Page 228 - Op Amps Design, Applications, and Troubleshooting

210 OSCILLATORS

If the oscillator circuit uses active components as part of the basic oscillator
loop (e.g., transistors), a shift in the DC levels in the circuit can cause the active
device to move out of its normal range of operation and introduce distortion.
If the waveform distortion is caused by clipping at one of the V SAT levels
(unless this is normal behavior for the circuit), look for defects that would affect
the DC operating point of the circuit. The first thing to do in this case is to verify
proper power supply voltages. Some oscillator configurations can continue to
oscillate with dramatic changes in power supply voltages. The symmetry and
purity of the output signal, however, may suffer.

Incorrect Frequency of Operation. In certain oscillator designs, nearly
every component in the circuit affects the frequency of operation. Troubleshooting
a circuit of this type can be streamlined by noting, but not concentrating on, the
frequency error. Rather, verify all other aspects of the oscillator's operation (e.g.,
DC level, waveshape, duty cycle), and if one of these other characteristics is found
to be abnormal, focus your attention on this latter problem. The off-frequency
problem is probably only a symptom and will be corrected when the other, more
easily detected problem is corrected. If all other characteristics appear to be nor-
mal, then suspect the components whose sole purpose is for frequency determina-
tion and that a change in value will not alter the DC levels in the circuit. There will
be very few components that can qualify for this category.

4.7 NONIDEAL CONSIDERATIONS

We have already discussed one of the most significant nonideal op amp character-
istics—the slew rate. If the op amp's slew rate is not high enough, the output will
be distorted (at best) and the frequency of operation will generally be lower than
expected.
Another constraint that can cause problems is the limited current capability
of the op amp output. For best performance, stability, and so forth, it is generally
wise to avoid heavy loading of an oscillator output—especially if the loads vary.
This limitation can easily be overcome by buffering the oscillator output with
another op amp configured as a voltage follower.
Unless the application is particularly demanding (e.g., low-frequency drift
with temperature changes), the other nonideal parameters do not generally create
major problems in oscillator circuits.