TRANSISTORS AND JUNCTION DIODES 



849 



a combination of the two methods is also feasible. Additional magnetic 

 amplifiers have the disadvantage of adding time delay. Transistor action 

 likewise is not instantaneous because it takes a finite amount of time 

 to move the charge over a finite distance in the crystal lattice. However 

 transistor action is much faster than the time required to change the 

 current in practical magnetic amplifiers. 



3.34. Current Regulators 



Fig. 17 shows a simple transistor control circuit to obtain constant 

 ciu"rent regulation with a magnetic amplifier regulated rectifier. The 

 operation of this circuit is similar to Fig. 15 and its description will not 

 be repeated. 



3.35. Temperature Effects 



One limitation of the foregoing transistor regulating circuits is the 

 sensitivity of collector current to ambient temperature variations. The 

 collector current increases ^^^th increasing temperature even if the base- 

 to-emitter bias is held constant. This is the result of three factors. (1) 7co , 

 the uncontrolled portion of Ic increases greatly as covered in Section 

 2.34; (2) the emitter resistance (r^) decreases causing /& to increase, and 

 (3) alpha changes. The effect of the temperature sensitivity of the col- 

 lector can be greatly reduced by using a differential or push-pull circuit 

 of the type illustrated in Fig. 18. 



3.36. "Push-Pull" DC Amplifier 



The pu.sh-puU circuit uses two emitter-coupled n-p-n transistors and 

 is in many respects similar to a cathode-coupled vacuum tube amplifier. 



AC 



LINE 



WINDING 



MAGNETIC AMPLIFIER 



I 



MINUS 



Fig. 18 — Push-pull transistor control circuit for a magnetic amplifier regulated 

 rectifier with constant voltage regulation. 



