Keithley SourceMeter 2425, Service Manual

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Model 2425 Service Manual Troubleshooting 4-9
Output stage
Figure 4-4 shows a simplified schematic of the output stage.
The Model 2425 output stage serves two purposes: (1) it converts signals from floating
common to output common, and (2) it provides both voltage and current amplification.
The output stage drive transistors are biased in class B configuration to prevent the possi-
bility of thermal runaway with high-current output values.
Output transistors Q518 and Q521 are cascoded with output MOSFETs Q516 and Q523.
All other MOSFETs and transistors are slaves, and the voltages across these devices are
determined by the resistor-capacitor ladder circuits shown. High-current drive capability
is provided by Q500-Q511. Coarse current limits are built into the output stage.