109G THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



Section 5.2. For constant voltage, variable current loads, transformer 

 coupled feedback with the output load in series with the feedback loop 

 results in low power dissipation, relatively small degree of stability 

 variations versus output current variations, and non-critical com- 

 ponents. The possible limitations are that transformers generally are 

 more expensive than other passive components and are not as readily 

 available in a variety of stock values. 



4. INPUT TRIGGER CIRCUITS 



The primary function of the input trigger circuit is to initiate the tran- 

 sition from the "off" to the "on" state when there is an input signal. At 

 all other times the input circuit must provide a threshold or margin 

 against false triggering due to noise or spurious disturbances. 



Although the input circuit must supply sufficient energy to establish 

 regeneration, it is unnecessary and undesirable that any additional energy 

 be supplied. To do so reduces the gain of the amplifier, since gain may be 

 defined as the ratio of the output power to the input power during one 

 cycle of operation. Because regeneration makes the input and output 

 power independent of each other, any reduction in input power results 

 in greater amplifier gain. 



In an amplifier with external feedback coupling it is possible, but not 

 always practical, to have the input circuit trigger the transistor at the 

 collector, base, or emitter terminal. The collector terminal seldom is 

 selected because then the input circuit must supply energy to the output 

 load as well as to the transistor. Also, the base is usually not used (ex- 

 cept occasionally with negative resistance feedback) because extra com- 

 ponents are required to steer the triggering energy into the transistor 

 and it is diffiult to apply a timing signal.* However, the following dis- 

 cussion and the dc input characteristic of Fig. 7 (a) are equally valid for 

 triggering at the base or emitter terminal of junction or point contact 

 transistors which are short-circuit stable. 



One of the simplest types of triggering circuits is shown in Fig. 7 (b). 

 The voltage and current increments assumed necessary to initiate regen- 

 eration are designated Vt and /, . Therefore, the required input signal 

 voltage Vs and current /« are : 



Vs ^ V, + IJix (1) 



^ Ri\ , Vt , Fi - V2 



'■^'■['+m) + w. + '^^ '■■'^ 



* Also, for junction tninsistors, about twice as much energy is required to trig- 

 ger at the base as at the emitter.' 



