a. Input. Input phase inversion is unnecessary since at zero base bias the NPN
transistor (Q1) and PNP transistor (Q2) conduct on alternate half-cycles of the
input.
When the input signal is positive going, Q1 conducts and Q2 is cut off.
When the input signal is negative going, Q2 conducts and Q1 is cut off.
Thus,
push-pull operation takes place as a result of the complementary action of the
transistors.
b. Base Current.
Note that Ib1 and Ib2 flow in opposite directions.
If the
transistors are properly matched, these two currents will be equal. Because they
are equal and opposite, the resultant charging current flowing into the coupling
capacitor is zero.
Since no charge will develop to reverse bias the bases,
discharge diodes are not needed.
c. Advantage.
The basic circuit just discussed illustrates how simplicity and
improvement can be attained by using NPN and PNP transistors together. Many varied
circuits take advantage of the complementary features of transistors.
Such
circuits have no electron-tube counterparts.
LESSON EXERCISES
In each of the following exercises, select the ONE answer that BEST completes
the statement or answers the question.
Indicate your solution by circling the
letter opposite the correct answers in the subcourse booklet.
SITUATION
Assume that the FM transmitter shown in figure 2-7 is being modified to increase
its efficiency.
Exercises 1 thru 5 are based on this situation.
1. Assume that the crystal in this FM transmitter operates at a frequency of 850
kHz. The center frequency of this transmitter output is
a. 27.2 MHz.
c.
81.6 MHz.
b. 54.4 MHz.
d.
122.4 MHz.
2. Assume that the transmitter is to be redesigned to have fewer stages and to
provide a multiplication of 256. The minimum number of frequency multiplier stages
that could be used to obtain this multiplication without excessive loss of output
is
a. three.
b. four.
c. five.
d. six.
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