MIXED FEEDBACK METHOD— PHASE SHIFT ACCEPTOR AMPLIFIER 



negligible phase shift at the resonant frequency, and the twin T resistances 

 should be about ten times higher than the anode loads and R^ in order not 

 to interfere with the stage gain without feedback. 

 A simpler and less efficient circuit is shown in Figure 13.11. The input 



HT+ 



Input .. 



o — \Y 



_£_ 



Output 





Figure 13.11 



and feedback voltage are mixed by resistors ^„;. These may conveniently 

 be made equal and as large as possible consistent with the prevention of 

 significant phase shift occurring between the output of the parallel Tand the 

 valve grid as a result of the valve input capacitance. Fleischer^ suggests that 

 if the resistance seen looking into the mixing resistors from the parallel T 

 is more than 2\ times the resistance looking back into the parallel F, the 

 loading effect on the parallel F will not be too serious. 



If the mixing resistors are equal the inputs to the valve will be half the 

 input signal and half the fed-back signal. The system will therefore behave 

 according to theory except that it is as if the valve gain had been halved. 

 Thus with this circuit the value for A is (^,„/?^)/2 and not g,„RL. 



A circuit of the Figure 13.10 type is published by Sowerby^. 



MIXED FEEDBACK METHOD— PHASE SHIFT ACCEPTOR 



AMPLIFIER 



This might be described as the poor man's tuned amplifier, as an indefinitely 

 large Q may be obtained with only 1 valve. The feedback is basically nega- 

 tive, in-so-far as the feedback loop is from the anode back to the grid (Figure 

 13.12). There is in the forward path a 3-section R-C high-pass filter, each 

 section having the same turn-over frequency, and a tapering factor between 

 sections of a. The method of operation is as follows. 



At very low frequencies the attenuation in the filter is so great that the 

 output is very small. At very high frequencies the filter offers no attenuation 

 and no phase shift, so that if a fraction of the output B is fed back to the 



203 



