Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 COLD CATHODE DIODE resistance, which is 70 V/1 mA = 70 kQ. Referring to Figure , it is clear that the goodness of the stabihzation depends on the attenuation of incre- ments in V^, that is, on R being much larger than 200 ohms. There is, however, a catch here. Suppose we choose a tube current of 1 mA as lying nicely in the centre of the makers' recommended range, and suppose Fj = 100 V. A 250-fold reduction in fluctuations of K,^ sounds attractive, so let us aim for
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 COLD CATHODE DIODE resistance, which is 70 V/1 mA = 70 kQ. Referring to Figure , it is clear that the goodness of the stabihzation depends on the attenuation of incre- ments in V^, that is, on R being much larger than 200 ohms. There is, however, a catch here. Suppose we choose a tube current of 1 mA as lying nicely in the centre of the makers' recommended range, and suppose Fj = 100 V. A 250-fold reduction in fluctuations of K,^ sounds attractive, so let us aim for this. Then R must be 50,000 ohms and the 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 per
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