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Support Circuit Design 371

Another low-cost, but higher-performance, regulator, is the series-pass tran-
sistor regulator of Fig. 8.46. If the voltage across the load attempts to increase
for any reason, the regulator’s output voltage will stay nearly steady. This is
due to the following action: The voltage across the load R must equal the volt-
L
age drop across the zener minus the voltage drop across the E-B junction of the
series-pass transistor Q . The transistor’s base voltage is set by the zener and,
1
since the voltage dropped across the zener cannot change, then any rise in the
regulator’s output voltage will force Q ’s emitter to be that much more positive
1
than the base, which is the equivalent of making the base less positive. This
ends in a smaller base-to-emitter voltage, resulting in less emitter current
through the transistor, and thus an increased voltage drop across Q . Any
1
attempted rise in voltage across the load R is substantially lowered in value,
L
and a steady output voltage is maintained for undemanding applications.
Figure 8.47 shows a series-pass regulator with feedback, which maintains a
far more consistent and steady output voltage than the two regulators dis-
cussed above. This circuit contains R , R , and R , which monitor the output
1 2 3
voltage as a voltage divider network. And since these resistors do form a volt-
age divider, we can also set the required output voltage to a wide range of val-
ues by simply moving the wiper of the adjustable R all the way from a little
2
above the zener’s V to just under the unregulated supply voltage. For
Z
instance, if we wish to increase the output voltage, we can move the poten-
tiometer’s wiper downward, and the voltage on Q ’s base decreases, lowering
2

Figure 8.45 A simple zener shunt regulator.

Figure 8.46 A series-pass transistor regulator.

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