Distributed power amplifiers

The potential of traveling-wave or distributed amplification for obtaining power gains over very wide frequency bands has been recognized yet in the mid-1930s when it was found that the gain-bandwidth performance is greatly affected by the capacitance and transconductance of the conventional vacuum tube [1]. However, the first theoretical analysis and its practical verification were obtained for very broadband vacuum-tube amplifiers more than a decade later [2,3]. The basic concept was based on the idea to combine the interelectrode capacitances of the amplifying vacuum tubes with series wire inductors to form two lumped-element artificial transmission lines coupled by the tube transconductances. As a result, the distributed amplifier overcomes the difficulty of a conventional amplifier, whose frequency limit is determined by the factor which is proportional to the ratio of the transconductance of the tube to the square root of the product of its input grid- cathode and output anode-cathode capacitances, by paralleling the tubes in a special way, in which the capacitances of the tubes can be separated, while the transconductances may be added almost without limit and not affect the input and output of the device. Since the grid-cathode and anode-cathode capacitances form part of low-pass filters which can be made to have a substantially uniform response up to filter cutoff frequencies, whose value can be conveniently set within a wide range by suitable choice of the values of the external inductor coils, it became possible to provide amplification over much wider bandwidths than was achievable with conventional amplifiers.