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drop at approximately 20 volts, leaving approximately 20 volts across the output terminals of the "piggy-back"
supply.
Agilent Technologies supplies may use any of three basic methods of controlling the high voltage output of the
Main Voltage Source: (1) the control signal from the High Voltage Control Circuit fires SCRs in the rectifier
circuit to vary the dc output, (2) the control signal varies the coupling of the high voltage input transformer to
adjust the ac input to the rectifiers or (3) the control signal pulse modulates the input to the rectifier to vary the
dc output.
High Performance Power Supplies
Agilent Technologies manufactures several types of high performance dc power supplies with specifications at
least an order of magnitude superior to the normal well-regulated laboratory supply. Foremost among these are
the precision voltage and current sources.
Precision Voltage Sources
This line includes both CV/CC and CV/CL supplies, similar to those described previously, with a
few important exceptions. The critical components of the supply, including the zener reference
diode for the voltage comparison amplifier and the low-level portions of the feedback amplifier, are
enclosed in a temperature-controlled oven. Moreover, the less critical components that are not oven
enclosed are high quality components with low temperature coefficients. These techniques, together
with the utilization of a high-gain feedback amplifier, result in an exceptionally stable and
well-regulated supply with a 0.1% accuracy.
Precision Constant Current Source
The concepts and circuits used in basic constant current power supplies were shown in Figure 16.
This section is devoted to the refinements necessary to upgrade a basic constant current supply to a
precision class, with characteristics that more closely approach an ideal current source.
An ideal current source is a current generator that has infinite internal impedance. It provides any voltage
necessary to deliver a constant current to a load, regardless of the size of the load impedance. It will supply this
same current to a short circuit, and in the case of an open circuit it will attempt to supply an infinite voltage (see
Figure 25).
In practical current sources, neither infinite internal impedance nor infinite output voltages are possible. In fact,
if the current source is to be used as a test instrument, it should have a control for limiting its maximum output
voltage, so its load will be protected against the application of excessive potentials. Its output impedance should
be as high as possible, of course, and should remain high with increasing frequency to limit current transients in
rapidly changing load. A capacitor across the output terminals should be avoided, since it will lower the output
impedance, store energy which can result in undesirable current transients, and decrease the programming
speed.
One approach to the design of a current source is to add a high series resistance to an ordinary voltage source.
However, it is difficult to achieve good current regulation with this design.
Typical applications for current sources call for output impedances of a few megohms to a few hundred
megohms and currents of tens or hundreds of milliamperes. This means the source voltage would have to be