GB773006A - Improvements relating to light detection apparatus - Google Patents

Abstract

773,006. Radiation detectors. GENERAL ELECTRIC CO. June 6, 1955 [June 9, 1954], No. 16207/55. Class 40 (3). Apparatus for detecting light intensity variations comprises a semi-conductor device having a pair of electrodes, the effective capacitance between which varies in accordance with the intensity of light falling on a photo-sensitive area in the device, connected in an oscillatory circuit so that variation in the said capacitance causes variation in the oscillatory frequency of the circuit. In Fig. 1, the oscillator circuit uses a transistor 11 having an emitter 13, base 15, and collector 17. A tuned circuit comprising capacitances 23 and 25 in parallel with an inductance 27 is connected between the collector 17 and one end of a choke 21, the other end of which is connected to the emitter 13. The base 15 is connected to the said one end of the choke 21 through a biasing network 22 comprising a capacitance 29 in parallel with a capacitance 31 and variable resistance 33 in series. A battery 19 is connected in series with the coil 11. In addition, the junction between capacitances 23 and 25 is connected to the emitter, providing feed-back from the collector-base circuit to the emitter-base circuit in the correct phase for maintaining oscillation. Light from a source 12, the variation of which is to be measured, is focused on to the photosensitive collector, varying the capacitance between the collector and other electrodes. Since this capacitance is effectively in parallel with the tuned circuit 23, 25, 27, the resonant frequency of oscillation varies. A coil 49 couples the tuned circuit to a frequency discriminator 14 and an indicator 16. In an alternative oscillator, Fig. 3 (not shown), there is inductive feed-back from the collector-base circuit to the emitter-base circuit.