150 Chapter 5
speaking, although their noise can be lessened by connecting a low ESR capacitor in
parallel with them. For applications where only a low voltage is needed and its actual
value is not very important but a low circuit noise is essential, a simple arrangement is to
use a string of silicon diodes, as shown in Figure 5.2(c). Each of these diodes will have a
forward direction voltage drop of about 0.6 V, depending on the current fl owing though
them. Light-emitting diodes have also been recommended in this application, for which
a typical forward voltage drop would be about 2.4 V, depending on the LED type and its
forward current. All of these simple shunt regulator circuits will perform better if the input
resistor (R1) is replaced by a constant current source, shown as CC1.
5.9 Series Regulator Layouts ......................................................................................
The problem with the shunt regulator arrangement is that the circuit must draw a current
that is always greater than would have been drawn by the load on its own. This is an
acceptable situation if the total current levels are small, but this would not be tolerable
if high output power levels were involved. In this situation it is necessary to use a series
regulator arrangement, of which some simple circuit layouts are shown in Figure 5.3.
The circuit of Figure 5.3(a) forms the basis for almost all of this type of regulator circuit,
with various degrees of elaboration. Essentially, it is a fi xed voltage source to which an
emitter–follower has been connected to provide an output voltage (that of the Zener diode
less the forward emitter bias of Q1) at a low output impedance. The main problem is that
for the circuit to work, the input voltage must exceed the output voltage—the difference
is termed the drop-out voltage—by enough voltage for the current fl ow through R1 to
provide the necessary base current for Q1 and also enough current through D1 for D1 to
reach its reference voltage. Practical considerations require that R1 shall not be too small.
In a well-designed regulator of this kind, such as the 78xx series voltage regulator IC, the
drop-out voltage will be about 2 V.
This drop-out voltage can be reduced by reversing the polarity of Q1, as shown in Figure
5.3(b) , so that the required base input current for Q1 is drawn from the 0-V rail. This
arrangement works quite well, except that the power supply output impedance is much
higher than that of Figure 5.3(a) , unless there is considerable gain in the NFB control
loop. In this particular instance Q2 will conduct and feed current into the Q1 base until
the voltage developed across R3 approaches the voltage on the base of Q2, when both Q2
and Q1 will be turned off. By augmenting Q2 with an op-amp, as shown in Figure 5.4 , a
very high performance can be obtained from this inverted type of regulator layout.