llOO THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



the "on" state and a high impedance in the "off" state. Since C must be 

 relatively large to make the voltage drop across it small during the pulse 

 duration, R3 must be equal to or smaller than Rl for satisfactory restora- 

 tion (50% duty cycle assumed). But then the current transmission 

 efficiency of the coupling network is less than 50 per cent because gener- 

 ally Rl is smaller than the input resistance of the driven circuits during 

 the pulse duration. Unless the pulse length is only a small fraction of the 

 pulse repetition period, it is seldom possible to effect a suitable compro- 

 mise. Also, it might be noted that variations of Ico current, which flows 

 through R3, cause variations in the output pulse amplitude. Finally it 

 is not possible to obtain pulse inversion. 



A transformer coupled circuit. Fig. 8 (b), works efficiently with a 

 transistor. Diode D2 isolates the transformer from the load and inter- 

 lead stray capacitance during the interdigit period* so that the restora- 

 tion time of the transformer is controlled by the value of R3. The restora- 

 tion time is approximately proportional to the mutual inductance divided 

 by the total shunting resistance. Diode Dl prevents R3 from shunting 

 down the output during the pulse duration, thus permitting high output 

 efficiency and proper restoration. f 



As noted in Section 2, the maximum output power from the transistor 

 is determined by the maximum collector voltage (as set by breakdown 

 or punch-through) and the maximum collector current consistent with 

 the permissible dissipation in the transistor. Usually this maximum 

 voltage exceeds the desired amplifier output voltage and, occasionally, 

 the maximum collector current is insufficient; in such instances a voltage 

 step down is desirable. When the transistor is not required to operate at 

 maximum power dissipation, it often is advantageous to balance the 

 "off" and "on" power dissipation. An increase in the collector supply 

 voltage increases the "off" power and decreases the "on" power (by 

 decreasing the required collector current for the same output power). 

 Thus the collector voltage may be adjusted to give the lowest total power 

 dissipation consistent with the average duty cycle of the amplifier. The 

 transformer turns ratio is specified to match the optimum collector 

 voltage to the desired output voltage. Furthermore, Ico variations have 

 negligible effect on the output voltage amplitude and pulse inversion (if 

 desired, for example, for inhibition) is possible. For these reasons trans- 

 former coupling appears to give optimimi output coupling performance. 



* The minimum time interval between the end of one pulse and the beginning 

 of a succeeding pulse; for a 50 per cent duty cycle the interdigit period is equal to 

 the pulse duration. 



t Occasionally it is possible to specify the collector impedance, the transformer 

 losses, and the reverse impedance of D2 so that Dl and R3 are not necessary. 



