TEANSlfcJTOKtf AND JUNCTION DIODES 



843 



a consequent increase in voltage drop in the legulating resistor, and a 

 decrease in the load voltage. The collecting process continues until the 

 load voltage returns to the regulated \'alue, and takes only a small 

 fraction of a second. The process is essentially the same for a decrease 

 in load voltage, exc(>pt that the base to emitter potential decreases, the 

 collector current decreases, the ^'oltage drop across the regulating re- 

 sistor decreases and the load \'oltage rises to the n^gulated value. 



The value of the regulated output voltage is determined by the ad- 

 justment of the potentiometer. Of course in a practical shunt regulator 

 circuit, the adjustable range of the potentiometer would have to be 

 limited to correspond with the operating range of the transistor. The 

 maximum allowable positive potential between the base and the emitter 

 is limited l\v the safe value of the maximum collector current. The maxi- 

 mum allowable negative potential between the base and the emitter is 

 limited by the saturation voltage of the emitter rectifier. 



The accuracy of this shunt regulator circuit is restricted by the 

 slope of the characteristic curves for the reference voltage diode. All of 

 the changes in base and collector currents required for regulation flow 

 through this diode and cause changes in the saturation voltage. The ad- 

 dition of the transistor does not increase the accuracy of regulation 

 but only allows adjustment of the regulated output potential to a value 

 which is greater than the standard potential. However additional stages 

 of transistor current amplification minimize the reference potential 

 changes b}' restricting the range of current excursions through the diode. 

 An example of a multistage shunt regulating circuit is given in Fig. 13. 



REGULATING 

 RESISTOR 



: — VA- 



UNREGULATED 

 INPUT VOLTS 



p-n-p 



TRANSISTOR 



SECOND 



CURRENT 



AMPLIFIER 



(INTERMEDIATE SIZE) 



n-p-n 



TRANSISTOR 



SHUNT 



AMPLIFIER 



(large Size) 



n-p-n 



TRANSISTOR! 



FIRST 



CURRENT 



AMPLIFIER 



(Small size) 



Fig. 13 — Transistor sliunt regulator using three transistors. 



