13 



TUNED AMPLIFIERS 



It often happens that amplifiers are required to amplify preferentially a 

 certain frequency (acceptor amplifiers) or else to amplify all except a certain 

 frequency (rejector amplifiers). In radio practice such amplifiers are the 

 rule rather than the exception, and tuning is obtained by resonant circuits 

 comprising inductance and capacitance. Q values of 100 or more are easily 



HT+ 



Output 



Input 



HT + 



Output 



Figure 13.1 



Figure 13.2 



obtainable. Thus a simple acceptor amplifier comprises a pentode having 

 for its anode load a parallel tuned circuit {Figure 13.1). The gain is^,„|Z, 

 and we saw in Chapter 5 that 



V\) 



w^L 



&)^ 



Q' 



i + e 





1/2 



Since g^ is constant, the gain of the stage has the same form as \Zj,\ in Graph 

 25. 



Similarly a rejector amplifier might have the form of Figure 13.2. The 

 series tuned circuit is effectively in parallel with the load resistance and will 

 have — at least near resonance — a much lower impedance than it. The gain 



where 



will be substantially g„i\Z 



2\l/2 



and the gain has the same form as \Zg\ in Graph 24. 



When we try to apply these circuits to the kind of frequencies commonly 

 encountered in electrobiology we run at once into difficulties. Suppose it is 

 required to build an attachment to an E.E.G. apparatus which selectively 

 amphfies a rhythm at 10 c/s. Then co is about 60 and the LC product 



196 



