TRANSISTORS 



across R^, which must in consequence be somewhere near 6 V/64 ^A = 

 94 kQ. 



The current gain, y^, of the output stage is 



The input resistance, Ri^, is R^, + y^R, = 500 + 44 x 7 ^ 800 Q. 



Rr 1,000 



The voltage gam, y^, is y,. -^ =. 44 X -^^ = 55 



The power gain is 44 x 55 = 2,400 f^ 34 dB's 



//7/7M? j'ra^e — The collector of transistor TRj sees two kinds of load: 

 so far as the mean working conditions are concerned, the load is the resistor 

 /?2, but to signals the load is given by i?2 i^i parallel with the input resistance 

 of TRg. A reasonable working point for TR^, and one for which the 

 makers of the OC 71 give data, is V^ = —2 V, /^ = 1 mA, The transistor 

 not being a completely linear device, the parameters are somewhat different, 

 viz. 



/5 = 41 



i?, = 18 Q 



R^ = 700 Q 



R, = 25 kQ 



For a collector voltage of —2 we have to drop 4 V across 7?2- If the current 

 is 1 mA, the value of R^, must be 4 kO.. As with the second stage, the 

 quickest way to find R^ to give the chosen collector current is by experiment. 

 To signals the load seen by TR^ is 4 kQ. in parallel with 800 Q, which is 

 670 D.. The current gain is therefore 



25 kQ 

 "^^ >^ 25 kQ + 670 Q ^ "^^ 



The input resistance = 700 Q + 40 X 18 Q 



= 1,400 Q 



670 

 The voltage gain is 40 X , .„„ i=v 19 

 ^ ^ 1,400 



The power gain is 40 X 19 = 760 = 29 dB 



Coupling capacitor — To compute the required coupling capacitor we 

 refer to Figure 45.17a; looking back from the capacitor into TRj we see 

 i?out» the output resistance of the transistor in parallel with R^, the static 

 load. Looking forward from the capacitor into TRg we see R^, the bias J 



resistor for TRg in parallel with its input resistance R\n. The time constant 

 of the coupling is given by CRrp, where Rj, = (-^out in parallel with R^ + ^ 



(/?3 in parallel with R^)- The value of /?out depends upon the resistance 

 of the signal source feeding TR^, but it cannot be lower than R^ for the 



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