THE DESIGN OF TETRODE TRANSISTOR AMPLIFIERS 



819 



Power Flow in a Two-Port Device 



A convenient point of departure in the analysis of power amplification 

 iu a transistor or other linear two-port device is the arrangement shown 

 ill Fig. 4. The two-port is supplied by a unit current at the frequency of 

 interest and at reference phase at the input terminal pair. The output of 

 the two-port is connected to a voltage source of the same frequency. The 

 input-current and output-voltage time functions are 



ti 



= Re\/2€'"' = ReV2li£'"' 



(7) 



iiid 



= ReV2(a + jb)e^"' = ReV2(L -f jM) (-AA 



\2h22r/ 



jolt 



(8) 



= Re\^E2e 



jat 



In (8), L and M are introduced for simplicity in some later relation- 

 ships. 



The whole analysis is essentially a study of power flow in the circuit 

 shown in Fig. 4 as L and M of (8) are varied. All possible terminations 

 and excitations can be simulated simply by varying L and M. Under 

 some conditions the voltage source will absorb power; under others it 

 w ill supply power to the two-port. Ordinarily the current source supplies 

 power to the two-port, but for appropriate ranges of L and M if the two- 

 l)ort is potentially unstable, the transistor may supply power both to the 

 current source and the voltage source. The problem of evaluating maxi- 

 mum power gain is simply finding the values of L and M corresponding 

 lo the greatest ratio of power out to power in. The load impedance to 

 which this situation corresponds is E-il — l-i . The input impedance for 

 t his condition is simply £"1//! , and the optimum source impedance is 

 the complex conjugate of the latter quantity. 



Ii=i+jo 



I 



£2= a+jb = 



Fig. 4 — A two-port device .supplied bj^ a current source and feeding into a 

 voltage source. 



