TRANSMISSION-LINE AMPLIFIERS

A thermionic vacuum-tube power amplifier is analyzed in which an extended and continuous grid and cathode serve,as an input transmission line that excites a corresponding traveling-wave signal on a similar transmission line between the screen grid and anode. The bandwidth is extremely wide because the usual gain-bandwidth product limitation is avoided by making the input and output capacitances part of the transmission lines. The gain mechanism is the same as that in the usual power tetrode in that bunched charges are delivered (or pumped) to the anode at the proper time and location along the anode transmission line to enhance the traveling-wave signal. If line attenuation losses are small, the power gain increases as the square of line length. Part of the backward wave can become a forward wave by tapering the impedance of the output line. A potential possibility is that this type of amplifier can employ a field-emission cathode and thus avoid the high heater power as well as the slow transit time between the cathode and grid of the thermionic emitter. Disadvantages of this type of amplifier are low gain and low input impedance.