Agilent Technologies B1500A Training Manual page 90

High Voltage SPGU
Effects of Load Impedance
Vset
The voltage actually applied to the DUT depends on it's impedance. This behavior is very common among the
instrumentations like DC power supply and pulse generators. In this meaning, SMU is very special because
the output of SMU is controlled by the feedback loop from the voltage monitored by the sense terminal.
If Rdut is large enough (ex. gate terminal) compared with 50 ohm output impedance, Vdut is almost equal to
Vset.
If Rdut is not so large (ex. drain terminal at the ON state), the difference between Vdut and Vset is not
negligible. In the case of 1 mA current with 1 V bias (= 1 kohm), the difference will be about 5 %.
The SPGU provides the function to adjust the output voltage automatically. To use this function properly, the
load impedance of DUT must be registered and it must be accurate. Then the SPGU can apply the voltage
near the setting value to the DUT.
Output Impedance
50 Ω
V
Pulse Source
Vset: Setting voltage with open load
=
Vdut Vset
13-6
Vdut
Rdut
Rdut
×
+
Rdut 50
Module 13
SPGU Control and Applications