TRANSISTORS AND JUNCTION DIODES 



845 



drop across the regulating resistance whereas the series regulator wastes 

 only the voltage drop across the series device. At light load the powoi- 

 dissipated in the shunt current is usually greater than the power dissi- 

 pated in the series circuit. With a transistor used as the series regulator 

 device this difference in efficiency is more pronounced because of the 

 small collector voltage that can be used for the full load ciu-rent. This 

 collector voltage is the voltage drop across the series transistor as shown 

 in Fig. 14. 



3.21. Simple Series Regulator 



P'ig. 14 shows a simple transistor series regulator circuit. A p-n-p 

 transistor is shown connected so that all the load current must pass 

 through it. The comparison of the output voltage to the reference po- 

 tential in the current amplifier of Fig. 14 is accomplished by holding the 

 emitter at a constant potential with respect to the positive output ter- 

 minal. Note the difference between this method and that covered in the 

 previous section on the shunt regulator 3.12, where the emitter was held 

 at a constant potential ^^ith respect to the negative output terminal. 

 Now, when the output potential increases by an amount AE, the base 

 voltage becomes more negative A\dth respect to the positive terminal by the 

 proportion of AE developed across points 2 and 3 of the potentiometer. 

 Since the emitter cannot change with respect to the point of reference 

 (the positive terminal), the net effect is to decrease the base to emitter 

 potential and the collector current for an increase in output voltage. 

 The collector current decrease is amplified by the current gain of the 

 p-n-p series transistor to decrease the load current, reducing the output 



UNREGULATED 

 INPUT VOLTS 



P-n-p 



TRANSISTOR 



SERIES c 



AMPLIFIER 



(intermediate size) 



REFERENCE 

 k VOLTAGE 

 DIODE 



Ic 



ADJUSTABLE 

 POTENTIOMETER 



n-p-n " 



TRANSISTOR 



CURRENT AMPLIFIER 



WITH PHASE REVERSAL 



(SWALL size) 



I_ 



REGULATED 

 OUTPUT 

 VOLTS 





Fig. 14 — Transistor series regulator constant voltage regulation. 



