)
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DC Current Gain (DC beta)
The DC or s�atic current gain of a transistor is
defined as the ratio of collector current to base cur
rent measured at one specific point of collector volt
age and current (hence the term "static" meaning
statibnary). The validity of this test is dependent
, upon the point of measurement and this point is
usually.. specified on manufacturer's data sheets; it
most often centers about the typical operating range
for whic4 the device was designed. The following
is an example:
CHARACTERISTIC
SYM MIN TYP MAX UNIT
DC Current Gain
hn� 40 125 400
le= 5mA, Ve= lOV
The conditions of test are clearly defined under
"DC Current Gain" as a collector current (le) of
5 mA and a collector voltage (V cl, of 10 volts. Since
current gain can vary substantially between devices
of even the same type, minimum-maximum condi
tions are usually specified. Note that in the example,
gain can vary by 10 to 1.
Figure 17 A demonstrates thQ technique for meas
uring DC beta. The point of measurement has been
conveniently centered on the
·
gr
aticule by using
1 mA/Division vertical and 2 Volts/division hori-
.
zontal calibration. With the STEP SELECTOR set at
10 µA, the third curve (30 ,u.A) just happens to pass
thru the measurement point. It is now obvious that
30 µA of base current produces 5 mA of collector
current at the 10 volt collector point. Using the simple
formula:
le
5 mA
DC beta=-=�-�= 166
In
.03 mA
The only difficulty that m
ay
be encountered is
conversion of units used in the calculation. Remem
ber that 1000
µ,
A = l mA or similarly, .001 mA = l µ,A.
The example in Figure 17B demonstrates how to
A
Point of MfftuNIMftt
le· !51110,
Ve·
10.
calculate DC beta even though none of the curves
pass through the measurement point. Simply approx
imate the percentage of distance between the curves
above and below the poin�; use it as a "percentage"
of one step to obtain total base current when added
to the number of current steps b
.
elow the point. In
the example, the measurement ·point. is above the 20
µA base steP' ,yet below the 30 µ,kstep. If its position
is approximated, it can be thought of as 30%
·
·
of the
distance between 20 and 30 11.A, or 23 fl.A total.
AC Current Gain (AC beta)
The AC or dynamic current gain of a transistor is
defined as the ratio of
change
in collector current to
the
change
in base current at a specified collector
voltage. This measurement is ultimately more useful
than DC beta because the transistor is tested under
actual operating conditions and from it, performance
can be predicted. AC beta may be calculated from
the display of curves as follows (refer to Fig. 18):
1. Measure the difference in collector current (.6
le) between two curves of the displ
ay
. The set
ting of the VERTICAL SENSITIVITY control
shows the amount of collector current repre
sented by each vertical division of the graticule
scale. Be sure that both readings are taken at
the same collector voltage (V c). Each horizontal
division of the graticule scale represents a
specific collector voltage. Make both readings
at one of the horizontal division markers.
2. Note the change in base current (.6 I
B
from the
STEP SELECTOR control. Each step produces
one of the curves of the display. Therefore, for
the 10 ,u.A position, .6 In equals 10 1.tA between
two curves of the display.
3. Calculate beta by dividing
6
le by
.6
In. For
example: if 6 Ic equals 2 mA and .6 In equals
10 ,u.A, beta equals 200.
8
10
1
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112311A
M::.:.�
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......... ( ...... ) ......... .!
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-----r,�=t===t:
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I
l�•O
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/I
T
3011A:, boN CUf'reftl / Vc��olls)
- !5111aof cel-
llctor _, ___ __.
tc
DCNta •
le
.�
.03111A
20
0
10
Vclvollsl
DC beta =
!�
.�
.023111A
JI•
lf>C
/1 •
217
·
Figure 17. DC Current Gain (DC Beta) Measurement
20
15