TUNED AMPLIFIERS 



In Graph 30 some performance curves for tuned amplifiers of the positive 

 feedback type are plotted. A is arbitrarily fixed at 100. Figure 13.6 shows a 

 possible circuit for a positive feedback tuned amplifier. 



Remember that the practical value of R' is the calculated value less the 

 resistance looking back into F^, i?^ in parallel with /-„. Similarly the practical 



HT* 



HT* 



Input 



Figure 13.6 



value of C is the theoretical value less the stray and input capacitance of 

 V^. Notice the un-bypassed cathode bias resistances which stabiUze the 

 gains of the two stages. Practical examples of circuits of this type have been 

 published by Morris and Dawe^. 



o Output 





 control 



Figure 13.7 



Figure 13.8 



Alternatively, positive feedback may be had by coupling back from the 

 second valve cathode to the first. This may be achieved by the delta resis- 

 tance network of Figure 13.7, or the equivalent T form {Figure 13.8). A 

 design employing this principle has been published by Beattie and Conn^. 



ACCEPTOR AMPLIFIER— NEGATIVE FEEDBACK METHOD 



Whereas in the positive feedback method we include a band-pass filter in 

 the amplifier, in the negative feedback method we include a null-transmission 

 filter — most conveniently a parallel T — in the feedback loop {Figure 13.9). 

 Away from the null-frequency of the parallel 7" there is 100 per cent negative 

 feedback with a consequent overall gain of only 1 . At the null-frequency, 

 the feedback disappears and the gain is A. 

 More precisely we have 



Fout A 



Vi 



in 



1 -\- AB 



200 



