Page 190 - Op Amps Design, Applications, and Troubleshooting

CHAPTER FOUR

Oscillators

4.1 OSCILLATOR FUNDAMENTALS

An oscillator is essentially an amplifier that produces its own input. That is, if we
connect an oscillator circuit to a DC power supply, it will generate a signal without
having a similar signal available as an input. One of the most fundamental ways
to classify oscillator circuits is by the shape of the waveform generated. In this
chapter, we will study oscillator circuits that produce waveforms such as
sinewave, rectangular wave, ramp wave, and triangular wave.
In general, in order for a circuit to operate as an oscillator, three basic factors
must be provided in the circuit. They are

1. Amplification
2. Positive feedback
3. Frequency determining network

Suppose that many random signal frequencies (e.g., noise voltages) are present at
the input of the amplifier shown in Figure 4.1. All of these frequencies are ampli-
fied by the amplifier and then enter the frequency selective circuit. This portion of
the circuit normally introduces a loss or reduction in signal amplitude. Essentially,
all frequencies can enter the frequency detenrtining network, but only a single fre-
quency (ideally) is allowed to pass through. In practice, a narrow band of frequen-
cies can pass with minimal attenuation. The narrower the passband of frequencies,
the more stable the output frequency of the oscillator.
Once the desired signal emerges from the frequency selective portion of the
circuit, it is returned to the input of the amplifier. The amplifier compensates for
losses in the frequency selective portion of the circuit. The overall closed-loop gain
of the circuit must be at least 1 (unity) in order for the circuit to sustain oscillation.
If the overall loop gain is less than 1, the oscillations quickly decay (ringing at
best); If it exceeds unity, then the amplitude of the output signal will continue to
increase until saturation is reached. If the circuit is intended to produce

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