Audio Engineering

304 Chapter 9

of VHF instability, which can, in the hands of an unwary amplifi er designer, lead to the
rapid destruction of the output devices. However, this excellent HF performance, when
handled properly, makes it much easier to design power amplifi ers with good gain and
phase margins in the feedback loop, where overall NFB is employed. In contrast, the
relatively sluggish and complex characteristics of the junction power transistor can lead
to diffi culties in the design of feedback amplifi ers with good stability margins.

Also, as has been noted, the power MOSFET is intrinsically free from the problem of
secondary breakdown, and an amplifi er based on these does not need the protective
circuitry that is essential in amplifi ers with BJT output devices if failure is to be avoided
when they are used at high power levels with very low impedance or reactive loads.
The problem here is that the protective circuitry may cut in during high-frequency
signal level peaks during the normal use of the amplifi er, which can lead to audible
clipping. (Incidentally, the proponents of thermionic valve-based audio amplifi ers have
claimed that the superior audible qualities of these, by comparison with transistor-based
designs, are due to the absence of any overload protection circuitry that could cause
premature clipping and to their generally more graceful behavior under sporadic overload
conditions).

4

I (amperes)d

3

2

1

0

0 1020304050

Vd (amperes)

IRFF110 25 C

7V

6V

5V

4V

3V

Figure 9.21 : MOSFET characteristics.