Variable gain amplifier

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.