BK Precision 501A, Instruction Manual

Page: 25 / 40

LEAKAGE MEASUREMENT (lcEo)
Collector leakage current is the collector-to-emitter
current that flows when the transistor is supposed
to be completely off. If the transistor is leaky, in­
creasing collector voltage causes the collector cur­
rent to increase independently of the base current.
Germanium transistors normally exhibit some
amount of leakage, but silicon transistors should
exhibit no measurable amount of leakage. When
measured in relation to specification data, tests
should be made at the indicated collector voltage
and temperature. Leakage current is normally tem­
perature dependent.
Leakage can be measured by observing the zero
base current line (neglect the remainder of the fam­
ily of curves). Any sloping of this line indicates a
leakage current (See Figure 22). However, a situa­
tion can occur that will cause apparent leakage
current even though the device under test has none.
The horizontal input of the oscilloscope used in con­
junction with the curve tracer will produce an actual
leakage current because it is connected across the
collector-emitter terminals. Typically, 100 Kn impe­
dance will cause 1 mA of apparent leakage at lOOV.
This can be taken into account by switching the
AGE
-CURRENT
_p
I
'
2
:)
t
........
'
__,
re
:--
.,/
-
:--
-
:.--
-
Figure 22. Leakage Current Measurement
transistor under test in and out while watching for
movement in the baseline.
SATURATION VOLTAGE [VcE(sat>]
The collector saturation region of a transistor is
that portion of the family of curves in the area of
low collector voltage and current below the knee of
each curve. Notice that the knee of each curve
occurs at approximately the same collector voltage,
regardless of base current. Notice, also, that collec­
tor voltage above the knee has little effect upon
collector current and base current has the predomi­
nant effect. Saturation voltage, V cd
sni
), is the col­
lector voltage at the knee of the curve. For measure­
ment in comparison to specifications, base current
and collector current should be stated. The specifica­
tion value is the maximum value at which the knee
should occur. Therefore, if the specification value is
on or above the knee, the transistor is acceptable.
To measure saturation voltage on the curve tracer,
only the saturation region need be displayed. This
is the low collector voltage portion up to and in­
cluding the knee of each curve. The display should
be "spread out" using a low voltage horizontal cali­
bration, such as
0.2
volt per division, to accurately
measure the low collector voltage value.
Saturation resistance, rcE<
$
n
1),
can be calculated
if desired. It equals the collector voltage divided by
collector current for a given value of base current
in the collector saturation region.
Ve
rci,:($at)
=
1;;-
.........
:
........... :. .. .. . . . .. .... ;
,
........ � ...........
/
ll . . . . : : :
a/ ... 1,, ) SATURATION VOLTAGE
:=:$
.15V ··'
. I
, ....
,
....
,
....
.i .
...
....... (
..
....
..
..
J
.........
(
......... :
si, .. •., ..... , ........... , ........... , ........... , .........
.
... ,,, .
..
.......... , ........... , ........... , ........... , ........... ,
i I ,
..i.
( �
)
i·
T ..
·i ... ,· · ··· / ..... · • ... • .. ·' ·+
.... ,
..... , ..... , ..... , ..... , ..... ,. +
4
/ J.1-...;.......-..-,!---'------...... ----·
....... ,
) I
' I
2:
I
I
)
..
+
0
.4
.8 1.2
1.6
2.0
Vc(vollsl
I B = SµA per step
Figure 23. Saturation Voltage
The specification values of saturation resistance
is usually stated as the maximum acceptable limit.
Dynamic resistance is found by calculating or plot­
ting the average saturation resistance over a range
of base current.
19