Parametric Diode Figure of Merit and Optimization

The noise figure and gain expressions for the diode parametric amplifier are presented and discussed which include the effects of diode losses. From these relationships, a new diode figure of merit is defined as follows: fD=an/ 4πa02C0RD, where an is the normalized Fourier coefficient of the time dependent (pumped) capacitance; C0, the diode capacitance at the operating bias point; and RD, the equivalent series diode resistance. It is shown that this figure of merit is directly applicable in comparing diodes of various types as well as in optimizing their respective design for low-noise amplifier use. Further, it is indicated how this figure of merit can be directly employed by the circuit designer to predict an amplifiers' noise figure performance including the effects of choice of bias, pump swing, and signal and idle frequency for a given diode. A complete evaluation of the proposed figure of merit is made for the abrupt junction diode by expressing fD in terms of device design parameters and examining as a function of bias and base doping. It is concluded, if no constraints are considered, that the maximum value of fD is obtained for a bias approximately equal to half the breakdown voltage with full pump swing and that fD(max) increases with decreasing base doping from 1018 to 3·1015 atoms/cc. It is further concluded that the maximum possible value of a1/a02 for the inverse square root of voltage capacitance law is approximately 0.557 such that the greatest value fD(max) can attain is 20% of fc, the diode cutoff frequency. Some possible limitations on this evaluation are discussed including the effects of diode deviation from model, pump power or diode dissipation restrictions, nonsinusoidal pumping waveform, and ambient temperature.