This curve will be obtained by the procedure
just
followed. Comparison between the drawing and the
actual curve will
be
helpful in recognizing the different portions
of
the curve.
No.
1.
This
is
the saturation region,
and
can
best
be
seen
with low horizontal voltage
settings. This
means
that
for
any
given
collector voltage the
same
collector current results
relatively independent
of
base
current. This
is
the operating region when the transistor
is
used
as
a saturated switch.
No.
2.
This
is
the portion
of
the curve where the majority
of
transistors
are
operated.
If
these
lines
are
perfectly horizontal, the collector current
is
a function
of
base
current only,
and
independent
of
collector voltage, thus, making
it
a perfect current source. The slope
present
here
is
due
to
the
leakage
represented
by
No.
3.
No.
3.
Generally speaking, the lower this slope, the better the transistor.
On
most silicon
transistors this
leakage
(represented
by
No.
3)
is
almost undetectable.
This
is
also theregion in which the
gain
(or hfe )
of
the transistor
is
read.
The
gain
read
is
in
reality hfe , but since the frequency
is
only 240
Hz,
it
can
be
taken
as
HFE
in
most
transistors. This
gain
is
obtainocl
by
applying a known current
to
the
base
(adjustable by the
base
current switch) and reading the corresponding collector current
on
the vertical
scale
of
the CRT
screen,
(this
scale
is
adjustable by the collector current switch). Now, by dividing
the
base
current into the collector current the hfe
or,#
can
be
obtained.
No.
4.
This portion
of
the curve
is
the breakdown region, and from
it
VcEO
can
be
obtained
by
reading the voltage at which the lowest curve No. 3
passes
through the specified
current
at
which the breakdown
is
measured.
If
while measuring this portion, the
set
of
curves
start
to
move upward rapidly, the
transistor under test
is
heating beyond its rated value,
and
can
be
burned
out
easily
if
left
at
thatvoltage. V
CB
can
be
measured
by shorting the
base
and
emitter terminals.
GROUNDED BASE: This
is
another important characteristic which
can
be
obtained
if
desired. This
is
done simply by reversing the
base
and
emitter terminals
of
the test transistor
to
the test terminals. The
base
generator (which must
be
of
reverse
polarity) now supplies the emitter current. The collector
voltage and current
are
read
as
before. In
some
cases
a blurr
of
the trace will occur
near
the breakdown
of
the transistor. This
is
normal and
difficult
to
prevent.
It
represents
an
oscillation
due
to
the very
high frequency
response
in this configuration,
and
the small
lead
inductance
and
capacitance
is
enough
to
form atuned circuit (which then oscillates
at
an
extremely high frequency). This
same
phenomenon
can
be
observed
in the negative resistance region
of
any tunnel diode characteristic.
GENERAL:
Almost any characteristic
of
any device
can
be
obtained with this curve tracer with the
addition
of
a
little
ingenuity. The primary method
of
obtaining uncommon characteristics
is
to
switch
the terminals around (including the horizontal input
to
the
scope).
In
some
cases,
additional power
supplies or transformers
can
augment the internal
ones.
It
should
be
remembered that the
unit
can
be
damaged.
However,
as
long
as
the current and voltage levels
are
kept low, no harm will come
to
it
or
the device under test.
-6-
I
I
I
I
I
I
i
I
