Low-frequency reactance amplifier

A reactance amplifier has been constructed which is capable of low-noise amplification at audio and subaudio frequencies [1]. Stable voltage amplification of 20-30 db and noise figures as low as 0.16 db (50-K source resistance) have been observed for signal frequencies between 2 and 50 cps. The circuit configuration is a double sideband up-converter or modulator in which a band of frequencies from dc to several hundred cycles is used to modulate a 150-kc carrier. An analysis for noise and gain is presented which considers the average, fundamental, and second-harmonic components of the time-varying diode capacitance. The amplifier input is a signal at low audio frequency, while the output is composed of the sum and difference sidebands of the pump. Gain is realized through up-converter action associated with the frequency shift from signal frequency to near pump frequency; the sum and difference sidebands are further amplified in the output tank by the negative resistance resulting from the action of the component of time-varying capacitance at the second harmonic of the pump frequency. Gain and equivalent input noise resistance are discussed as functions of detuning in the passive output tank; measured and calculated results are compared. A scheme is also presented for stabilizing the amplifier at high values of negative resistance gain.