Power Supply Design 147chosen so that its stray magnetic fi eld will be at right angles to the plane of the amplifi er
PCB.
5.6 Half-Wave and Full-Wave Rectifi cation ...............................................................
Because the reservoir capacitor recharge current must replace the current drawn from
it during the nonconducting portion of the input cycle, both the peak recharge current
and the residual ripple will be twice as large if half-wave rectifi cation is employed, such
as that shown in the circuit of Figure 5.1(h) , in which the rectifi er diode only conducts
during every other half cycle of the secondary output voltage rather than on both cycles,
as would be the case in Figure 5.1(b). A drawback with the layouts of both Figures 5.1(a)
and 5.1(b) is that the transformer secondary windings only deliver power to the load
every other half cycle, which means that when they do conduct, they must pass twice the
current they would have had to supply in, for example, the bridge rectifi er circuit shown
in Figure 5.1(e). The importance of this is that the winding losses are related to the square
of the output current (P i R) so that the transformer copper losses would be four times
as great in the circuit of Figure 5.1(b) as they would be for either of the bridge rectifi er
circuits of Figure 5.1(f). However, in the layout of Figure 5.1(b) , during the conduction
cycle in which the reservoir capacitor is recharged, only one conducting diode is in the
current path, as compared with two in the bridge rectifi er setups.
Many contemporary audio amplifi er systems require symmetrical ve and ve power
supply rails. If a mains transformer with a center-tapped secondary winding is available,
such a pair of split-rail supplies can be provided by the layout of Figure 5.1(e) or, if
component cost is of no importance, by the double bridge circuit of Figure 5.1(f). The
half-wave voltage doubler circuit shown in Figure 5.1(g) is used mainly in low current
applications where its output voltage characteristic is of value, such as perhaps a higher
voltage, low-current source for a three-terminal voltage regulator.
5.7 Direct Current Supply Line Ripple Rejection .......................................................
Avoidance of the intrusion of AC ripple or other unwanted signal components from the
DC supply rails can be helped in two ways: by the use of voltage regulator circuitry
to maintain these rails at a constant voltage or by choosing the design of the amplifi er
circuitry that is used so that there is a measure of inherent supply line signal rejection.
In a typical audio power amplifi er, there will be very little signal intrusion from the ve