
nected directly to ground, then the output should go to its normal DC level. The
normal value, of course, depends on the circuit being considered. In any case, if
the output does go to the correct DC value with no input signal, then the problem
is most likely caused by a defect in one of the AC branches. An AC branch will
contain a series capacitor.
If the DC level on the output is abnormal when the input is shorted to
ground, then apply the bask analytical techniques described in Chapter 2. When
the DC output level is incorrect, more often than not the output will be saturated.
Comparison of the polarity of the differential input voltage of the op amp with the
output polarity will quickly reveal a defective op amp.
If you suspect an open, 2-terminal component, you can momentarily parallel
the suspected part with a known good one while monitoring the output. If the
problem is corrected, you have located the defect.

REVIEW QUESTIONS

1. What is the name of the signal processing circuit that is used to shift the DC level of
the input signal without altering its waveshape?
2. Refer to Figure 7.2. In your own words, explain why the output waveform for the
silicon diode is a constant 0 volts.
3. Refer to Figure 7.3. What is the effect on circuit operation if diode Dj opens?
4. Refer to Figure 7.3. What is the effect on circuit operation if resistor R t decreases in
value?
5. Refer to Figure 7.5. While monitoring the output waveform on an AC-coupled
oscilloscope, you momentarily short resistor K 4. Describe the effects, if any, that are
noted on the oscilloscope display.
6. Refer to Figure 7.5. Describe the effect on output waveshape if resistor R 3 is increased
to 45 kilohms.
7. Refer to Figure 7.8. What is the effect on output waveshape if resistor R 2 is increased in
value?
8. Refer to Figure 7.8. What is the effect on circuit operation if capacitor Q develops a
short circuit?
9. Refer to Figure 7.16. Describe the effect on output waveshape if resistor R 3 is increased
in value.
10. Refer to Figure 7.19. Describe the effect on circuit operation if resistor R 3 is changed to
4.7 kilohms.