Audio Engineering

374 Chapter 12

and shows only damped oscillation. Figure 12.7 shows over 50 μ s what happens when
the amplifi er is made very unstable (there are degrees of this) by setting P 2  5 μ s and
P 3  500 ns. It still takes time for the oscillation to develop, but exponentially diverging
oscillation like this is a sure sign of disaster. Even in the short time examined here the
amplitude has exceeded a rather theoretical half a kilovolt. In reality, oscillation cannot
increase indefi nitely, if only because the supply rail voltages would limit the amplitude.
In practice, slew rate limiting is probably the major controlling factor in the amplitude of
high-frequency oscillation.

We have now modeled a system that will show instability. But does it do it right? Sadly, no.
The oscillation is about 200 kHz, which is a rather lower frequency than is usually seen
when an amplifi er misbehaves. This low frequency stems from the low P 2 frequency we
have to use to provoke oscillation; apart from anything else this seems out of line with the

40

20

0

 20

 40

10 H 100 H 1.0 kH 10 kH 100 kH 1.0 MH 10 MH 100 MH

Frequency

db(v(7))

50p 100p 220p

Figure 12.5: The frequency responses that go with the transient plots of Figure 12.4. The

response peaking for Cdom 50 pF is very small compared with the transient overshoot.