Patent GB-592402-A (GB)
Title: Improvements in apparatus for amplifying, generating, or changing the frequency of electric oscillations of ultra-high frequency
IPC: H03F3/52
Filed: 1944-01-31
Abstract: 592,402. Thermionic amplification. WESTERN ELECTRIC. CO., Inc. Jan. 31, 1944, No. 1817.. Convention. date, Sept. 27, 1941. (Classes 40 (iii) and 40 (v)] In an ultra-high-frequency amplifier, frequency-changer or oscillation-generator, a thermionic valve is enclosed in a loop of shielded transmission line whose dimensions are appropriate to the frequency. In a grounded-grid valve, the anode current in the input circuit constitutes negative feedback, and this is neutralized to 'the desired extent by positive feedback applied through a retarding-line of the requisite electrical length and characteristic impedance, the load being tapped off at such a point that variations in it do not affect the positive feedback. In the amplifier shown in Fig. 2, the grid is a perforated septum 6 across the outer conductor of a coaxial pair while the cathode 5 and anode 7 are at the ends of two inner conductors, glass or other seals 23 being provided while the heater leads 31 are located inside the inner conductor. Positive feedback from the anode 7 to the cathode 5 goes round the complete loop of coaxial conductor, and if this is one wavelength long and of uniform characteristic impedance, the inherent negative feedback is neutralized. The input connection 4 is half a wavelength away from the valve, and the output connection 9 is a quarter of a wavelength away so that the feedbaek is unaffected by variations in the load. Capacitance between the grid 6 and anode 7 may be neutralized by providing an inductance which is anti-resonant with it; for this purpose a lumped capacitance is located at the end of the quarter-wave section 20 and is consequently inverted into an inductance at the anode. Capacitance between the cathode 5 and grid 6 is similarly neutralized by an inductive stub 25 at the end of the half-wavelength section 22. This stub provides an entry for the heater leads 31 and a connection for a grid-biassing resistance 36. A high-frequency short-circuit with direct-current insulation is provided by a piston 32 and a metallic sleeve 33 outside an insulating sleeve 34. Anode voltage is supplied through a stub 39 with a quarter-wave capacitive shunt 41. The output line 9 is insulated for direct current by a capacitive high-frequency connection 42. The inherent negative feedback may be over-compensated, to allow for the effects of input loss and grid current, by enlarging the diameter of the inner conductor of section 21 or section 22 so as to diminish its impedance as compared with that of section 20. The output line 9 may constitute the input line of a second stage, Fig. 3 (not shown), in a mmlti-stage. amplifier. An oscillation generator may be constructed, Fig. 4 (not shown), by omitting the input connection 4 and enlarging the diameter of the inner conductor of section 21, the glass seals 23 being replaced, by a single seal in the output connection 9 where the radio-frequency potential is relatively low. In a frequency-converter or modulator, mixed frequencies, such as those of a signal and a local oscillator, are supplied through the input connection 4, Fig. 5, and the desired frequency is selected from among the modulation products by suitably locating the short-circuiting piston 41 in the stub 39.