THE DESIGN OF TETRODE TRANSISTOR AMPLIFIERS 



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a: 



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0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 



FREQUENCY IN MEGACYCLES PER SECOND 



50 100 



Fig. 15 — Computed and measured response of a common emitter amplifier. 



In the design of the IF amphfier one is mterested in a moderate range 

 of frequencies. It will generally be true that the most frequency de- 

 , pendent parameters are the output and load admittances, since the load 

 is to be tuned. One can take as a suitable load a parallel combination of 

 a fixed conductance with a frequency dependent susceptance, the sort 

 I of termination typical of tuned circuits. Thus on Fig. 8, the locus of 

 !(r2 + JB2 is one of the G2 = Const, circles. 



Superposition of Fig. 9(b) on Fig. 8 with the 



huh 



urm 



2h 



22r 



axis making an angle of —64° with the L axis reveals that Z,„ — hn 

 has a negative real part on the upper left edges of all of the contours of 

 constant G2 . On the G2 = 2/i22r contour, Re{Zin) reaches a minimum of 

 22.5 ohms. We select a load with Gl = 2/i22r((?2 = 3/i22r) to avoid low 

 values of input resistance resulting from the internal feedback. 



Superposition of Fig. 9(a) on Fig. 8 with the gradient line making an 

 angle of —64° through the point L, Tlf = 1, reveals that the maximum 

 value of Pa/ Pi on the G2 = 3/i22r circle is 1.87 Poo/Pio and it occurs for 

 B2 = — 2/i22r . The input impedance at this point is 36 + jS7 ohms. 



For an amplifier one is primarily interested in 



Po 



Power Available from Source 



(which is called transducer gain) rather than Po/Pi , the quantities just 



