2-22
forward bias. Any increase in signal level causes an increase in agc voltage. An increase in agc voltage
increases conduction, which in turn drives the transistor to or near saturation. As the transistor approaches
saturation, its gain is correspondingly reduced.
On the other hand, if the input signal level decreases, the negative agc voltage decreases. The
forward bias is then reduced, and the transistor operates on a lower portion of its characteristic curve
where gain is higher.
Forward agc provides you with better signal-handling capabilities; however, reverse agc is simpler to
use, causes less loading of the tuned circuits, and produces smaller variations in input and output
capacitance.
Q17. What does manual gain control do to strong and weak signals, respectively?
Q18. What is the purpose of agc/avc in a receiver?
Delayed Automatic Gain Control
The disadvantage of automatic gain control, attenuating even the weak signal, is overcome by the use
of delayed automatic gain control (dagc). Let’s take a look at the typical dagc circuitry in figure 2-19. This
type of system develops no agc feedback until an established received signal strength is attained. For
signals weaker than this value, no agc is developed. For sufficiently strong signals, the delayed agc circuit
operates essentially the same as ordinary agc.
Figure 2-19.—Delayed agc action.
Our circuit uses two separate diodes; one is the detector diode and the other the agc diode. The agc
diode is connected to the primary of the last IF transformer and the detector diode to its secondary. A
positive bias is applied to the cathode of the agc diode. This keeps it from conducting until a prearranged
signal level has been reached. The adjust delay control allows manual control of the agc diode bias.
Manual control allows you to select the signal level at which agc is applied. If mostly weak stations are to
