PROPERTIES AND APPLICATIONS OF fl-p-fl TRANSISTORS 543 



If Ri = Rg and Ro = Rl then impedances are matched at the input and 

 output terminals and the gain is a maximum. The conditions are: 



Matched input impedance = 



Rim = Rn Vl - Ri2R2i/RnRr2 , (12) 



Matched output impedance = 



Rom — R22 V 1 — Rl2R2\/ R11R22 ) (1^) 



Maximum available gain = 



^•^•^' " R^2 [1 + \/T^=^2i?2l/i?lli?22l' ^^^^ 



The Grounded Base Stage 



In this and the following two sections we will put into equations (7) 

 through (14) the appropriate 4-pole r's to obtain expressions for impedances 

 and gains. As a numerical example we will substitute into the resulting equa- 

 tions the measured values of these r's for Transistor No. I working at V ^ = 

 4.5v. and /c = 1 ma'. It must be understood that the numerical values 

 may vary appreciably from transistor to transistor and that these numerical 

 calculations are intended only for illustration and not as a basis for final 

 circuit design. The numerical values to be used are 



Te = 25.9 ohms 



rb = 240 ohms 



re = 13.4 (10)« ohms (15) 



re- rm = 0.288 (10)« ohms 



a = 0.9785 



In this section it will be shown that the grounded base connection is 

 suitable for working between a low impedance source and a high impedance 

 load. The input impedance may be of the order of a hundred ohms and the 

 output impedance of the order of one or more megohms. In this connection 

 the current amplification is always less than unity but the voltage amplifica- 

 tion may be very large indeed. Power gains of the order of 40 to 50 db can 

 be obtained between matched impedances, and appreciable gains can still 

 be obtained if the load resistance is reduced to a few thousand ohms (be- 

 cause the current gain is then almost equal to unity). In this case the gain 

 of the stage is almost completely independent of those transistor properties 



