Audio Engineering

Negative Feedback 375

known Ft of power transistors. Practical amplifi ers are likely to take off at around 500 kHz
to 1 MHz when Cdom is reduced, which seems to suggest that a phase shift is accumulating
quickly at this sort of frequency. One possible explanation is that there are a large number of
poles close together at a relatively high frequency.

A fourth pole can be simply added to Figure 12.1 by inserting another RC–buffer combination
into the system. With P 2  0.5 μ s and P 3  P 4  0.2 μ s, instability occurs at 345 kHz, which
is a step toward a realistic frequency of oscillation. This is case B in Table 12.1.

When a fi fth output stage pole is grafted on, so that P 3  P 4  P 5  0.2 μ s, the system
just oscillates at 500 kHz with P 2 set to 0.01 μ s. This takes us close to a realistic frequency
of oscillation. Rearranging the order of poles so thatP 2  P 3  P 4  0.2 μ s, while
P 5  0.01 μ s, is tidier and the stability results are of course the same; this is a linear
system so the order does not matter. This is case C in Table 12.1.

40 V

30 V

20 V

10 V

0V

0s 10 μs20μs30μs40μs50μs

Time

v(3) v(7)

100n200n500n 1 μ 2 μ 5 μ

Figure 12.6: Manipulating the P 2 frequency can make ringing more prolonged but it is still

not possible to provoke sustained oscillation.