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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



voltage, and thus regulating it. The value of the regulated output 

 voltage is again determined by the adjustment of the potentiometer. 

 The ohmic value of the R^ resistor in Fig. 14 is selected to keep the cur- 

 rent flowing through the reference voltage diode in its saturation voltage 

 region. 



Fig. 14 is the simplest form of a transistor series regulator circuit. It 

 requires two transistors whereas the most simple form of a transistor 

 shunt regulator shown (Fig. 12) reciuires only one transistor. But the 

 added current gain of the second transistor in Fig. 14 results in better 

 regulation than can be obtained with Fig. 12. If desired the circuit in 

 Fig. 14 can be modified to change the series transistor to the negative 

 output lead by using the complementary p-n-p first current amplifier 

 and an n-p-n series transistor. This illustrates another advantage of the 

 complementary symmetry of the two types of transistors. Also, if more 

 gain is rec^uired, additional transistor stages can be used employing the 

 principles outlined above. 



3.22. Series Current Regulator 



The circuits covered so far regulate for constant output voltage. 

 Similar transistor regulator circuits can be developed which will regulate 

 for constant output current. One of these is shown in Fig. 15. In this 

 circuit the load current produces a voltage drop across the regulating 

 resistance and, in the n-p-n transistor, this voltage drop is compared to 

 the reference voltage. The difference between these two potentials 

 controls the n-p-n transistor base current and this base current is am- 

 plified by the current gain of both transistors to control the load current. 



REGULATING 

 RESISTOR 



I LOAD 



REGULATED 

 OUTPUT 

 CURRENT 



Fig. 15 — Transistor series regulator constant current regulation. 



