How do I select between a fixed gain amplifier and a variable gain amplifier for my receiver?

Selecting between a fixed-gain amplifier and a variable-gain amplifier (VGA) for a receiver depends on the input signal dynamic range, the system architecture, and the performance requirements for noise figure, linearity, and gain control resolution. Use a fixed-gain amplifier when: the input signal range is narrow (< 20 dB variation) and the downstream stages (mixer, ADC) can handle the resulting output range, the lowest noise figure is required (fixed-gain LNAs are optimized for minimum noise at a specific bias point; VGAs inherently compromise noise figure for gain variability), the design simplicity is important (no control circuitry, no AGC loop, no gain calibration), and the frequency is very high (> 30 GHz, where VGA options are limited). Use a VGA when: the input signal range is wide (> 30-40 dB variation) and the ADC or downstream stages have limited dynamic range, the receiver must handle both weak and strong signals simultaneously or sequentially without saturation or loss of sensitivity (cellular base stations, military receivers), the output level must be precisely controlled for consistent ADC operation (the ADC performs best when the signal fills a specific portion of its full-scale range), and the system requires programmable gain for different operating modes or standards. A common compromise is: a fixed-gain LNA for the front end (lowest noise figure) followed by a VGA or digital step attenuator (DSA) in the IF chain (provides gain control without degrading the receiver noise figure because the LNA gain suppresses the VGA's noise contribution).