THE DESIGN OF TETRODE TRANSISTOR AMPLIFIERS 817 



From the approach to the problem taken in this paper, one solves first 

 for the maximum power gain and subsequently determines the optimum 

 terminations. It turns out that the solutions leads to explicit relation- 

 ships for optimum performance and terminations and also leads to 

 charts from which power gains and input impedance can be read for any 

 terminations. 



In all expressions to be developed, the h parameters are used. Pre- 

 cisely the same expressions can be obtained for z's, y's, or ^'s provided 

 that one uses the corresponding quantities in the table of Fig. 1. 



The maximum power gain is a quantity of primary interest in tran- 

 sistors since the transistor ordinarily has a resistive component in its 

 driving-point impedance. Thus voltage or current amplification is con- 

 strained by the limited power gain attainable. In some cases, however, 

 because of the inherent feedback internal to the device, instability can 

 result simply from proper passive terminations without application of 

 any additional feedback. Such cases are distinct because of this property. 

 Transistors exhibiting this possibility are said to be potentially unstable 

 at the frequency in question. 



A quantity of interest presented here and derived later is a particular 

 power gain defined for /i-parameters as 



power out _ Poo _ | h 



21 



power in Pm 4:hnrh22r — 2ReQinh2i) 



(1) 



where hnr and h^ir mean the real part of hn and of /122 • ReQinhi^ means 

 the real part of the product of /ii2 and hn . Unless the amplifier is po- 

 tentially unstable, the quantity Poo/Pio is M'ithin 3 db of the maximum 

 available gain for the transistor. 



The matter of potential instability of the transistor is of great interest. 

 Certainly the transistor is potentially unstable if Poo/-Pio is negative. 

 Otherwise potential instability is indicated by greater than unity values 

 of the criticalness factor 



C = 2^ 



PiO 



h2i 



(2) 



If the transistor is not potentially unstable the maximum available 

 gain is Ko(Poo/Pio) where 



K, = ^(1 - ^[^'^ (3) 



For O^C^ 1,1 ^ Ko ^ 2. A plot of Kg as a function of C is shown 

 in Fig. 3. The function is seen to be exceedingly flat near Kg = 1 for 



