Low-noise microwave reactance amplifiers with large gain-bandwidth products

Basically, there are two modes in which reactance (parametric) amplifiers can be operated. In one mode, there is no negative conductance associated with the terminal impedances of the amplifier, and consequently absolute stability results. However, the maximum available power gain for this mode of operation is limited to the ratio of the output signal frequency (the sum of the input signal frequency and the pump frequency) to the input signal frequency. In the second basic mode of operation, the terminal impedances of the amplifier contain negative conductance components. Consequently, the amplifier is potentially unstable but can give large gain at the expense of reduced bandwidth. In designing low-noise receiving systems for microwave frequencies, one is led to consider the use of the negative conductance type of reactance amplifier in order to obtain sufficient gain so that the overall system noise factor is determined by the low-noise reactance amplifier stage. Until recently, however, the values of gain-bandwidth products achieved in practice with this type of amplifier have been disappointingly low. A theory is presented in which voltage gain-fractional bandwidth products for two negative conductance types of reactance amplifiers are analyzed in terms of pertinent nonlinear-capacitance diode and circuit parameters. Low-frequency prototype models of these types of reactance amplifiers were designed in accordance with the theory, and the measured operating characteristics were found to be in excellent agreement with the large values predicted by theory. At microwave frequencies, gain-bandwidth products substantially lower than those obtained at low frequencies may result because of the effects of diode lead inductance and the selectivity of the associated microwave circuits. Designs for minimizing these effects are discussed, and experimental results obtained at microwave frequencies are presented. Designing amplifiers in accordance with the theory also provides a nearly optimum noise figure.