Direct Current Power Systems 201
ly, if the voltage across the load increases, the rise in emitter voltage makes
the base less positive. The current through Q1 will then be reduced, which
results in an increase in the collector-emitter resistance of Q1. The increase
in resistance will cause a larger voltage drop across transistor Q1, which
will now compensate for the change in voltage across the load. Opposite
conditions would occur if the load voltage were to decrease. Many varia-
tions of this circuit are used in regulated power supplies today.
Shunt regulators are also used in DC power supplies. The circuit of
Figure 7-33 is a shunt voltage regulator. Again, the zener diode (D1) is
used to establish a constant DC bias level. Therefore, voltage variations
across the DC output will be sensed only by resistor R2. If the DC output
voltage rises, an increased positive voltage will be present at the base of
transistor Q2. The increased forward bias on transistor Q2 will cause it to
conduct more. This makes the base of transistor Q1 more positive, and
Q1 will then conduct more heavily. Increased current flow through bothFigure 7-31. Zener diode voltage regulator: (A) Circuit diagram, (B) Zener diode
characteristic curve