The first varactor parametric amplifier, invented at Bell Labs in the late 1950s. Using 4 varactor diodes, it achieved 10 dB gain at 400 MHz. Parametric amplifiers are used as extremely low noise microwave amplifier stages in radio telescopes, spacecraft communication antennas, and military communications. The input microwave signal is amplified in a semiconductor varactor diode by applying a "pump" microwave signal at another frequency from a local oscillator.