Specifications

2-10
2.4.2 DC bias
In addition to the AC test signal, a DC voltage can be output through the Hc terminal and applied to
the DUT. A simplified output circuit, with a DC bias source, is shown in Figure 2-8. Many of the
conventional impedance measurement instruments have a voltage bias function, which assumes that
almost no bias current flows (the DUT has a high resistance.) If the DUT’s DC resistance is low, a
bias current flows through the DUT and into the resistor (Rr) thereby raising the DC potential of the
virtual ground point. Also, the bias voltage is dropped at source resistor (Rs.) As a result, the speci-
fied bias voltage is not applied to the DUT and, in some cases, it may cause measurement error.
This must be taken into consideration when a low-resistivity semiconductor device is measured.
The Agilent 4294A precision impedance analyzer (and some other impedance analyzers) has an
advanced DC bias function that can be set to either voltage source mode or current source mode.
Because the bias output is automatically regulated according to the monitored bias voltage and cur-
rent, the actual bias voltage or current applied across the DUT is always maintained at the setting
value regardless of the DUT’s DC resistance. The bias voltage or current can be regulated when the
output is within the specified compliance range.
Inductors are conductive at DC. Often a DC current dependency of inductance needs to be mea-
sured. Generally the internal bias output current is not enough to bias the inductor at the required
current levels. To apply a high DC bias current to the DUT, an external current bias unit or adapter
can be used with specific instruments. The 42841A and its bias accessories are available for high
current bias measurements using the Agilent E4980A, 4284A, and 4285A precision LCR meters.
Figure 2-8. DC bias applied to DUT referenced to virtual ground