.S'().1/A; (7A'(77/" ASrF.CTS or THE TK.WSISTOR 



.V).^ 



one ()l)tains power ct'ticitMuics (oniparahlf to ( 'lass A flcclron tultt- (»|H'ratic)ii, 

 that is, 20 to .^5''^' cnu inu \- with a lew percent harmonic distortion. As 

 contrasted to recommendations lor good low-level j^ain for the Type A 

 transistor, the optimum conditions for power output have usually involved 

 lower load impedances and higher currents. Representative values may be: 

 load impedance, 5000 ohms; collector current, —8 milliami)eres at —35 

 volts bias; emitter current, 3 milliamperes; power output, 60 milliwatts, 

 with distortion less than ten percent. 



One complication of the power transistor is that, when the optimum load 

 imi)edance is low, the operating point gets nearer to the region where the 

 transistor may tend to oscillate if it happens to be one of the kind which 

 is short-circuit unstable. A saving circumstance here is available in that 



Fig. 25 — Some power transistors. 



added resistance in the emitter lead tends to promote stability, so that the 

 transistor may be stabilized by operating out of a higher generator im- 

 pedance, possibly at some cost in reduced gain. A corollary aspect of the 

 same phenomenon is that the input impedance of a high-power transistor 

 may become very low or even negative. 



Higher power output from the transistor can also be obtained by in- 

 creasing the permissible collector dissipation. This has been accomplished 

 by using a thin wafer of germanium directly soldered to a copper base 

 equipped with suitable fins to facilitate the removal of heat generated in 

 the vicinity of the collector point. An increase in allowable dissipation from 

 200 to 600 milliwatts has been thereby obtained. Output powers of ap- 

 j)ro.ximately 200 milliwatts at a conversion ef^ciency of ?<?>% have been 

 realized. 



The photograph of Fig. 25 shows on the left the type A transistor, in 

 the center the power version of this unit, and on the right is shown a double 



