c. AF Output Circuit.
Resistors R1 and R2 form the detector load.
Voltage
developed across the load resistors causes current to flow in the primary circuit
of AF output transformer T2.
Resistor R1 serves as the volume control since its
setting determines the amount of voltage applied to the primary of T2.
3-3.
TRANSISTOR AGC CIRCUIT
(fig.
3-3)
a. IF Input Circuit.
The same transformer (T1) that serves the detector also
supplies the IF voltage necessary for operation of the AGC circuit.
A third
winding on T1 couples voltage into the base-emitter circuit of the AGC amplifier.
The signal developed across this winding causes diode Z2 to develop the AGC voltage
across R7 with the polarity indicated. Capacitor C1 serves to shunt the IF signal
pulsations away from the AGC amplifier.
b. AGC Output Circuit.
Resistors R2 in the mixer and R5 in the first IF
amplifier emitter circuits provide ground returns for the AGC amplifier.
Current
flow (dc), shown by arrows, produces a voltage drop proportional to signal
strength, across each of the resistors. The dc potential developed by the current
flow makes the emitter of each stage more positive. In turn, the forward bias of
each of these stages is reduced, driving the transistors closer to cutoff and
reducing their gain.
The stronger the antenna signal, the greater will be the
voltage drops across the resistors (R2, and R5) and the lower the gain of these
stages. The AGC circuit thereby controls the gain of the receiver.
c. AGC Delay Circuit. Resistor R1 in the RF amplifier provides a ground return
for the ACC amplifier. A delay diode (Z1) delays the application of AGC voltage to
the RF stage only.
This RF amplifier delay keeps the amplifier operating
efficiently over a wider change of signal amplitude than the mixer and IF stages.
The net result is that the RF amplifier receives no AGC voltage until the signal
amplitude rises to a value high enough to exceed the positive voltage on the RF
Consequently, the AGC voltage does not control the gain of the RF amplifier signal
amplitude until its output voltage equals the threshold value. However, the IF and
mixer stages always receive a value of AGC voltage proportional to the IF signal
strength.
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 answer in the subcourse booklet.
1. The type of detector which develops an output proportional to the amplitude
of the RF input voltage is a
322 L3
20
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