variablegain

Variable gain refers to the ability of a system to change its amplification factor in response to external control. In electronics, this capability is most commonly implemented with variable-gain amplifiers (VGAs), whose gain can be adjusted by an external control signal. Gains are typically expressed in decibels and can cover wide ranges depending on the band and technology involved. Variable gain enables systems to maintain signal levels within a desired range despite large input variations.

VGAs are implemented through several architectures. Common approaches include attenuator-based designs that use PIN diodes or MESFETs to vary attenuation, transconductance-based VGAs that adjust input transconductance, and mixed architectures that combine fixed gain blocks with controllable attenuators. Control signals can be analog voltages or currents for continuous adjustment or digital codes for stepped gain changes. Some designs prioritize wide dynamic range with logarithmic or exponential control laws to preserve resolution across the range.

Key performance characteristics of VGAs include gain range and gain flatness across frequency, noise figure, linearity metrics such as third-order intercept (IIP3) and input/output compression points, settling time, and temperature stability. The choice of technology (for example, GaAs, SiGe, or CMOS) and architecture affects bandwidth, noise, and linearity.

Applications span radio and wireless receivers (often as part of automatic gain control), radar and sonar systems, instrumentation and test equipment, and audio processing. In many systems, VGAs are combined with feedback loops to maintain constant output levels or to optimize dynamic range in changing conditions.