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(2) The use of a fixed dc input voltage means that the output voltage can only be one polarity, the opposite of
the reference polarity.**
(3) The series regulator can conduct current in only one direction. This, together with the fact that the rectifier
has a given polarity, means that the power supply can only deliver current to the load, and cannot absorb
current from the load.
* Special design steps have been added to the design of most Agilent low voltage supplies to permit a
significant reduction in the size of the output capacitor merely by changing straps on the rear terminal strip (see
page 89).
** In Agilent’s line of Bipolar Power Supply/Amplifiers this output capacitor is virtually eliminated by using a
special feedback design. In addition, BPS/A instruments are capable of ac output, conduct current in either
direction, and their outputs are continuously variable through zero (see page 54).
Figure 6. Operational Amplifier Representation of Adjustable CV Power Supply
Series Regulator with Preregulator
Adding a preregulator ahead of the series regulator allows the circuit techniques already developed for low
power supplies to be extended readily to medium and higher power designs. The preregulator minimizes the
power dissipated in the series regulating elements by maintaining the voltage drop across the regulator at a low
and constant level. This improves efficiency by 10 to 20% while still retaining the excellent regulation and low
ripple and noise of a series regulated supply. In addition, fewer series regulating transistors are required, thus