Relation between amplification and bandwidth of parametric travelling-wave amplifiers using varactor diodes

Abstract

The paper discusses parametric travelling-wave amplifiers whose wave-guide has the character of a band-pass filter. For two band-pass structures of practical importance which contain as lumped capacitances merely the basic capacitances of varactor diodes, the maximum product is estimated of the gain per diode at the mid-band frequency by the relative bandwidth that can be attained with parametric travelling-wave amplification. It is assumed that the mid-band frequency of the amplifying action equals half the pump frequency, that it is at the centre of the pass-band of the band-pass guide, and that the bandwidth of the amplifying action agrees with the bandwidth of the band-pass guide. It is further assumed that the amplitude of the controlled diode capacitance variation is 50% of the basic diode capacitance. A maximum of about 2 dB per diode results for the aforementioned product which we term “effectiveness”. Unlike the bandwidth-gain product of conventional amplifiers, it is thus for the here discussed parametric travelling-wave amplifiers no longer the absolute, but the relative bandwidth which is representative. Effectiveness values of about 0·5 to 1·4 dB per diode can be gathered from existing experimental results of the author and others.