Eliminating Oscillations - Keithley 4200A-SCS Reference Manual

Section 12: Maintenance
In general, oscillations can be classified in two categories: high-frequency oscillations (100 kHz
through 200 MHz), and low frequency-oscillations (below 100 kHz). For solutions to both types of
oscillation, refer to

Eliminating oscillations

The measures needed to eliminate oscillations depend on whether the oscillations are high frequency
or low frequency.
Eliminating high-frequency oscillations
One or more of the following remedies may help to eliminate high frequency oscillations; the
remedies are listed in order of preference:
•
Mount the preamplifiers as close to the DUT as possible.
•
Connect the COMMONs (outer shields) of all cables together at the DUT.
•
Use lossy ferrite beads or 100 Ω resistors in series with the DUT leads.
•
Disconnect the ground link between GNDU COMMON and chassis ground on the rear panel of
the mainframe. Connect the cable shields to the prober chassis.
•
Add a high-quality capacitor between the base and emitter of a bipolar junction transistor (BJT) or
between the gate and source of an FET. Use a 100 pF to 1000 pF capacitor (Keithley Instruments
part number C-138-100 pF).
Eliminating low frequency oscillations
Oscillations at low frequencies (DC to 100 kHz) occur when the gain of a transistor under test
interacts with the output impedances of the connected SMUs. The following ratios of impedance (Z)
determine the gains of the transistors:
•
For a FET: ZDrain SMU / ZSource SMU
•
For a BJT (bipolar junction transistor): ZCollector SMU / ZEmitter SMU
A SMU measures current through the voltage drop across a resistance, which is in series with the
DUT. This series resistance is high for low current ranges and low for high current ranges. Therefore,
for two SMUs connected to the transistor BJT collector and emitter terminals, or FET source and
drain terminals, a large current-range difference (oscillation) results in the following:
•
A large series-resistance difference
•
A large impedance ratio between the two series resistances connected to the transistor
•
A large circuit gain (potentially, the maximum, intrinsic transistor gain)
•
A potentially unstable circuit
To avoid oscillations for a FET, try the following:
•
Set (Drain-SMU current measure range) = (Source-SMU current measure range)
•
If necessary, set both SMUs to autorange.
•
For the source SMU, do not select the Common operation mode. The mode prevents you from
configuring a current measurement range for the source SMU, which results in a lower
impedance than at the drain SMU; a potentially high gain; and an increased likelihood of low-
frequency oscillation. Instead, configure the source SMU for the Voltage Bias operation mode and
set it to 0 V. This allows you to configure the current measurement range.
12-10
Eliminating oscillations (on page 12-10).
Model 4200A-SCS Parameter Analyzer Reference Manual
4200A-901-01 Rev. C / February 2017