840 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



50 MILLIWATT 2 WATT 



Fig. 10 — Typical junction transistors. 



Thus, note that there are two kinds of transistors with respect to the 

 polarity of the electrodes. The n-p-n transistor operates with positive 

 collector potential and the p-n-p requires negative potential on the col- 

 lector. Both will amplify current changes in the base circuit into much 

 larger current changes in the collector circuit. The transistors have simi- 

 lar equivalent circuits and parameters but all of their operating poten- 

 tials and currents are reversed. It is also significant that the normal 

 direction of current flow is out of the base terminal of the p-n-p transistor 

 and that the normal direction of current flow is into the collector ter- 

 minal of the n-p-n transistor. Likewise, the normal direction of current 

 flow is out of the collector terminal of the p-n-p transistor and into the 

 base terminal of the n-p-n transistor. This relationship between direction 

 of current flow in n-p-n and p-n-p transistors is cafled reversed or com- 

 plementary symmetry, and enables the circuit designer to cascade direct 

 coupled transistors, alternating n-p-n and p-n-p. This is not possible 

 with vacuum tubes because there is no tube that will operate \\ath 

 negative plate potential. It ^\nll be sho^\^l how this complementary sym- 

 metry can be used to advantage in multistage direct current amplifier 

 circuits. 



3. TYPICAL REGULATING CIRCUITS 



3.1. Shunt Regulators 



3.11. Simple Diode Regulator 



If a load is connected to a source of power, the current through the 

 load and thus the voltage drop across the load will depend on the po- 

 tential of the source of power, the internal impedance of the power supply 

 and the load impedance. The voltage drop across the load can be made 

 very nearly independent of these three parameters by employing a cir- 

 cuit known as a shunt regulator. 



