Minimum Noise Figure of the Variable-Capacitance Amplifier

The variable-capacitance parametric amplifier is of interest primarily because it shows promise of very low noise amplification. However, variable capacitance diodes have a small but finite series resistance which limits the obtainable minimum noise figure; for the better diodes, the shunt conductance can be neglected. Taking the contribution of this resistance into account, Leenov 1 has discussed the noise figure of upper sideband up-converter, and Haus and Penfield 2 and others 3,4 ' 5 have discussed the lower sideband circulatortype amplifier. This paper discusses the lower sideband idler output 095 710 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , MAY 19(il amplifier and the degenerate amplifier, in addition to the amplifiers mentioned above, and compares them to one another. Further, we shall demonstrate the unique importance of the dynamic quality factor Q, as defined here, in characterizing the diode at a given temperature for noise figure considerations. The diode is assumed to be a series connection of a junction capacitance C(t), which is a periodic function of time, and a spreading resistance R a . Leenov, as well as Haus and Penfield, used the open-circuit assumption for the unwanted frequencies at the diode junction. However, there are a number of published papers 3,5 ' 6,7,8 in which the short-circuit assumption is used. We shall discuss both of these cases simultaneously, and show that the two assumptions give the same expressions for the noise figure, if the dynamic quality factor Q is redefined for each case.