Input Stage
Electronic amplification circuits positioned at the very front of a receiver chain must boost weak signals while adding minimal noise. Designing a low noise amplifier requires careful selection of transistor bias points and impedance matching networks to minimize the overall noise figure. Because this stage determines the sensitivity of the entire measurement or communication system, it is designed with high gain to suppress noise contributions from subsequent stages.
Impedance Matching
Achieving optimal noise performance requires matching the input impedance of the amplifier to the source impedance of the sensor. The low noise amplifier must balance the need for maximum power transfer with the need for minimum noise generation, which do not occur at the same impedance point. Engineers use specialized source pull measurements to find the optimum source reflection coefficient that yields the lowest noise figure.
Noise Factor
Quantifying the noise added by the circuit involves measuring the ratio of the output signal-to-noise ratio to the input signal-to-noise ratio. The low noise amplifier has a noise figure specified in decibels, which represents this ratio at a standard reference temperature of 290 kelvin. Thermal noise from resistors and shot noise from semiconductor junctions are the primary physical sources that must be managed to maintain high signal integrity.
In sub-kelvin applications, cryogenic cooling is sometimes applied to the amplifier to further reduce thermal noise and achieve extreme sensitivity levels.
System Optimization
Metrological calibration of the amplifier involves using a calibrated noise source to measure the gain and noise figure across the operating bandwidth. Adjusting the bias current of the low noise amplifier optimizes performance for the targeted frequency range.
