Variable-gain amplifiers (VGAs) can be used in may types of systems, such as radio receivers. In this system, the input signal voltage depends on an uncontrolled variable, such as distance to the transmitter. To ensure that the input signal amplitude matches the input voltage range of key components, such as ADCs and DACs, thereby maximizing the converter’s dynamic range, VGA can be used in this type of system.

Digitally Controlled High Frequency VGA1 circuit schematic

The bandwidth of VGA in the following figure has high bandwidth, ranging from 115MHz at high gain to 225MHz at low gain, so we can use this circuit in the RF-signal path without degrading the signal. The DAC in this circuit can be upgraded, but the level-shifter op amp limits the speed at which you can update the gain of the VGA to 2MHz. As configured, a calibration function with a 3MHz DAC update rate is implemented by this VGA. [Source: Intersil Application Note]