.SOMK CIRCriT ASPECTS Of HIE TR.WSLSTOR 391 



Measurement of the characteristics can be by conventional ]Joint-by-point 

 plots or by oscilloscope presentation. An oscilloscopic curve tracer has been 

 built whicii can show any of the four characteristics for any of the six pairs 

 of independent parameters of the Type A transistor, as well as any two-pole 

 characteristic wliich might be of interest (such as a negative resistance 

 characteristic). 



Occasionally the static characteristics are affected by effects of a thermal 

 nature such that an oscilloscope trace does not give the same results as a 

 slow point-by-])oint plot. These thermal effects are small in the usual region 

 of operation of the Type A transistor but may become appreciable if the 

 unit is heated by excessive power dissipation in it. 



Power Output and Distortion 



The problem of obtaining good "undistorted" power output from a tran- 

 sistor at low frequencies is one which is conveniently discussed by means 

 of the static characteristics. Analytically this question belongs to the class of 

 slightly non-linear problems but, for descriptive purposes, it is illustrated 

 by the curves of Fig. 24. The family of collector characteristics of a Type A 

 transistor is shown. The region of linear operation is substantially that part 

 of the plot where the curves are uniformly spaced, have constant slope, and 

 lie within the permitted power dissipation of the unit. 



In driving a Type A transistor harder and harder in an attempt to get 

 greater power output, one may encounter four types of overload distortion, 

 analogous to the types found in tubes. 



1. One may drive the emitter negative into the cutoff region where the 

 collector current fails to respond to changes in emitter potential, correspond- 

 ing to grid cut-off in a tube. 



2. One may drive the emitter positive into an emitter overload region 

 where non-linear distortion may be encountered because the emitter im- 

 pedance changes with its voltage. The corresponding tube phenomenon is 

 positive grid distortion. For both tubes and transistors this effect is a minor 

 one which may be actually beneficial in practical cases. 



3. The collector may be driven down to low potential where it can no 

 longer draw the current required to follow the impressed emitter current 

 variations. This distortion corresponds to plate "bottoming" in electron 

 tubes. 



4. The collector may be driven up to high currents where it overlixids 

 because of the non-linear voltage response in that region arising from heating 

 effects. This effect has practical consequences something like the overloading 

 of electron tubes which may arise from insufficient cathode emission. 



In other words, either emitter or collector may be driven into overload 

 or cut-off and the problem of getting good jM)wer output reduces to choosing 



