Keithley SourceMeter 2400, Service Manual, Page: 61 / 137

4-7 Model 2400 Service Manual
Troubleshooting
2400-902-01 (G - Feb 2006)
Output stage
Figure 4-3 shows a simplified schematic of the output stage.
The Model 2400 output stage serves two purposes: (1) it converts signals from floating com-
mon 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 possibility of thermal
runaway with high-current output values. High-current taps for the ±20V outputs are provided
to reduce power dissipation on the 20V and lower ranges.
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 deter-
mined by the resistor-capacitor ladder circuits shown. Coarse current limits are built into the
output stage.
225
+36
Q514
Q516
Q518
Q521
Q523
Q525
-36
-225
+17VF
+15VF
-15VF
-17VF
Maindrive
300k
F
F
O
Figure 4-3
Output stage
simplified
schematic