as insufficient gain to be detected. Until equipment
manufacturers begin to supply such information on
their schematics or servicing diagrams, servicemen
must rely on the go no-go
.•
test or develop their own
reference files of normal waveform displays. For
shops which specialize in servicing a specific brand
of solid state products, the time required to produce
a reference file may be well worthwhile.
CURVE TRACER APPLICATIONS
TESTING BIPOLAR TRAN$1STORS
The most common application of the B & K Model
501 A Semiconductor Curve Tracer is the testing of
NPN and PNP junction transistors made of either
germanium or silicon. The instrument will accurately
measure several parameters of such transistors un
der dynamic conditions. Range adjustments allow
measurements from the smallest signal transistor to
high current power transistors. An extensive list of
parameters include:
-Current Gain (DC and AC beta)
-Collector-emitter breakdown
-Collector-base breakdown
-Base-emitter reverse breakdown
-Output admittance
-Saturation Voltage
-Saturation Resistance
-Cutoff current
-Leakage current
-Linearity and distortion
-Effects of temperature
-Identification of germanium or silicon
-Matching
-Sorting and substitution
The application and analysis of results described
in this section of the manual assume the use of
previous "Operating Instructions" for properly con
necting the transistor and setting controls to obtain
a display of a family of curves. However, the follow
ing precautions should be observed:
PRECAUTIONS
1. Keep the SWEEP VOLTAGE control: below col
lector breakdown level except during the short
period of a collector voltage breakdown test.
Although the current limiter prevents destruction
of the transistor, high internal temperatures from
longer periods of operation m
ay
cause failure.
2. Limit testing of power transistors without heat
sinks to a few seconds, just long enough to
accurately make a reading. Excessive tempera
tures may result from longer periods of opera
tion. Tests may be stopped and started without
disconnecting the transistor by using the SOCK
ET switch.
3. Keep the VERTICAL SENSITIVITY control as
low as will adequately perform the test. The
higher settings use higher values of current
limiting which could damage small transistors.
It is good practice to return the VERTICAL
SENSITIVITY to the 1 mA/Div position after
each test so that the next test begins with full
protection.
NPN vs PNP Transistors
As described previously in the "Typical Test"
section, the family of curves of an NPN transistor
is in a positive direction. That is, zero volts is at the
left and zero current is at the bottom of the displ
ay
.
The curves sweep to the right and upward as col
lector voltage and current increases from zero. The
collector sweep voltage is of positive polarity.
The f.amily of curves of a PNP transistor, by con
trast, is in the negative direction. That is, zero volts
is at the right and zero current is at the top of the
display. The curves sweep to the left and downward
as collector voltage and current increase from zero.
The collector sweep voltage is of negative polarity.
All transistor measurements described in this man
ual apply equally to NPN and PNP transistors. Any
examples showing only an NPN or PNP type should
be understood to apply to its counterpart as well;
basic characteristics of both types are the same.
Their displays are merely inverted with reference
to one another. Therefore, any measurement that
can be made for NPN transistors can also be made
for PNP transistors and vice versa.
CURRENT GAIN MEASUREMENT
The current gain of a transistor is its single most
important characteristic and is usually measured
before any other tests are performed. The general
condition of a device can most often be determined
while testing for current gain.
Transistors are said to amplify because a small
change in base current causes a proportionately
larger change in collector current. The ratio of
change, called "current gain", is stated numerically.
A current gain of 100 means that a base current
change of 1 part will produce 100 parts of collector
current change. More specifically, current gain can
be measured two w
ay
s:
a. DC current gain (symbolized h
Fr,:
)-also known
14
as
static
current gain, DC beta or static forward
current transfer ratio.
b. AC current gain (symbolized hr,)-also known
as dynamic current gain, AC beta or small
·
signal short circuit forward current transfer
ratio.
Note the difference in symbols for both types. The
DC parameter subscript is written in capitals; the
AC parameter subscript in lower case letters, so as
to distinguish between the two. For convenience
purposes, this manual will refer mainly
\
to DC beta
and AC beta respectively. Lastly, "bet' is often
symbolized by the lower case Greek alph,�t letter {3.
,.
'
i
I
�
.
J
