842 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



3.12. Transistor-Diode Regular 



To examine the operation of shunt transistor regulators consider the 

 circuit shown in Fig. 12. In this, the transistor is shown by its stand- 

 ard convention where the upper slanting line represents the collector 

 rectifier shown in Fig. 7 and the lower slanting line with the arrow on it 

 represents the emitter rectifier. The direction of the arrow shows that, 

 in this transistor, current flows out of the emitter, so it is an n-p-n 

 transistor. The (c), (5) and (e) designations also help to locate the col- 

 lector, base and emitter. Notice that the emitter current, shown by the 

 arrow le , flows through the reference voltage diode in its reverse direc- 

 tion. The rectifier symbol with an adjacent "s" is a convention for this 

 diode. 



In Fig. 12 a portion of the load voltage is applied to the base of the 

 transistor by means of the adjustable potentiometer. The potential of 

 the emitter is held constant with respect to the negative output potential 

 by the saturation voltage of the diode. The base-to-emitter voltage is 

 thus ecjual to a proportion of the load voltage minus the saturation volt- 

 age. The potentiometer is adjusted so that the base potential is slightly 

 positive with respect to the emitter when the desired value of voltage 

 appears across the load. This value of load voltage is called the regulated 

 voltage. Current h then flows into the base, current Ic flows through the 

 regulating resistance, and h + Ic combine to form /« . Now assume that 

 the regulated voltage (E) increases by an amount AE. The base voltage 

 becomes more positive with respect to the negative terminal by the pro- 

 portion of AE developed across points 1 and 2 of the potentiometer. 

 Since the emitter potential is held constant by diode "s" and cannot 

 change, the net effect is to increase the base to emitter potential. This 

 change in base to emitter potential causes an increase in collector current. 



REGULATING 

 RESISTANCE 



AAV 



UNREGULATED 

 INPUT VOLTS 



minus! 



n-p-n 



TRANSISTOR 



SHUNT 



AMPLIFIER 



lb 



r^ 



ADJUSTABLE , 

 POTENTIOMETER 



REFERENCE 

 :: VOLTAGE 

 DIODE 



REGULATED 

 OUTPUT 

 VOLTS 





Fig. 12 — Transistor shunt ic^gulator using one transistor. 



