Page 419 - Op Amps Design, Applications, and Troubleshooting

Nonideal AC Characteristics 395

The op amp has a varying gain depending upon the operating frequency, but the
gain requirement for oscillation is certainly a possibility.
Additionally, the op amp inherently has a 180° phase shift between its inverting
input and the output by virtue of its operation. Now if we provide an additional 180°
phase shift and provide a loop gain of at least unity, we will have built an oscillator,
In an earlier chapter, we provided this extra phase shift externally at a specific
frequency to construct an op amp oscillator. All of the internal stages of an op amp
also have certain frequency and phase characteristics. As frequency increases, the
cumulative phase shift of these internal stages also increases. If, at some frequency,
the total phase shift reaches 360° (or 0°) and at the same time we have a loop gain
of at least unity, the circuit will oscillate even though that may not be the intended
behavior.

Principles of Frequency Compensation. So far in the text we have primar-
ily focused on the frequency characteristics of the op amp with only occasional
reference to the phase characteristics. Figure 10.7 shows a simple RC circuit and
graphs showing its frequency and phase performance. Table 10.1 shows this same
data in tabular form.

FIGURE 10.7 The frequency/phase response of a simple RC circuit.