TUNED AMPLIFIERS 



who points out the difficulty in trying to use more than rather moderate 

 tapering factors. The impedance of the first fiher section ought to be 

 appreciably larger than the anode load; the impedance of the last filter 

 section ought to be appreciably less than the mixing resistances: hence the 

 limitation on a. 



It is not necessary to use a high-pass filter but it is quite useful to do so 

 for filters peaking at frequencies below a kilocycle or so, because the first 

 capacitor forms the HT blocking capacitance. At high frequencies the stray 

 capacitance of the output circuit becomes significant and it is better to use 

 low-pass sections, the last section being designed bearing the presence of 

 the stray capacitances in mind. The effect is to reverse the gain/frequency 

 characteristic left-right. 



REJECTOR AMPLIFIER 



Where the rejector characteristic of a simple parallel T filter is insufficiently 

 sharp, improved performance may be had by following it with an acceptor 

 amplifier of appropriate characteristic {Figure 13.13). The appropriate 



-f- 



log frequency 



c 

 o 



"^ l_ 

 .11 o 



^■^ 



log frequency 

 Figure 13.13 



log frequency 



Input 



If 



f 



•Output 



Figure 13.14 



characteristic here is given by an acceptor of the negative feedback type, i.e. 

 one having another parallel Tin the feedback loop {Figure 13.14). 

 The characteristic of the parallel T is given by 1/(1 — Ajo) 



where 



a = 



CO 



(JO, 



and a)„ = 



CR 



COg CO 



as we have seen. Further, the characteristic of the acceptor amplifier is 

 given, as we have also seen, by 



A 



1 + A 



1 



1 - 4ja 

 206 



