Audio Engineering

372 Chapter 12

NFB amplifi er depends on the open-loop gain available. Once more elementary feedback
theory is brought to life. The value of Cdom controls the bandwidth, and it can be seen that
the values used in the simulation do not give a very extended response compared with a
20-kHz audio bandwidth.

In Figure 12.4 , one extra pole P 2 at 1.59 MHz (a time constant of only 100 ns) is added to
the output stage, andCdom stepped through 50, 100, and 200 pF as before. 100 pF shows
a slight overshoot that was not there before; with 50 pF there is a serious overshoot that
does not bode well for the frequency response. Actually, it’s not that bad; Figure 12.5
returns to the frequency-response domain to show that an apparently vicious overshoot is
actually associated with a very mild peaking in the frequency domain.

From here on Cdom is left set to 100 pF, which is its real value in most cases. In Figure
12.6 , P 2 is stepped instead, increasing from 100 n s t o 5 μ s, and while the response gets
slower and shows more overshoot, the system does not become unstable. The reason is
simple: sustained oscillation (as opposed to transient ringing) in a feedback loop requires

40

20

0

 20

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

Frequency

50p 100p 200p

db(v(7))

Figure 12.3: SPICE simulation in the frequency domain. As the compensation capacitor is

increased, the closed-loop bandwidth decreases proportionally.