文档介绍：TUTORIAL
DESIGN AND OPERATION
OF AUTOMATIC GAIN
CONTROL LOOPS
FOR RECEIVERS IN MODERN
COMMUNICATIONS SYSTEMS
his article is intended to provide insight VGA TYPES
into the effective operation of variable There are two major classes of VGA in use
Tgain amplifiers (VGA) in automatic gain today. The first is the so-called IVGA (input
control (AGC) applications. Figure 1 is a gen- VGA), which can be regarded as a passive
eral block diagram for an AGC loop. The in- variable attenuator followed by a fixed-gain
put signal passes through the VGA to produce amplifier. The second type is the output VGA
the output level to be stabilized. The detec- (OVGA), which is essentially equivalent to a
tor’s output pared against a setpoint fixed-gain amplifier followed by a passive at-
voltage to produce an error signal, which is tenuator.
then integrated to produce a gain control volt- An IVGA is the preferred choice for a re-
Fig. 1 VGA-based AGC age. This is applied to the control input of the ceive AGC system because the available out-
loop block diagram. ▼ VGA. The attenuator shown be-
tween the VGA and the detec-
VGA (LINEAR IN dB)
SIGNAL tor is used to align the maxi- This article is intended
OUTPUT mum output level of the VGA
SIGNAL
INPUT with the maximum input level to provide insight into the
of the detector. effective operation of variable