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Operation
DC
parameter checks
With
a
transistor
configured
as
a
common-emitter am-
plifier,
the
test
system
shown
in Figure 4-8
can be used
to
determine
the
following E)C parameters:
CoUector
current
(Ic),
base
current
(Ig),
current
gain,
emitter
cur-
rent
(Ig)
and
base-to-emitter voltage
(Vgg).
Figure
4-8
shows which
crosspoints
to
close to config-
ure
the amplifier
circuit.
SMU
#1
is
configured
to
source voltage
and measure
current.
It
is
used
to
power
the
collector
circuit
(Vce)
and
measure
the
collector
current
(Ic)-
SMU
#2
is
configured
to
source
current
and measure
voltage.
It is
used
to
provide
the
base
cur-
rent
(Ig)
for
the
transistor,
and
will also
measure
the
base-to-emitter voltage (Vgg).
With
collector
current
(1^)
and
base
current
(Ig)
known,
the current
gain can
be
calculated as follows:
Gain
—
—
Common-emitter
characteristic
curves
A
profile of
the
transistor
operating
characteristics
can
be
obtained
by
measuring
the collector current
over a
specified voltage
range
(Vcg)
for different
base
bias
currents
(Ig).
For example. Figure
4-9
shows
the charac-
teristics
of
a
typical
MPN
silicon
transistor at
base
bias
currents
(Ig)
of 20}iA, 40jxA,
60pA,
and
80jiA.
Extensive
trigger capabilities
facilitate
synchronization
of
the Keithley Source
Measure Unit
operations.
By
performing
a subordinate
sweep,
SMU
#1
will
perform
a
staircase
sweep
at
every base
bias
current
level set
by
SMU
#2.
On
every
step of
each
staircase
sweep,
SMU
#1
will
source a voltage
level
(Vce)
measure
the
subsequent
collector
current
(1^).
For
the characteris-
tics
shown
in
Figure
4-9,
four
staircase
sweeps were
performed; one
staircase
sweep
at
each base
bias
level.
Refer
to
a Keithley
Source
Measure
Unit
instruction
manual
for details
on
perforrning
sweeps.
The
emitter current
(Ig)
can be determined
by
using
Kirchoff's
Current
Law
as follows:
4-11
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