. Electronic apparatus for biological research . Figure voltage drop across it will be 50,000 ohms X 1 mA = 50 V, so K„ = 150 V. Now let us try to do four times better. For a 1,000-fold reduction we want R to be 200,000 ohms, hence the drop is 200 V and so F„ = 300 V. Suppose now K„ undergoes a 10 per cent upward change in value. In the first case it alters from 150 to 165 V, so the increment is 15 V and the proportion that is added to V^ is 1/250 x 15 = 0-06 V. In the second case the change in F„ is 30 V and the proportion of the change added to K,, is 1/1,000 X 30 = 0-03 V, only twi
. Electronic apparatus for biological research . Figure voltage drop across it will be 50,000 ohms X 1 mA = 50 V, so K„ = 150 V. Now let us try to do four times better. For a 1,000-fold reduction we want R to be 200,000 ohms, hence the drop is 200 V and so F„ = 300 V. Suppose now K„ undergoes a 10 per cent upward change in value. In the first case it alters from 150 to 165 V, so the increment is 15 V and the proportion that is added to V^ is 1/250 x 15 = 0-06 V. In the second case the change in F„ is 30 V and the proportion of the change added to K,, is 1/1,000 X 30 = 0-03 V, only twice as good. If we aim for a 4,000-fold reduction we get R = 200 ohms X 4,000 = 800,000 ohms, and the drop across it would be 800 V and F„ would have to be 900 V. A 10 per cent increase would then be a change of 90 V, and the fraction appearing at the output is 1/4,000 X 90 = 0-0225 V, an improvement hardly commensurate with requiring 900 V to produce 100. In point of fact there is little to be gained by making F„ more than two or three times F„ and the proper thing to do is to get F„ from some con- venient point in the circuit and then to decide on the glow current, rather than the other way round. The performance of the circuit is improved if R is increased, and R can be increased until the tube current approaches the lower limit advised by the makers, below which the tube performance will become unsatisfactory. It is a good general rule in any case with soft valves to have low currents if possible, because the hfe falls off inversely as a high power of the current—about the fifth. Voltage stabilizer tubes As with the voltage reference tube, one or more of these are arranged in series to produce the required steady output voltage K, from a supply voltage K„ subject to fluctuation, but in this case it is intended that they be used to stabilize the voltage supplied to some sort of load, represented by Rj^ in Figure If the load resistance is also liable t
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