DYNAMIC MEASUREMENTS ON ELECTROMA(JM IK Dl \ K i - 1451 



like those of the amphfier just described. Similar coiisideriitioiis for the 

 overall design were used here and the looj) jrain culolT control uscss tlie 

 same principles, modified to take account of the inductor load and its 

 constant current feedback eciuilization. 



The output tube is a GLO with an 874 voltage regulator tul)e in the 

 cathode return to the —250 volts. The GL6 is operated at 40 miUiam- 

 peres, which is within the current rating of the 874. This stage wiis 

 designed for the widest possible output voltage swing and for plate cur- 

 rent cutoff independent of plate voltage, both characteristic of pentodes. 



The transformer has a 900-cycle self resonance with its ecjuivalenl 

 shunt capacitance. At higher frequencies it behaves as a capacitor as far 

 as the tube and loop gain are concerned. Its Q is unity at 50 cycles. 



The function of the output amplifier is to convert the input voltage 

 signal into proportional current in the inductor. Constant voltage gain 

 would not do this. For frequencies above 56 cycles, the current would 

 drop at a 6 db per octave rate with increasing frequency, because of the 

 reactance. 



Note that if the voltage across the inductor rises with increasing fre- 

 quency, then the current through the winding will be proportional to a 

 constant input voltage regardless of the equivalent shunt capacitor. The 

 capacitor will draw greatly increased current but as long as it is uni- 

 formly distributed, it will not affect the winding current itself. If series 

 output feedback were used, the tw^o currents could not be separated and 

 a more complex equalizer would be necessary to provide the necessary 

 frequency response. The choice of the value of inductance for the output 

 transformer will be discussed after errors in the system have been con- 

 sidered. 



The rising gain is produced by shaping the feedback, starting an inser- 

 tion gain rise at 56 cycles with the 0.057-m/ capacitor, and continuing to 

 10 kc. At this frequency the rising characteristic is discontinued by the 

 280-ohm series resistor, and the capacitors shunting the feedback re- 

 sistors. Over the rising characteristic range the equilization introduces 

 90° adverse loop phase shift and above 900 cycles the output trans- 

 former adds another 90° phase shift. Thus from 900 cycles to 10 kc the 

 amplifier just skirts being Nyquist stable.^ 



The internal gain of this amplifier is 80 db. At dc the external gain is 

 21 db. It rises to 67 db at 10 kc because of the feedback equilization. 



Output Amplifier Square Wave Response 



The square wave response of the output amplifier is shown in Fig. 20. 

 Two effects are evident (1) a finite time for the major change to occur, 



