Page 147 - Op Amps Design, Applications, and Troubleshooting

130 AMPLIFIERS

2.10 TROUBLESHOOTING TIPS FOR AMPLIFIER CIRCUITS

In order for an amplifier to operate properly, it must be biased in its linear range of
operation. That is, the output must be between the two saturation limits with no
signal applied. Many, if not most, of the problems encountered when trouble-
shooting op amps configured as linear amplifiers result in the output being driven
to one of the saturation limits. Your task, then, is to recognize the symptoms and to
locate the defective component.
If the amplifier circuit is properly designed (i.e., capable of achieving the
desired performance), you can generally diagnose the problem by comparing the
actual to ideal op amp behavior. The following are two critical characteristics to
remember when troubleshooting amplifier circuits utilizing op amps:

1. The output should be between the saturation limits.
2, The differential input voltage (%) should be very near 0.

2.10.1 Basic Troubleshooting Concepts

When troubleshooting any type of circuit, it is important to use a logical, systematic
technique. Although there are several accepted methods, the following sequence of
activities is a common and effective procedure:

1. Observation
2. Signal injection/tracing
3. Voltage measurements
4. Resistance measurements

Observation. This is probably the most important step in the process if done
effectively. Observation means more than just looking at the circuit. It includes all
of the following actions:

1. Interrogate the owner, user, or operator for clues regarding how the trouble
developed.
2. Operate the user controls and observe the behavior for clues.
3. Use your senses. Do you see any visible damage? Do you smell burned
components? Do you hear suspicious sounds?
4. Be alert to similarities between observable symptoms on the defective unit
and the symptoms of previously diagnosed circuits.

Many problems can be identified during the observation stage. How many of us
have "successfully" traced a malfunction throughout a complex circuit until we
located a suspected switch or variable resistor on the schematic? Then, when we
physically locate the suspected component on the system, it turns out to be a front
panel control! Had we applied the preceding procedure faithfully, we could have
reduced our efforts dramatically.