Bias circuit for a photo-avalanche diode

Claim 2

A photo-avalanche diode bias circuit comprising: a. a photo-avalanche diode for receiving a light signal and generating an electrical output in response thereto; b. capacitor means for providing a bias voltage for said diode, said diode being connected to a first plate of said capacitor means in shunt circuit relationship with said capacitor means; c. energy supply circuit means for charging said capacitor means to a voltage level with respect to ground which is equal to the avalanche breakdown voltage of said diode in response to a first timing pulse; and d. reference voltage circuit means connected to the second plate of said capacitor means for reducing said diode bias voltage provided by said capacitor by a predetermined amount to a level with respect to ground which is below that of said avalanche breakdown voltage of said diode in response to a second timing pulse.

Claim 3

A circuit as in claim 1 wherein said energy supply circuit means comprises a flyback transformer having a first end of its primary winding connected through a voltage source to ground, having a second end of its primary winding connected through the normally nonconductive collector-emitter circuit of a first transistor to ground, having a first end of its secondary winding connected through a diode to the junction point of a first plate of said capacitor means and said photo-avalanche diode and having a second end of its secondary winding connected to ground, said first timing pulse being applied to the base electrode of said first transistor to render said transistor conductive to thereby draw current from said voltage source and store energy in the primary winding of said transformer.

Claim 4

A circuit as in claim 1 wherein said reference voltage circuit means comprises a resistor and a clamping diode connected in parallel with each other and both being connected in series with a voltage source to form a series-parallel circuit, said series-parallel circuit being connected between the second plate of said capacitor means and ground; and a pulse responsive second transistor also connected between said second plate of said capacitor and ground.

Claim 5

A circuit as in claim 2 wherein said reference voltage circuit means comprises a resistor and a clamping diode connected in parallel with each other and both being connected in series with a voltage source to form a series-parallel circuit, said series-parallel circuit being connected between the second plate of said capacitor and ground and said circuit further including a second normally nonconductive transistor having its collector-emitter circuit also connected between said second plate of said capacitor and ground and said second timing pulse being applied to the base electrode of said second transistor to render said second transistor conductive to thereby reduce the voltage with respect to ground on said first plate of said biasing capacitor from a value equal to the breakdown voltage of said photo-avalanche diode to which said capacitor is originally charged by said gransformer after said first timing pulse to a value equal to said breakdown voltage minus the voltage of said reference voltage source.

Claim 6

A circuit as in claim 4 wherein said first timing pulse is derived from a first one shot multivibrator and said second timing pulse is derived from a second one shot multivibrator, both of said multivibrators being triggered by a laser synchronizing pulse, the leading edge of said synchronizing pulse triggering said first multivibrator which has a pulse period shorter than that of said synchronizing pulse and the second multivibrator being triggered by the trailing edge of said synchronizing pulse.