paco T-65 Assembly And Operating Manual page 33

ment having the required sensitivity; full scale deflection is obtained at 100 microamperes with
the maximum -sensitivity instrument setting.
Transistor manufacturers have standardized a number of nomenclature symbols for
various transistor characteristics. Three most commonly encountered terms are:
1. lebo This symbol indicates current flowing between collector and base with the
emitter open. See Figure 16. The letter I of course, is the symbol for current; the next two
symbols represent collector and base, the two parts of the transistor in which current is meas­
ured;
0
indicates the remaining element of the transistor, the emitter, is open for this test.
In similar manner, a test requiring the current measurement between emitter and base, with
the collector open, is represented by the symbol lebo. In fact, this test is sometimes used by
transistor manufacturers.
2. lebs -
In
this symbol, I, c, and b mean the same as before; s indicates the misstng
element (the emitter in this case) is shorted to the base.
3. Beta - Beta describes current gain and is analogous to amplification factor in re­
ceiving tube nomenclature. Specifically, it is current gain from collector to base with output
shorted and a constant collector voltage. Beta refers to either AC or DC current gain. In the
Model T-65, Beta or gain is indicated by first observing a reading of emitter to collector
current with base open, then obtaining a second reading with a predetermined value of current
injected into the base to produce an increased collector current reading. The increase in
current indicates the gain or Beta of the transistor. Extensive tests show close correlation
between AC and DC Beta readings under these conditions.
Other symbols may be encountered by the technician. They will, however, folloW the same
pattern as those just described.
A diffused junction PNP transistor can be seen in Figure 23. Both physical and schematic
representations are shown. The physical sketch is much larger than the actual size of the
transistor elements and shows that emitter and collector can be spaced close to each other and
hence become shorted. Therefore, the "SHORT" test is the first important test which should be
perfo~med with the Model T-65 tester. A shorted or low resistance transistor should be re­
jected without further testing. A simplified schematic of the circuit for the "SHORT" test is
shown in Figure 15.
DIFFUSION PRODUCES
P TYPE GERMANIUM
REGIONS
PNP NPN
EMITTER COLLECTOR EMITTER COLLECTOR
Y
Y
N TYPE
BASE
BASE
GERMANIUM
BASE
FIGURE 23
As noted previously, lebo is the basic transistor test parameter. It should be the second
test performed on all transistors with the Model T-65 tester. A simplified circuit for this test
is pictured in Figure 16.
The third transistor test is
ilL"
or leakage. The circuit for this test appears in Figure 17.
The leakage test in itself has no special significance insofar as the condition of the transistor
is concerned but is used to arrive at a final figure for transistor gain or Beta.
The circuit for the Beta test is shown in Figure 18. The reading obtained during the leak­
age test is subtracted from the reading obtained by the gain test to yield an actual gain number.
Tetrode transistors are similar in construction to the usual three terminal transistor
except that an additional base connection which functions as a tetrode connection is made exter­
nally. Before testing for tetrode action, the usual tests for lebo, leakage, and gain are per-
Page 31