INTERSTAGE COUPLING 



The tap at X is chosen to be equal to the mean anode potential of V^ so that 

 there is no difference of potential between the output terminals when there 

 is no difference of potential at the input. Chmbing amplifiers work well, 

 but have at least three disadvantages when more than about two stages are 

 used: 



(1) An inconveniently large total HT voltage is required. 



(2) If earth is connected to the amplifier as shown in Figure 9.1, then the 

 output is at a high positive potential with respect to earth. If the position 

 of the earth connection is moved to, say, the point X, then the input terminals 

 are inconveniently negative with respect to earth. 



(3) If a common heater supply is used for all valves it may be difficult to 

 arrange that the maximum allowable heater-cathode voltage for the valves is 

 not exceeded, leading to breakdown of the alumina insulation. 



All three difficulties are overcome if the amplifier be prevented from climb- 

 ing, that is, if all the valves are operated with their cathodes at approximately 

 earth potential. To do this we have to transfer the signal from valve to valve 

 with a fall in voltage from the considerably positive anode of valve 1 to the 

 slightly negative grid of valve 2. 



Coupling battery 



One way of doing this is with a battery. If the anode potential of V^ in 

 Figure 9.2 is +75 V, and the bias required by Ko is —3 V, then the correct 



Figure 9.2 



conditions are obtained when the e.m.f. of the coupHng battery is 78 V. 



The coupling battery does not have to supply any current and so can com- 

 prise small cells of the deaf-aid type. It is a bad thing to use a physically 

 large coupling battery as the high frequency performance of the amplifier will 

 be spoiled by the capacitance of the battery to earth. Deaf-aid batteries 

 do not possess taps, so the design of battery-coupled stages is restricted by 

 the available battery voltages. As the smallest and most convenient batteries 

 can be made up from units giving multiples of 15 V, the mean anode voltage 

 finally chosen in a design may have to be altered ±7| V from the provisional 

 value. This is not usually a very serious matter. A disadvantage with coup- 

 Hng batteries is that before they are many months old their voltage becomes 

 rather erratic, so that spurious signals are generated in the amplifier and the 

 batteries have to be replaced. With a large number of stages this is a tedious 

 and expensive business. 



150 



