THE DESIGN OF TETRODE TRANSISTOE AMPLIFIEES 



829 



passive impedances this puts an upper limit on the level of impedance 

 presented by the load over a band of frequencies. The greatest possible 

 constant level of load impedance over 20 mc is 



^1 ~C~o.- 



2.10-12. 2.10^- 27r 



= 7,96012 



(30) 



Thus number, though not strictly applicable to this case, nonetheless 

 gives a measure of the sort of value which one can expect. Hence one 

 observes that for a broad-band video amplifier the load impedance is 

 certainly going to be considerably less than 1/| /i22 1 which up to 10 mc 

 is not less than 15,000 ohms. Moreover, the gain, if it is to be uniform, 

 will certainly be limited by the gain obtainable at 20 mc. 



Recognition that the load admittance will be a number of times h^ir , 



\.SfJ.IJ.P 



\^z\ ~0. 84-0. 93 



Fig. 10 ^ — A rough approximant for the common base video amplifier. The 

 variation of | /i. 21 | is a function of frequency and not variation between units. 



five to ten, means that in Fig. 8 one will be operating near the origin 

 where (L + jM) is much less than one. Thus Z,-„ in (28) will be approxi- 

 mately hn • Moreover, by superposing Fig. 9(a) on Fig. 8 at the correct 

 angle for a frequency of 30 mc {d = —64°) one observes that negative 

 power input occurs only in the small section of circle cut-off by a chord 

 running from the 80° to the 155° points on the periphery. This region is 

 quite a way from the likely point of operation. Thus, this points out that 

 the low impedance termination precludes instability due to internal 

 feedback. 



If the amplifier is supplied by a 75-ohm source, its output admittance 

 at 30 mc (Equation 4, Figure 1) is (7.0 + ^20) -10"^ mho. At 10 mc the 

 output admittance is (4.2 -f- j8.8)-10~ mhos. 



These computations reveal that the amplifier in the common-base 

 connection appears quite like the model shown in Fig. 10. Clearly, the 



^ H. W. Bode, Network Analysis and Feedback Amplifier Design, D. Van 

 Nostrand Co., New York, 1945. 



