TUNED AMPLIFIERS 



the incorporation of frequency-selective networks in valve circuits possessing 

 feedback. 



ACCEPTOR AMPLIFIER— POSITIVE FEEDBACK METHOD 



We have seen that the application of negative feedback round an amplifier 

 tends to reduce the effect of variations of amplifier gain with frequency (or 

 with anything else). It is therefore intuitive that the employment of positive 

 feedback will serve to exaggerate them. This is exactly what we want to 

 improve the selectivity of our band-pass circuit. 



Suppose we have an amplifier of gain A and insert into it somewhere in 

 the signal path a band-pass filter whose transmission factor we will for the 

 present designate /(w). Then the gain without feedback becomes Af((x>) and 

 if we now feed back positively a fraction of the output B (Figure J 3.4) then 



the gain with feedback is 



which simplifies to 



Figure 13.4 

 Afico) 



Kout_ 



Kin 1 - Af{oj)B 

 A 



1 



/(«>) 



AB 



Now we can see from the results in Chapter 3 that for the R-C band-pass 

 filter having a = 1, 



3 +j{^ - ^ 



where 



Hence 



and 



^"^C^ 



Kin 

 Kout 



"A-AB 



Kin 



U3-ABf + 



(OJ 



211/2 



This is clearly a curve which passes through a maximum at a 'resonant' 

 frequency to — oj„ and at resonance the gain will be just Al(3 — AB). It 



198 



