AC beta
=
610
=
2 mA
=
200
.6I
8
.01 mA
The .6 Ic measurement is usually made between
the centermost curves of the display. The
.6
Ic
measurement may be made between two non-ad
jacent curves if desired. For example, the difference
between the collector current of the second and
fourth curves may be used for measurement of
.6
I
0•
If this method is used, be sure to include two
steps of base current for determining
.6
In when
calculating beta (refer to Fig. l8B).
If the transistor data sheet is available, beta
should be measured at the approximate collector
current and voltage specified. If not, the STEP
SELECTOR is usually adjusted for a display of the
most evenly and widely spaced curves.
Summary of Transistor Current Gain
Note that the beta of a transistor is not constant,
but is dependent upon the point of measurement.
The distance between all curves is seldom equal,
which means that .6 le is not the same at various
regions of base current and, to some lesser extent,
at various collector voltages. Base current values
that produce curves that are closer together are in
the region of lower gain. Gain is usually higher in
the normal operating region of the transistor and is
lower at collector currents above or below the nor
mal operating region.
Current Gain Linearity vs. Distortion
Measuring the linearity and distortion of a tran
sistor is an extension of the gain or beta test. As
pointed out in the beta test procedure, beta is not
necessarily constant, but may vary with collector
current. When such variation of gain occurs, the
transistor in non-linear and will introduce distor
tion if operated in the non-linear region. Typically,
transistors have lower current gain at high values
of collector current as shown in Figure ISA. This is
apparent by the fact that the curves are closer to
gether at higher collector current. Each base current
step has precisely the same amount of increase,
which should cause the collector current curves to
be separated by equal amounts if the gain were
constant. When gain decreases, the curves are closer
together. Some transistors have an opposite charac
teristic in that their gain increases at high values
of collector current (Figure l9B). A few transistor
types have nearly uniform gain over a very wide
range of collector current. Such transistors have low
distortion and a wide region of linearity. The curves
from such transistors are equally spaced.
Not all applications require that the transistors
have good linearity and distortion-free characteris
tics. Examples are switching transistors, some class
C amplifiers, and frequency multipliers. In fact, some
applications require non-linearity and depend upon
this characteristic for operation. Of course, most
class A and B amplifiers, including audio and video
amplifiers, do require linear and distortion-free oper
ation over the dynamic range of the input signal.
For most small signal applications, a linear region
can usually be found. For larger signals, a wider
linear region is required.
To measure linearity and distortion, adjust the
STEP SELECTOR control for the most evenly spaced
display of curves, or so that the centermost curves
of the display are the most evenly spaced. Note that
it will not be possible to obtain evenly spaced curves
for some transistor types. Set the sweep voltage to
approach, but not reach, collector breakdown. Note
that the curves are somewhat shorter at higher col
lector currents. Plot an imaginary line along the
ends of the curves as shown in Figure 20. This line
is called the "Test Load Line". Plot an "Operating
( l _
�le
AC CURRENT GAIN hfe - �lb
/J=�
4111
0
2
16
2,nA
fl:
ro;;.
4
6
Ve
!volts)
A
fJ:
200 at Ve of 5voltt
8
10
R: 3.5mA
f-' 20µA
fl: 175 at \le af 5voltt
10r··
········· ,,··=:.,
�'::::::F·
�=Faor-T
j,,
.
:
.. j ..
L ....
... : ·····-··'··----.1 ...........
�_= .... .
!
0
2
4
6
Vclvoltsl
B
8
10
Figure 18. AC Current Gain (AC Beta) Measurement
.
i
!
