Electronic Design

Variable-Gain Amplifiers: A Look At The Larger Picture

Programmable-gain amplifiers (PGAs) are actually a subset of variable-gain amplifiers (VGAs). Widely used in wireless communications, industrial scanning, radar, ultrasound, and speech-analysis applications that require a wide dynamic range of a continuous voltage, VGAs have much more daunting design challenges than PGAs. As such, modern VGAs make use of analog circuit techniques like variable-voltage attenuation, multiplication, and gain interpolation.

Impressive performance gains are constantly being achieved with VGAs. Among the leaders in this field are Analog Devices, National Semiconductor, and Texas Instruments/Burr-Brown. Notable VGA ICs include Analog Devices' AD83xx family, Texas Instruments/Burr-Brown's VCA8613, and National Semiconductor's CLC5526.

Analog Devices' 120-MHz AD8332 features extremely low-noise performance. Its dual-channel front end is rated for a noise level of just 0.74 nV/√Hz and current noise of a mere 2.5 pA/√Hz. Like devices in Analog Devices' AD83xx family, it's based on the company's innovative X-Amp architecture, which originated about 10 years ago in the firm's AD600/602 VGAs. The architecture consists of a resistive ladder network and highly linear amplifier and interpolator circuits. This arrangement enables linear-in-dB gain control that's essentially independent of temperature. Analog Devices is developing the next-generation Z-Amp architecture for VGAs that can operate at up to 4 GHz, which features even better noise and gain performance.

Texas Instruments/Burr-Brown's VCA8613 is a highly integrated VGA IC with eight voltage-controlled attenuators, eight PGAs, and eight output filters. Texas Instruments/Burr-Brown says the 3-V IC is the lowest-power dissipating device available at just 600 mW (75 mW/channel). The 5-MHz VGA features low noise of 1.2 nV/√Hz.

And, National Semiconductor's CLC5526 is a 350-MHz VGA IC. It features differential inputs and outputs for large signal swings from a 5-V power-supply rail, and it's digitally controlled.

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