SECTION2
Operation
10MHz-
40MHz; Fair-Rite
2673000301
40MHz-
200MHz; Fair-Rite
2643000301
2.
Add
100pF polystyrene capacitor (Keithley
C-138-100pF) from base (gate) to collector (drain).
This
reduces the loop gain
at
high frequencies,
thus
increasing gain margin.
This
capacitor can also
"swamp
out'' the effects of
unwanted
feedback net-
works arising from floating
or
long
ground
returns.
3.
Add
330pF polystyrene capacitor (Keithley
C-138-330pF) from base (gate) to emitter (source).
This provides capacitive divider effect
on
feedback
from collector (drain) to base (gate), reducing loop
gain
at
higher frequencies. This technique
may
be
very effective
when
the Source Measure
Unit
that is
supplying power to the base (gate) of the DUT is
on
a
low current range. The capacitor "stiffens" the base-
emitter impedance
at
high frequencies.
4.
Use 100pF to
1000pF
polystyrene capacitors to by-
pass collector
(drain) to emitter (source). This helps
decouple inductances of cables
or
output
imped-
ances of other
Source
Measure Units.
5.
Install the
ground
link connecting
OUTPUT LO
to
chassis
ground
on
the Source Measure
Unit
that is
providing current
to the base (gate) of the DUT. Only
one Source Measure
Unit
should have the
ground
link installed in order to avoid
ground
loops. The
ground link prevents high
and
low frequency oscil-
lations.
2.29
MEASURE ONLY
The
Source
Measure
Unit
can
be
used
as a stand-alone
voltmeter
or
current meter.
Using the
Source
Measure
2-110
+
+
Unit to make routine DMM type measurements is cov-
ered
in
Getting
Started
(paragraph 1.2). In general, meas-
ure
only requires that the Source
Measure
Unit
be
set to
source
O.OOOOV
to measure current,
or
O.OOOOnA
to meas-
ure
voltage. Program compliance for the desired meas-
urement range.
OUTPUT
HI
and
OUTPUT LO
are then
used
to make the measurement.
Special
input
characteristics of the
Source
Measure
Unit
allow it
to
be
used
to make electrometer type measure-
ments. These characteristics include:
>10
14
0
Input
Impedance-
The high
input
impedance
prevents circuit loading
when
making voltage measure-
ments of high impedance devices.
Low Current Measurement Ranges -
On
the 1nA
range, measurement resolution (at 5-d.igits) is 10fA.
<1m V Voltage
Burden
-The
low voltage
burden
pre-
vents circuit loading
when
measuring currents from
sources of a few volts
or
less.
GUARD-
The driven
guard
is available to eliminate the
effects of leakage current
in
test cables that can adversely
affect low level measurements.
Figure 2-76
and
Figure 2-77
show
how
to configure the
Source
Measure Unit
for electrometer type measure-
ments.
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