Audio Engineering

156 Chapter 5

In the circuit of Figure 5.6 , which is used as the power supply for an 80- to 100-W
power MOSFET audio amplifi er—again only one channel is shown—a P-channel power
MOSFET is used as the pass transistor and a circuit design based on discrete components
is used to control the output voltage. In this, transistor Q21 is used to monitor the
potential developed across R33 through the R35/RV3 resistor chain. If this is below the
target value, current is drawn through Q19 and R29 to increase the current fl ow through
the pass transistor (Q17). If either the output current or the voltage across Q17 is too
high, Q7 is cut off and there is no current fl ow through Q18 into Q17 gate.

This regulator circuit allows electronic shut down of the power supply if an abnormal
output voltage is detected across the LS terminals (due, perhaps, to a component failure).
This monitoring circuit (one for each channel) is shown in Figure 5.7. This uses a pair of
small-signal transistors, Q1 and Q2, in a thyristor confi guration, which, if Q2 is turned

70 V in R15

0R15 R23

10k

R17

120R

ZD1

4V7

Q17 P-MOSFET 55 V to

power amp

D1

D2

D3

R13

15k

D4 R2912k

R31

12k

RV3.

15k

Q21

D5

D6

R33. 15K

0V 0V

To LS 12 V ref.

10k

Q19

R35

68k

C10.

0.47 uF

C9.

220 uF

R21 Set V out

Q7

LS trip cct

Figure 5.6 : S/C-protected PSU.