THE DESIGN OF TETRODE TRANSISTOR AMPLIFIERS 



831 



will be within an order of magnitude of /?ii , perhaps in the vicinity of 500 

 ohms. One sees that in this case if the termination impedances are equal, 

 Yl ^ h22 . Again, with reference to Fig. 8, the point of operation will be 

 close to the origin of the L-M plane. Again the input impedance approxi- 

 mates All . The output admittance is given by 



^22 — 



hnh 



12't21 



An + Zs 



^-4 



(31) 



land if Zs = 500 12, Yo is (3.3 + il.0)10~ mhos. A rough approximant to 

 the common-emitter transistor is shown in Fig. 13. On an order of mag- 

 initude basis, one expects an iterative power gain of | hn f per stage. 

 Final choice of elements amounts to computation using the approxi- 

 mant of Fig. 13 and experimental adjustment. 



A video amplifier circuit employing the common emitter connection 

 is show-n in Fig. 14. If it is assumed that Ri is zero and no compensating 

 metwork is used in the output circuit, the gain characterictic can be com- 

 iputed from the approximant of Fig. 13, or if desired, using the exact 

 expression (Equation (5) of Fig. 1). The two methods agree to within 



-4 5 V 



Fig. 12 — Circuit of a common base video amplifier. The series coil compen- 

 sates for the decrease of | hn \ with increasing frequency. 



:C 



4.5 



c 



3-9/J.fJ.P 



Fig, 13 — An approximant for a common-emitter stage video amplifier when 

 terminated in a few hundred ohms. The variation of | hii \ and C are a function 

 of frequency. 



