542 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



and if a load of resistance Rl is connected to the output terminals 



V2 = — -Kl^*2 . 



The equations for the circuit of Fig. 13 are, therefore, 



{Rn + Rg)ii + Rui2 = v^ 

 R2iii + (R22 + RL)i2 = 

 Solving for the voltage developed across the load (= —RlIx) gives 



Rl R21 



V2 = 



(Rn + Rg){R22 + Rl) — R12R2 



(6) 



(7) 



(8) 



Fig. 12 — A three-terminal network representing either grounded emitter, grounded base, 

 or grounded collector connection of a transistor. Note the convention of signs. 



3-TERMINAL 



NETWORK 



REPRESENTING 



TRANSISTOR 



L2 Rl 



^=-A/\An 



V2 



Fig. 13 — The three-terminal network of Fig. 12 connected between a generator and a load. 



The power gain in the circuit is the power delivered to the load (vI/Rl) 

 divided by the power available from the generator {i^g/4Rg). From equation 

 (8), this gives 



G = 



4i?oi?jr,i?21 



[{Rn + Rg)(R22-h Rl) - R12R21V 



(9) 



The gain depends on Rg and Rl and will be maximum when these are 

 chosen to match the input and output impedances of the transistor stage. 

 But the input impedance depends on Rl and the output impedance de- 

 pends on Rg in the following way: 



Input impedance = Ri = i^n — ^ ^^ '^L and 



Output impedance = Ro = R22 



R22 + Rl 



Rn R21 

 Rn + Ro 



(10) 



(11) 



