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from
that
of
Fig.
2-4
in that
the
trace is so
slow
the effect of the delay line in the oscillo-
scope
and
the
delay
time
of
the
transistor
are
not observable.
In this display,
the input
pulse
starts
at
the
point
marked
O
on
the
TIME
axis.
You
may
determine
the
starting
point of the input pulse by pressing the PULSE
ZERO
TIME
REFERENCE
button.
Zero
time
for
turn-on
and
turn-off
may
not
lie
in
a
vertical
plane
because
of
a slight
difference
in
sweep
triggering.
Since
the
display
here
is almost symmetrical, thetransistor is operat-
ing "Class
A" , i.e., somewhere between cutoff
and saturation.
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zero
Time
REFERENCE
bas
Fig.
2-5.
Pulse
response
of
a type
2N109
p-n-p
transistor operating
Class
A,
Sweep
speed
is 10 wsec/em.
Measuring
the Risetime
Adjust
the
BIAS
VOLTS
control
so
that
the
right-hand
portion
of
the
display
is
exactly
two divisions high. Movethedisplay horizontally
so
that
the
upper
trace
intersects
the
center,
vertical
graticule
line
at
the
point shown
in
Fig. 2-6.
Recall
that
the
risetime
is
defined
as the
time interval between the 10% and 90% amplitude
levels
in the direction
of increasing
collector
current.
By
making
the
total
height
of
the
double triggered display 2 centimeters, the 10%
and
90%
amplitude levels are marked by minor
graticule divisions.
To determine the risetime,
measure
the
distance
in centimeters
between
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bas
Fig.
2-6.
A
method
of
positioning
the
display
so
that
the risetime
can be measured
accurately.
the
center
graticule
line and the point where
the
trace
would
intersect
a
10%
amplitude
mark,
For
the
display
shown,
with
a hori-
zontal
sweep
rate of 10 wsec
per
centimeter,
the
risetime
is
approximately
2.4 x 10 psec
or
24
wsec.
Falltime
is
calculated
in
the
same manner.
Using
the Series
Resistor
Move
the
SERIES
RESISTOR
control
from
1K
to
20K
and
watch
the effect on the trace
at
each
intermediate
switch
position.
Two
things
will
be
apparent.
One is that the ampli-
tude of the display will diminish.
This happens
because
the
bias
voltage
at
the
base
of the
transistor is approaching zero (cut-off),
Recall
that
the
biasing
current
flows
through
the
SERIES
RESISTOR
and that the input resistance
of a transistor is relatively low in the grounded-
emitter configuration.
The other
effect you will
notice
is
that
the
risetime
and falltime
will
increase
since
the
input
capacitance
of
the
transistor has more effect on the pulse- response
time
as
the
driving
impedance
increases,
The
two
causes
mentioned
are
not the
only
ones
in
operation
since
the
current
gain
and
other
factors
within
the transistor
are varying also.
SATURATION
HPN
TAH epee rerys vera crerarereeayers teres
NO
ZERO TIME
REFERENCE
paz
Fig.
2-7.
Response
of
a type
2N109
transistor driven
into saturation.
Now
move
the
SERIES
RESISTOR
control
from
1K
to
50
ohms
and
observe
the effect
on
the
trace
at
each
switch
position.
The
upper
trace
should
flatten
out.
If
it doesn't
move
the
BIAS
VOLTS
control
to
a
more
negative
setting
by
a small
amount
until you
get
a pronounced
flattening of the upper trace
in the 50-ohm position of the SERIES RESISTOR
control.
A typical display is shown in Fig. 2-7.
The flat portion of the trace presents transistor
saturation.
During this timethe collector voltage
is almost
zero
and the current is limited only
by
the
Current-Measuring
Resistor
(100
Q)
and the external resistor.
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