DIRECT COUPLING METHODS 



Potential divider 



For this method {Figure 9.3) it is necessary to have a negative supply 

 voltage, but as one source can serve a whole amplifier it is much more econo- 

 mical in batteries than the coupling-battery method. It is clear that if the ratio 

 of R^ to i?2 is correctly chosen the grid potential of V^ may be arranged to 



HT+ supply 



HT-supply 

 Figure 9.3 



come out at the right bias. The fraction of the signal appearing at V^ anode 

 which is transferred to V^ grid is RJi^i + ^2)' so that i?2 should be large; 

 if R2 is large, the negative supply must be large in order that Kg grid potential 

 be correct. R^ and R2 alter the working conditions of V^, for F^ now has an 

 effective load of R^ + ^2 ^^ parallel with Rj^ — thus reducing the gain — and 

 effectively a reduced HT. Exact design becomes complicated but fortunately 

 is not necessary, for it is clear that the reduction of V^ gain and HT is mini- 

 mal when i?i + ^2 is large compared with Rj^. We therefore arrange that 

 this is so, then F^ working conditions may be regarded as substantially 

 unaffected. 



Suppose the anode potential of V-^ is provisionally +60 V, the negative 

 supply is —100 V, 7?i = 50,000 Q, and the bias required by V^ = —2 V. 



Then 



Ro 



R. 



V 



B, 



62 



98 



If we make 7?i = 620,000 Q and R^ = 980,000 O, then /?i + i?2 = 1-6 MQ 

 which is certainly ^/?2.- The fraction of the signal transferred by the coupling 

 will be 98/(62 + 98) = 0-6. This is rather a serious loss. Suppose it is found 

 that Vi will give almost as much gain if the mean anode potential is dropped 

 to +30 V. 



Then RJR^ = 32/98. Suitable values would be 320,000 O and 980,000 Q, 

 for i?i + -^2 = 1*^ MO is still ^Rl- The fraction of signal transferred will 

 be 98/(98 + 32) == 0-755, a considerable improvement. A still better perfor- 

 mance will result if the circuit be re-designed for a negative supply of, say, 

 -150 V. 



The potential divider coupling method is cheap, simple and reliable. It 



151 



