MICROWAVE REPEATER RESEARCH 



229 



figure consisting of two of these experimental grounded grid tubes in tandem. 

 The circuits were designed so that there is a mismatch at the input of each 

 tube. The overall noise figure is 4.0 db with the amplifier connected be- 

 tween a 75-ohm generator and a 75-ohm load; the gain is I7f db; and the 

 bandwidth at the 0.1 db down points is about 13.5 megacycles. 



Main IF Amplifier — In order to obtain the required output power over 

 the desired bandwidth it was necessary, at the beginning of this work, to 

 use the WE 367-A tube in the output stage. Since the gain of a stage using 

 this tube is very low, it must be driven by a tube similar to the 6AG7 and 

 to prevent compression in this latter tube a special high gain, triple-tuned 

 interstage transformer was designed, with a relatively low coupling coef- 

 ficient. 



Methods of paralleling tubes to get more power output or the same power 

 output with a much wider bandwidth have been worked out, but recent 



^f^-- 



Fig. V-8. — Schematic of a "low noise figure" preamplifier. 



developments in power output tubes and in transmitting converters have 

 made them unnecessary, except for appHcations requiring very wide band- 

 widths. 



Automatic gain control can be applied to the IF amplifiers by controlling 

 the grid bias of the various stages. However, it is not advisable to apply 

 the gain control bias to either of those stages which are preceded by the 

 special high-gain transformers. In these stages the slight changes in input 

 impedance of the tubes caused by variations in the grid bias would be suf- 

 ficient to alter significantly the band-pass characteristics of the transformer. 



Figure V-9 shows an amplifier with about 55 db gain developed for the 

 New York-Boston circuit.* Automatic gain control bias is applied to the 

 grids of the first three stages which employ wide band (K = 0.7) matched 

 interstage transformers. 



It will be noted that the first interstage transformer of the preamplifier 

 and the last interstage transformer of the main IF amplifier require low 

 coupHng coefficients to obtain the gain desired at these points. For this 



* Developed by A. L. Hopper and B. C. Bellows, Jr. 



