HT SUPPLIES 



Alternative methods of improving stabilizer performance revolve round 

 the necessity to get the maximum possible amplification from V^. One 

 approach lies in the use of positive feedback ; this is done in the circuit of 

 Figure 37.7, which was shown to the author by J. A. Popple. The method 

 seems to originate in a paper by Gray^. A definite fraction of the output 



+ o 



Stabilized 

 supply out 



— o 



o + 



Unstabilized 

 supply in 



o — 



Figure 37.6 



2 807's 

 in parallel 

 V2^ — r 



+ 300 



+ A50 



Stabilized 68k 

 supply out 



2 5k 



Figure'J7.7 



is applied via i\ to V^, which controls V^ in the same way as in Figure 37.5. 

 The reference potential is applied not direct to V^ cathode but to V^ grid. 

 Vi and K3 form a differential amplifier with the 47k resistor as common 

 'tail'. Clearly when Pg slider is at the top there is no positive feedback, but 

 as P2 slider is moved down feedback comes into action; for an increase in 

 output voltage raises V^ grid and hence the common cathode potential. 

 Kg anode current therefore falls, reducing the voltage drop across that 

 fraction of /*2 above the slider and hence increasing the slider potential. 

 This increase in potential must be communicated down the chain to V^ grid, 

 hence the feedback is positive. 



In use, /*! is used to set the output voltage to the required value, whilst 

 P2 determines the output impedance. After altering P2, Pi will require 

 re-adjustment. The output impedance can be made zero, or even negative, 

 but instabihty can occur if this is done. 



An important advantage possessed by this circuit, and not by those of 

 Figures 37.5 and 37.6, is that the use of differential amplification between 

 the reference potential and P^ slider confers the usual advantage that the 

 circuit is relatively unaff"ected by the variations in cathode emission which 

 follow from feeding V^ from an unstabilized 6-3 V a.c. supply. 



589 



