908 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



put current between the load impedance and the internal plate impedance 

 of the upper triode, is very nearly the transconductance of the lower 

 triode. Since no current is lost to a screen, this is higher than that of a 

 pentode of the same spacing. The output impedance of the upper triode 

 is multiplied by the local feedback consequent on its cathode being off 

 ground by the plate impedance of the lower triode. Since the load pre- 

 sented to the lower triode is a low impedance, approaching the reciprocal 

 of the transconductance of the upper triode, the grid-plate capacity of 

 the lower triode is not enhanced by feedback. The higher value of trans- 

 conductance means less grid drive for the same output current, and 

 more obtainable feedback, both contributing to superior modulation 

 performance. 



The modulation of the upper triode, and the changes in transcon- 

 ductance of this tube, are suppressed by approximately the sum, in db, 

 of the local and loop feedback. In consequence, the modulation and mu- 

 beta effect contributions of this tube to the complete amplifier are 

 negligibly small. 



It will be observed that the grid of the upper triode is not connected 

 to ground, but to the top of the beta circuit. In consequence, the grid- 

 plate capacity of the upper triode appears as part of the end-capacity of 

 the coupling network rather than as a parasitic capacity to ground. This 

 results in a gentle potentiometer term in the output amplifier. This 

 connection, however, also has the effect of vitiating to some extent the 

 desirable qualities of the circuit, particularly at the higher frequencies, 

 where the grid-cathode capacity of the upper triode becomes important. 



Because of this gentle output potentiometer term, it is not necessary 

 to tune the interstage of the output amplifier, which consists simply of 

 the circuit capacity plus a network which has the characteristics of a 

 10-mmf capacity in the transmitted band. Above the band this network 

 shapes the gain and phase characteristics of the feedback loop to obtain 

 the desired stability margins. The incorporation of this network reduces 

 the in-band feedback by about 3 db, a sacrifice which unfortunately is 

 necesary to assure stabihty when the thermistor in the regulating net- 

 work is at its minimum value. For this value of thermistor, the phase 

 and magnitude relations of the regulating network impedance and the 

 grid to cathode capacity of VT3 produce a potentiometer term in the 

 feedback loop which appreciably reduces the margins around 30 mc. The 

 stability margins for the mid-range value of thermistor would be satis- 

 factory without this sacrifice of in-band feedback. When the thermistor 

 is at maximum resistance, some degradation of phase margin at 10 mc 

 occurs, again because of the potentiometer term effect mentioned above, 



