A NEW MICROWAVE TRIODE 



545 



Fig. 20 illustrates this point. The low-level gain of this tube was 12.3 

 db but when the tube was driven so as to have an output power of 400 

 mw the gain was only about 3 db. At this point, retuning the circuit to 

 rematch the tube at the high output level increased the gain to about 5 

 db. Now, returning to low level, the gain was only 10 db. Presumably 

 in between these two points, 5 db at 500 mw output and 12.3 db at less 

 than one milliwatt output, the performance could have been better than 

 either of these two curves shows, i.e., the performance could have been 

 improved by rematching at each intermediate power level. 



100 150 200 250 300 350 400 450 500 

 POWER OUTPUT IN MILLIWATTS 



Fig. 20. — "Compression" in a one-stage microwave amplifier Ip = 30 ma. 



This tube is not representative of all of the tubes tested. It is rather 

 poorer in the spread of the two curves than most. It was picked merely 

 to illustrate that besides a drop in gain also a detuning effect takes place 

 when the driving power is changed. In the example given here the cathode 

 current was held at or near 30 ma by the stabilizing bias circuit. 



Without the stabilizing circuit, these so-called "compression" curves 

 would be quite different. For instance, if the bias were held constant, we 

 should expect that the gain would not drop as fast as indicated here, 

 since the plate current would rise as the drive was increased. 



At any rate, in an F.M. system, we are not concerned with how much 



