ADVANTAGES OF ELECTRON MULTIPLIERS OVER PHOTOCELLS

Zworykin, Morton, and Malter showed1 that the use of electron multipliers in place of photocells makes it possible to increase the signal-to-noise ratio. However, until now there had been no general consideration of this question. Such a consideration was recently given by F. Preisach2.

The author considers the operation of a photocell and of an electron multiplier in the amplifying circuit shown in Fig. 1, where \(P\) is the photocell (or electron multiplier), \(R\) is the input resistance of the amplifier tube, and the capacitance \(C\), shown by a dotted line, represents the shunting capacitance of the connecting leads, together with the internal capacitance of the photocell and the tube. Thus \(C\) determines the upper limit of the amplifier frequency band; in the calculations it is taken equal to \(20\,\mu\mu\text{F}\).

Fig. 1

The principal causes of noise considered are: (a) the noise produced by the photocathode itself as a consequence of the shot effect in photoemission, and (b) the noise caused by the Johnson effect of the resistance \(R\).

Taking the ratio of the mean square of the potential difference across the resistance \(R\) due to shot-effect noise to the same quantity for the thermal noise of the resistance \(R\), the author obtains the expression

\[ \frac{I_a R}{\dfrac{2kT}{e}}, \]

where \(I_a\) is the photocurrent, \(R\) is the value of the input resistance, \(T\) is the absolute temperature, \(k\) is Boltzmann’s constant, and \(e\) is the charge of the electron. For room temperature \((T = 300^\circ \text{K})\) this ratio gives

\[ \frac{I_a R}{50}, \]

where \(I_a R\) must be expressed in millivolts.

From this relation it is already clear that, in the case of weak light fluxes, when the question of the noise level is of essential importance, the determining factor is the Johnson noise of the input resistance, while the shot-effect noise of the photocathode plays practically no role. This is the case for an ordinary photocell.

The use of electron multiplication leads, obviously, to the fact that the current from the photocell (in the present case, the electron multiplier) entering the input resistance of the amplifier tube is increased. Accordingly, the Johnson noise loses its dominant role and, with a sufficiently large number of stages,

multiplication, the shot-effect noise becomes the determining factor. For this to be the case, the total amplification factor of the electron multiplier must be about 10,000.

Thus, the use of electron multipliers makes it possible to lower the lower limit of sensitivity of photoelectric circuits by eliminating the interfering action of Johnson-effect noise. This gain in threshold sensitivity, which the author estimates as 200-fold, constitutes the principal advantage of electron multipliers over ordinary photocells—an advantage that is already being realized in practice, for example in television. The significance of Preisach’s article lies in the fact that it gives a general analysis of the operation of both amplification systems and thereby puts an end to the misleading assertions that replacing photocells with electron multipliers offers no advantage in terms of increasing sensitivity.

N. Khlebnikov, Moscow

References

1. K. Zvorykin, G. A. Morton and L. Malter, Proc. Inst. Radio Eng., 24, 351, 1936.
2. F. Preisach, Wireless Eng., 16, 169, 1939.