6 mA, a 2mA change. As it swings negatively to
10 p,A (1st curve), the collector current decreases
for 4 mA to 2.1 mA, a 1.9 mA change. Out of the
3.9 mA total collector current swing, the imbalance
between the positive and negative swing is 0.1 mA.
This amounts to approximately 2 ½
%
distortion (0.1
mA--;- 3.9 mA = .0256), acceptable for some applica
tions-not acceptable for others.
The measurement should be taken along the oper
ating load line rather than at a specific V c, because
it more nearly duplicates the dynamic conditions of
operation. The transistor will operate with a load,
not at a specific fixed V
0•
The load causes operation
along the load line, since an increase in collector
current will reduce collector voltage and vice versa.
With transistors which exhibit near horizontal col
lector current curves such as shown in Figure 19B,
there is little difference between using the load line
or a specific V
0•
However, in transistors with more
slope to the collector current curves, the collector
voltage affects the collector current and could pro
duce considerably different results in the linearity
and distortion measurement.
BREAKDOWN VOLTAGE MEASUREMENT
As sweep voltage is increased, a collector break
down will be reached. The value at which this occurs
depends upon the transistor type. The curve tracer
tests breakdown up to 100 volts, which is sufficient
to test all but the high voltage rated transistors. At
collector br.eakdown voltage, the collector current
becomes independent of base current and rises
sharply to the current limiting protection limit of the
curve tracer. Except for this feature of the curve
tracer, the transistor would be destroyed by the test.
Figure 21 shows a typical family of curves with the
sweep voltage set high to cause collector break
down. In the examples shown in the figure, break
down occurs at a collector voltage of approximately
40 volts for both transistors. Note that base current
has little effect upon the point at which the increase
in collector current occurs. Keep the test as short
as possible to prevent excessive temperature dam
age to the transistor. Even with current limiting, the
current value is much higher than normal and
causes temperature increase. If operated in this
condition for an extended period, damage could
occur. Be sure the VERTICAL SENSITIVITY control
10 ........................ , ......... .......................... , ........................ , .......... , ............ , ........... ,
Ie
(,no)
.. :,
........... ,
...........
,
...........
,,
......... ,
........... , ...........
,
...........
, ,.,,,u-
.........
,
...........
,
, ........... ,
...........
, ........... , ........................ , ..........
v.i
..
�
.
_v //.
·
·
, .... , ..... , ..... , ..... ,
y )
J
_;...,....-.' 1
···
·
·
·
�/
)·
............. ,
...........
,
.,,)'
0
10
20
30
40
..... ,
..........
.
Ve
!volts)
ABRUPT BREAKDOWN
is in a proper range for the type of transistor being
tested. If the setting is too high, the current limiting
action may be too high to protect the transistor.
Figure 21. Typical Transistor Breakdown Curves
1. To perform the measurement, first adjust for a
normal family of curves on the display. This
display should not fill the graticule scale hori
zontally.
2. Next, increase the SWEEP VOLTAGE control
until the upturn in collector current at the tail
of the curves is observed. This upturn will be
very sharp for most transistors, but more grad
ual for other types.
3. Read the collector voltage value at which the
upturn occurs. Read this value from the hori
zontal graticule scale. If a breakdown voltage
specification for the transistor is available, use
the figure to determine whether or not the tran
sistor is acceptable. If specifications are not
available, a good rule of thumb is that the
transistor should withstand approximately twice
the collector supply voltage of the circuit in
which
it
is to be used.
Ve (voltal
GRADUAL BREAKDOWN
Figure 21. Typical Transistor Breakdown Curves
18