Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1991-10-28
1992-12-01
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
307311, G01J 144, H01L 3102, H04B 900
Patent
active
051681544
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to electrical quenching circuits. Such circuits may, for example, be used in conjunction with an avalanche-type photo-detecting device for detecting a photon-induced avalanche in the device and for re-setting the device to enable detection of a following avalanche, each avalanche detected producing a corresponding electrical output.
SUMMARY OF THE INVENTION
According to the invention, there is provided an electrical quenching circuit for detecting, and quenching, avalanches in an avalanching device, comprising a comparator having a first input connected to the avalanching device to detect avalanching by comparing the potential at the device with a reference potential, a switchable device connected to the avalanching device and switchable from a normal state into a quenching state in which it quenches avalanching of the device, and circuit means responsive to the output of the comparator when avalanching of the device is detected to switch the switchable device into the quenching state, characterised in that the switchable device is a switchable current source whose normal state is a high current state and which is also connected to a further circuit path which receives current from the source when the avalanching device is not avalanching, whereby avalanching of the device causes a diversion of the current from the further circuit path and into and through the avalanching device so as to change the potential at the avalanching device and to cause detection of the avalanching by the comparator, and in that the quenching state of the current source is a state in which the current is switched off.
BRIEF DESCRIPTION OF THE DRAWING
An electrical avalanche-detecting and quenching circuit embodying the invention will now be described, by way of example only, with reference to the accompanying drawing which is a circuit diagram of the circuit.
DESCRIPTION OF PREFERRED EMBODIMENTS
The circuit is for detecting avalanches in a photo-detector device 5, such avalanches being induced by a detected photon event or a thermally generated event in the device. The circuit produces an output which records each such avalanche (and thus the event which caused it) and then rapidly quenches the avalanche so as to reset the device and permit it to detect the next event.
The circuit comprises a current source 6 which is switchable between low and high impedance states, the current source 6 being connected to supply current via line 7 to the photo-detector device 5 when it avalanches, thus acting as a low impedance (approximately 1K ohm) load. The current source 6 is also connected to ground through a resistor 8 and a high speed switching diode 10, their junction being connected to the non-inverting input 12 of a comparator 14. The comparator 14 is of the transistor-transistor logic (TTL) type. A parallel path to ground is connected via a high speed switching diode 16 and resistors 18 and 20. The junction between diode 16 and resistor 18 is connected to the output of a voltage reference unit 22. The junction between resistors 18 and 20 is connected to the inverting input 24 of comparator 14. The inverting output 26 of the comparator 14 is connected via a line 28 to control the switching of the current source 6 between its low and high impedance states. In addition, this output is fed through a time delay circuit 30 and a diode 32 back to the inverting input 12 of the comparator 14 in order to reset the circuit as will be explained.
A line 34 connected to the non-inverting output 36 of comparator 14 produces the avalanche-indicating output of the circuit.
In the quiescent state of the photo-detecting device 5, current source 6 is in its low impedance state. Current from the source flows through diode 16 to the voltage reference 22 and also flows through diode 10 and resistor 8 to ground. The value of resistor 8 is such that the current divides substantially equally between the two diodes 16 and 10. Line 7 is held substantially at the voltage of the voltage reference unit 22
REFERENCES:
patent: 4001614 (1977-01-01), Dahlinger
patent: 4479052 (1984-10-01), Suzuki
patent: 4730128 (1988-03-01), Seki
patent: 4945227 (1990-07-01), Jones et al.
patent: 4963727 (1990-10-01), Cova
Allen S. B.
Kidde-Graviner Limited
Nelms David C.
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