INTERSTAGE COUPLING 



is also effective within the limitations already outlined. As it works best 

 when Vi anode potential is low it is particularly suited to the coupling of 

 low-level amplifier stages. 



'Neon lamp' coupling 



This makes use of the ability of a cold cathode diode to maintain across 

 itself a rather constant potential difference ; clearly as the potential at A in 

 Figure 9.4 fluctuates the current in the ballast resistor R^, fluctuates also and 



HT+ HT+ 



HT- 

 Figure 9.4 



this current flows through the lamp, so that the constant voltage properties 

 are essential. The advantage of the method is that it is almost as cheap and 

 rehable as the potential divider method, but, like the couphng battery tech- 

 nique, provides almost no reduction in signal transferred. 



In order that the performance of Fi be not degraded by the coupUng, R^ 

 must be ^R^, which implies that the lamp current must be much smaUer 

 than F] anode current. About 100 fxA is typical. Voltage reference tubes 

 and small neon indicator bulbs will work satisfactorily at this current, but 

 stabilizer tubes on the whole will not. Design procedure is therefore: having 

 fixed a provisional mean anode potential K„ for Kj and grid bias — Vg for Kg, 

 find the reference tube or indicator neon whose running voltage is the nearest 

 to (^a + ^g)- Take the makers' minimum current and divide it into the 

 negative supply voltage. This gives with sufficient accuracy the value of R,^. 

 Now re-design the first stage for an anode current of at least five times the 

 neon current and for the corrected anode potential as dictated by the particu- 

 lar neon lamp chosen. 



The negative supply must be at least more negative than the voltage 

 differential of the reference tube or lamp, otherwise the latter will not strike. 

 It ought to be considerably more negative than this in order that the glow 

 shall not extinguish during large negative excursions of V^ anode. 



The field of application for neon coupling is in the high level stages of 

 amphfiers for two reasons : 



(1) the voltage across a soft diode is subject to small fluctuations and these 

 must be swamped by typical signals ; 



152 



