TRANSISTORS 



In our particular case it lies between 25 and 50 k. To keep the amplifier 

 general in purpose we do not want to tie down Ra too tightly. A reasonable 

 procedure is to take the output resistance as the geometric mean of 25 and 

 50 k, ?«34 k, and design a transformer so that the second stage presents a 

 matching load resistance of 33 k to the first. 



The circuit might have the appearance of Figure 45.18. R^ and Q are 

 decoupling components and R^ drops the battery voltage from 6 to 2 for 



Figure 45.18 



TRi. It is assumed that there is negligible voltage drop across the primary 

 resistance of the transformer. Then the turns-ratio required for the trans- 

 former is 



/i?outforTRi\i/2 



i?in for TR 



34 k 



2 / 

 1/2 



\80o a J 



= 6-5 to 1 

 We can now re-assess the performance of TRj. 



iS. 



R, 



41 X 



33 k 



Rc + Rl 

 The input resistance is R^ + YiRe 



= 700 ^ + (21 X 18 a) 



33 k + 33 k 



The current gain, y,, is 

 =-21 



1,10012 



R, 



The voltage gain, y^, is y^ X ^ 



= 21 X 



33k 



TTk 



='630 

 The power gain is 7^7^ = 21 X 630 = 13,000 



= 41 dB's 



The total gain of the amplifier is now 41 + 34 = 75 dB's. We have secured 



a 12 dB improvement merely by paying attention to the matching conditions. 



Temperature effects — The amplifiers in Figures 45.15 and 45.18, whilst 



serving to illustrate the design procedure so far as gains, and input and 



688 



