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variable gain amplifier


VGA Introduction

A variable gain amplifier (VGA) is a device having a control input that can vary the gain of the device. In the wireless communication industry, particularly for wireless communications, variable gain amplifiers are well known as being used to provide amplification of either intermediate frequency (IF) or radio frequency (RF) signals. Variable gain amplifiers are frequently used in modern radio receivers to amplify or attenuate incoming signals to properly drive an associated analog-to-digital converter (A/D). Typically, the variable gain is distributed among radio frequency (RF), intermediate frequency (IF), and low-frequency or baseband circuits. Radio receivers, or tuners, are widely used in applications requiring the reception of electromagnetic energy. Applications can include broadcast receivers such as radio and television, set top boxes for cable television, receivers in local area networks, test and measurement equipment, radar receivers, air traffic control receivers, and microwave communication links among others. Transmission of the electromagnetic energy may be wirelined over a communication media or wireless by electromagnetic radio waves. In a radio frequency (RF) transceiver, the received signal typically has a high dynamic range. In order to supply a signal of constant amplitude to a baseband section of the transceiver, a variable gain amplifier (VGA) with equivalent or better dynamic range is required. In a variable gain amplifier, a control unit will provide a gain signal to the variable gain amplifier, and, based upon the gain signal, the variable gain amplifier will accordingly amplify an input signal by an amount corresponding to the gain signal, to obtain an amplifier output signal. In order for a signal of a constant level to be supplied to a base band terminal of the received signal, the variable gain amplifier must also have a high dynamic range. Variable gain amplifiers may be based on voltage, current or charge. Voltage mode amplifiers are probably the most widely used. Examples of such include complex circuits where the amplification is provided by discrete transconductance stages. Charge mode amplifiers are one alternative. However, such a circuit utilizes a discrete time technique that is not suitable for high-speed operation. In contrast, current mode amplifiers are less constrained by reduced power supplies and are able to operate at very high speeds.

Variable gain amplifiers (VGA) are required for radio communicated systems, and the VGA with exponential
gain control characteristics is preferred for its large dynamic control range and control loop is settling time
independent of the absolute gain. With the increasing demand for lower dissipation, downsizing to mobile
communication systems, the VGA consisted of only CMOS process is needed for a system-on-a-chip LSI that
has various ability on a single chip. There are two main approaches for realizing an exponential function circuit
using MOSFETs. One approach is the use of an exponential characteristic of a MOSFET in the weak inversion
The other is that the square-law characteristics of MOSFET in the saturation region approximate an
exponential function Latter approach is suitable for high-speed applications.

Variable gain amplifiers have a wide range of applications in electronic
systems, especially in those requiring an automatic gain control loop. In communication
systems, they play an indispensable role in receivers by controlling the incoming signal?s
power level and normalizing the average amplitude of the signal to a reference value.
This helps in optimizing system capabilities and reducing the complexity of circuits
designed to extract the correct timing information and data at the receiving end.





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