Page 148 - Op Amps Design, Applications, and Troubleshooting

Troubleshooting Tips for Amplifier Circuits 131

Signal Injection/Tracing. All electronic circuits can be diagnosed to some
extent by signal injection, signal tracing, or a combination of the two methods. The
underlying goal for this process is to reduce the number of possible culprits down
from a set consisting of every component in the system to a smaller set consisting
of only a few components.
Signal injection requires us to inject a known, good signal at some point in the
circuit and observe the effects. If the circuits that utilize this signal then appear to
operate normally, we can infer that the malfunction is located ahead of our injec-
tion point. We then move our injection point closer to the source of the trouble and
inject another signal. Again, the behavior of the subsequent circuits will provide
guidance as to our next injection point. Two common types of test equipment for
signal injection are signal generators and logic pulsers.
Signal tracing is similar in concept, except we put a known good signal at the
input and verify (trace) its presence throughout the circuit. If we lose the signal (or
it becomes distorted) at a certain point, then we can infer that the trouble lies
ahead of the monitored point. The oscilloscope and logic probe are two common
types of signal tracing equipment.
Both signal tracing and signal injection can be enhanced by using the split-
half method of troubleshooting. By selecting your injection or monitor point to be
approximately halfway through the suspected range of components, each mea-
surement effectively reduces the number of possible components by half.

Voltage Measurements. Voltage measurements normally occur after you
have isolated the problem down to a particular stage consisting of up to perhaps
10 components. The voltage checks contrasted with normal values should result
in the narrowing of suspects down to one or two possibilities. Distinction between
the signal tracing and voltage measuring phases often becomes blurred when an
oscilloscope is used. The concept remains valid, however.

Resistance Measurements. Resistance checks are performed last because
accurate measurements often require desoldering of a component. Desoldering
not only is time consuming but also risks damage to an expensive printed circuit
board in many cases. The resistance checks are done to verify that you have in fact
located the defective component. Component testers can also be used at this point
if available and appropriate.

2.10.2 Specific Techniques for Op Amps
Through observation and signal injection/tracing, the technician can normally
isolate the problem down to a specific circuit. For this discussion, we will assume
the problem has been isolated to an amplifier circuit built around an op amp. The
following sequence of activities will normally isolate the defective component:
1. Verify the power supply voltage on the op amp.
2. Measure output voltage.
3. Measure differential input voltage (V D).
4. Compare the results of steps 2 and 3. If the results violate basic theory (e.g.,
noninverting input is more positive than inverting input, but output is