THE EARTHED EMITTER TRANSISTOR 

 and in Figure 45.13b we have 



^V,^ = (Re + RbWin + /?e<5/out 

 and <5Ko,.t = (Re - ^RcWin + (Re + RcWout 



Comparing the coefficients of 61^ and <5/out we see that 



(Re + Rb) = Re-b 

 Re = B . Rg_^ 



(Re + Re) — Re-c 

 ^Rc ~ Re — ^Re-c 



but with practical transistors it is sufficiently accurate to write 



(Re + Rb) = Re-b 

 Re = B . Rg_j, 



Re = Re-c 



^ =A 



If we provide a transistor with a load resistance R]^ and feed it from a 



fi/in R^ 







^c 



Figure 45.14 



signal generator of internal resistance Rq (Figure 45.14) then these exact 

 expressions^ follow: 

 The current gain 



_ a/o„t _ ^Re - Re 



^' 61,, R, + R, + Rl 



The input resistance, /?{„, that is, the resistance seen looking in at the input 

 terminals with the generator temporarily disconnected but with the load 

 connected, equals R^ + RX\ + 7^- 

 The voltage gain, 



_^nut__ Rl 



The output resistance, Rq^u that is, the resistance seen looking back into 

 the output terminals with the load temporarily disconnected but with the 

 generator connected, equals 



K + R,i\+ ^^'-^- ^ 



^b + -^e + Rg) 



683 



