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uses a network consisting of Cc and Rc
to compensate for the output inductive
reactance.
However, the value of Cc
cannot be increased enough to make the
output impedance capacitive because the
settling time would become too long.
Therefore, the SMU may oscillate if an
unusually large inductance is connected
to it. This could occur if the OUT is a
superbeta transistor (hFE
SMU connected to the emitter is set to
one of the low current ranges.
(2 )
Oscillation due to strays.
The test sample itself may also oscillate
due to the residual inductance and stray
capacitance of the connection cables,
test fixture or probe card.
wide bandwidth bipolar transistors are
especially susceptible to OSCillation.
3.
OSCILLATION COUNTERMEASURES
The type of devices which tend to cause oscil
lation are:
(1)
Superbeta (High hFE) bipolar transistors
(2 )
High gm MOSFETs
(3)
Devices
which
capacitance
(4)
Devices operated at very high frequency
(5)
Negative resistance devices
Some effective methods for eliminating oscilla
tions are:
For (1). (2). (3). and (4) above, position ferrite
beads as close as possible to the OUT to kill
high frequency oscillations, and to kill low
frequency oscillations, insert
shown in Figure 3. In some instances it may
be necessary to use more than one ferrite
bead.
Figure 4 shows two methods of stopping a
SMU from oscillating when measuring (5) of
negative resistance devices.
3000) and the
>
Superbeta,
have
large
stray
resistor as
a
2
"Ferrite Be a ds or Resistor
PN 9170-0029)
( HP
Figure
3.
Preventing Transistor Oscillation
G
Figure 4. Negative Resistance Measurements
R
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