Low-Frequency Parametric Amplifier for Small Voltage Measurements

This paper analyzes the low-frequency parametric amplifier, which, in opposition to the parametric amplifiers in the high-frequency range, is wide band and can be realized only simultaneously as an upper and lower sideband up-converter. Three derived basic equations give the possibility of drawing equivalent circuits, in order to calculate gain and stability conditions. The influence of detuning on characteristics of amplifiers is analyzed and investigated experimentally. Cascading more stages of parametric amplifiers is meaningless. The best results can be obtained when the second stage is an upper sideband up-converter. LF parametric amplifiers enable us to achieve a low noise (<0.3 μV for 100-Hz bandwidth), free from flicker noise, a high input resistance and a high common-mode rejection ratio. For this reason, the amplifier can be applied as the measuring preamplifier of brain voltages.