Noise induced gain saturation in the ultrasonic traveling wave amplifier

A relationship has been established between departure from small signal theory gain, the onset of nonohmic I-V characteristics, and gain saturation on broadband acoustic noise flux in the CdS ultrasonic amplifier. Noise figure measurements in material whose experimental gain behaves according to small signal theory have allowed evaluation of the level of acoustic flux at the amplifier output plane when noise saturation occurs. Extrapolation from measured points yields a relationship between the acoustic flux saturation level and the material carrier concentration. Measurements also indicate that, even for noise flux levels approaching saturation, signal saturation occurs for bunching parameters slightly greater than 1. Second harmonic distortion is more than 20 decibels down in this condition. The interrelationships between signal power levels and noise flux levels are examined for their effect on an amplifier's maximum possible gain and dynamic range. In particular, maximum possible ultrasonic gain is found to be limited to approximately 80 decibels.