DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 383 



are applied, however, the principal difference between the two devices be- 

 comes apparent and turns out again to be associated with the difference 

 between forward and reverse. This is because biases applied to the grid affect 

 only the forward part of the plate circuit characteristic while biases apphed 

 to the emitter affect only the reverse part of the collector circuit character- 

 istic. 



Thus the grid and plate are normally biased in the reverse and forward 

 directions, respectively, with the result that the vacuum tube input im- 

 pedance is high and the output impedance is relatively low. The emitter 

 and collector, on the other hand, are normally biased in the forward and 

 reverse directions, respectively, with the result that the transistor input 

 impedance is low and the output impedance is relatively high. 



The comparison of vacuum tube and transistor properties can be carried 

 further with the help of Fig. 1 (a) which shows the plate circuit characteristics 

 of a particular vacuum tube triode and Fig. 1 (b) which shows the collector 

 circuit characteristics of a particular transistor. The axes in these two figures 

 have been chosen to faciUtate comparison of transistor currents with vacuum 

 tube voltages and vice versa. The result is that the two families look quite 

 similar. It is seen that the quantities to be compared are 



Vp with —ic, 



ip with —Vc , 

 — Vg with ie , and, though not shown, 

 —ig with Ve . 



The consistent difference in sign between vacuum tube and transistor 

 quantities holds only when the transistor is made from an iV-type semicon- 

 ductor. If the transistor is made of P-type material there is no difference 

 in sign between corresponding transistor and vacuum tube quantities. 



By referring to Fig. 1(a) it can be seen that to a first approximation the 

 effect of applying a negative voltage bias to the grid is simply to shift the 

 plate circuit characteristic to the right along the Vp axis. The number of 

 volts shift caused by a change of one volt on the grid is called the voltage 

 amplification factor, /x, of the triode. Similarly, it can be seen from Fig. 

 1 (b) that the principal effect of applying a positive current bias to the emitter 

 is shnply to shift the collector ckcuit characteristic to the right along the 

 — ic acis. The number of miUiamperes shift caused by a change in emitter 

 current of one milHampere is called the current amplification factor, a, 

 of the transistor. Thus, a of the transistor corresponds to fi of the vacuum 

 tube. 



1 The p-Germanium Transistor, W. G. Pfann and J. H. Scaff, Proc. I.R.E., 38, 1151. 



