Power Supply Design 153In the inverted stabilizer circuit shown in Figure 5.4 , R1 monitors the output current, and
if this is large enough to cause Q1 to conduct, then the output voltage will progressively
collapse, causing the PSU to behave as a constant current source at whatever output
voltage causes the load to draw the current determined by R1. (I know this protection
technique works because this is the circuit I designed for my workshop bench power
supply 20 years ago,^1 which has been in use every working day since then, having
endured countless inadvertent output short-circuits during normal use, as well as
surviving my son having left it on overnight, at maximum current output, connected to
a nickel-plating bath that he had hooked up, but which had inadvertently become short-
circuited.) In the particular layout shown, the characteristics of the pass transistors used
(Q3 and its opposite number) are such that no current/voltage combinations that can be
applied will cause Q3 to exceed its safe operating area boundaries, but this is an aspect
that must be borne in mind. Although I use this supply for the initial testing of nearly all
my amplifi er designs, it would not have an acceptable performance, for reasons given
earlier, as the power supply for the output stage of a modern hi-fi amplifi er.
However, there is no such demand for a completely unlimited supply current for voltage
amplifi er stages or preamplifi er supply rails, and in these positions, a high-quality
regulator circuit can be of considerable value in avoiding potential problems due to
hum and distortion components breaking through from the PSU rails. Indeed, there is
a trend in modern amplifi er design to divide the power supplies to the amplifi er into
several separate groupings: one pair for the gain stages, a second pair for the output
driver transistors, and a fi nal pair of unregulated supplies to drive the output transistors
themselves. Only this last pair of supplies normally needs to be fed directly from a simple
high current rectifi er/reservoir capacitor type of DC supply system.
A further possibility that arises from the availability of more than one power supply to the
power amplifi er is that it allows the designer, by the choice of the individual supply voltages
provided, to determine whereabouts in the power amplifi er the circuit will overload when
driven too hard since, in general, it is better if it is not the output stage that clips. This was
an option that I took advantage of in my 80-W power MOSFET design of 1984.^2
5.11 Integrated Circuit (Three Terminals) Voltage Regulator ICs ................................
For output voltages up to 24 V and currents up to 5 A, depending on voltage rating,
a range of highly developed IC voltage regulator packages are now offered, having