6. The noise figure expresses the relative merit
of a receiver which, if
perfect, would have a noise figure of 0 decibel (db).
The noise figure of an RF
amplifier can be reduced by using
a. step-up input transformers,
low
input
conductance
tubes,
or
high
transconductance tubes.
b. low input conductance tubes, high transconductance tubes, or bandwidth-
increasing circuits.
c. high transconductance
tubes,
bandwidth-increasing
circuits,
or
step-up
input transformers.
d. bandwidth-increasing circuits, step-up input transformers, or low input
conductance tubes.
SITUATION
Assume that the circuit shown in figure 1-3 is the amplifier in an FM tuner.
is shown in figure 1-4. Assume also that the screen-grid current is 3 ma.
Exercises 7 and 8 are based on the above situation.
7. When the circuit is operating normally as an RF amplifier in an FM receiver,
approximately
a. 80 ohms.
c.
330 ohms.
b. 100 ohms.
d.
1,200 ohms.
8. Assume that the control-grid bias on the 6BJ6 tube is changed to -2 volts.
Without automatic volume control (AVC), the plate current will increase 4 ma when
the signal on the grid increases 1 volt.
When an AVC potential of -4 volts is
applied, an increase of 1 volt in the signal will cause the plate current to
increase
a. less than 0.25 ma.
b. between 0.25 ma and 0.50 ma.
c. between 0.50 ma and 1 ma.
d. more than 1 ma.
9. The bandwidth of the input circuits of an RF amplifier such as shown in A of
figure 112 (TM 11-668) might be increased by either
a. moving the coils closer together or using coils with higher Q.
b. using coils with higher Q or incorporating additional tuned circuits.
7
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