1098 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



The single terminal AND-type circuit^ ■ ^° Fig. 7 (c) has the desirable 

 characteristics of the previous circuit, and is relatively insensitive to 

 input signal variations. In this circuit the input pulse switches the cur- 

 rent through R3 into the transistor input and then encounters the rela- 

 tively high resistance R2, as compared to the parallel resistance of R2 

 and El in Fig. 7 (b). The blocking action of D2 thus reduces variations in 

 the input signal current. However, R2, R3, F3 and V2 cannot be increased 

 without limit to reduce the variations; the dc power dissipated in R2 

 and R3 would become excessive. 



Another advantage of the AND-type circuit is that several inputs may 

 be paralleled with a common R3 to provide an AND logic function as 

 well as an input trigger function. This feature, when desired, saves com- 

 ponents and does not reduce the gain of the amplifier. 



When both the input circuit and the feedback circuit terminate at the 

 same transistor input terminal, as is usually the case, some additional 

 components are generally required to prevent one circuit from shunting 

 the other circuit. To steer the trigger current into the transistor, a diode 

 may be placed in the feedback path so that the diode is reverse biased 

 except when there is feedback current. Similarly, a diode or a resistor 

 may be placed in the input circuit so as to prevent the feedback current 

 from flowing into the input circuit.* 



Although the discussion has assumed positive polarity input pulses, 

 the remarks apply equally well to negative pulses if the polarity of the 

 diodes and the supply voltages are reversed. 



It is recognized that the preceding remarks assume that the minimum 

 triggering energy is known and that a step function of current or voltage 

 is the optimum form of the triggering energy. Actually, until a study is 

 made of the circuit and transistor parameters (including the non-linear 

 aspects) that affect the initial triggering before the feedback is estab- 

 lished, the design of an optimum input trigger circuit will remain an 

 experimental art. Experience with the AND-type input circuit has indi- 

 cated that appreciably more current is required to trigger junction luiits 

 than point contact units. 



5. OUTPUT COUPLING CIRCUITS 



In addition to the obvious function of efficient power transfer from 

 the amplifier to a load, the output coupling circuit is a convenient point 

 at which to perform other functions, as for example, dc level restoration 



i 



* This precaution is not necessary if the transistor input exhibits appreciablr 

 negative resistance. 



