FPGA Based Optimum-Settling Automatic Gain Control Circuit Design for Multistage Amplification
No Thumbnail Available
Date
2018-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Many electronic applications require their input signal to be of constant amplitude. Especially, in
communication systems, the transmitted signal may be faded and the amplitude of the received
signal may be below the desired level due to multi-path fading and different positions. This thesis
work solves problems of inconsistent amplitude signal arrival at receiving sides of electronic
systems. Automatic Gain Control (AGC) circuits are mainly used in such applications to adjust
the gain of a Variable Gain Amplifier (VGA) and to provide relatively constant amplitude signal
to the receivers.
This work makes use of multi-stage amplification circuits to increase the dynamic range of the
AGC. Three amplification stages are designed which increased the gain from 21 dB in single stage
to 63 dB in three stages. Due to their flexibility better than ASICs as well as high performance
speed and better power efficiency than microcontrollers, FPGAs are preferred in modern high
speed technologies. Consequently, this thesis work used FPGAs to generate control voltages to
adjust the gain of VGAs in the multistage amplification circuit. The designed AGC employs both
feedback and feedforward loops to optimize the settling time. Feedback loop is used in input stage
and feedforward loop is used in intermediate and output stages. When the amplitude level of the
input signal diverges from the desired level, the FPGA generates a control voltage that adjusts the
gain of the VGA and the result is an output signal with consistent amplitude level. A reference
input of 25 mVpeak is considered and the AGC circuit is tested with input signals of amplitude
below and above the chosen reference value. For all tested inputs a constant amplitude signal of
87 mVpeak is achieved.
Description
Keywords
AGC, FPGA, VGA, feedback, feedforward, multistage, amplification