. Electronic apparatus for biological research . HT- Figure 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 wi
. Electronic apparatus for biological research . HT- Figure 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
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