Optical sensing circuit and method for driving optical sensing circuit
An optical sensing circuit includes a light detection circuit, a differential circuit, and a controller. The light detection circuit includes a photodiode with a cathode electrode and a first amplifier transistor with a gate electrode connected to the cathode electrode. The light detection circuit outputs a voltage corresponding to a voltage at the gate electrode of the first amplifier transistor. The differential circuit determines a difference between a voltage read from the light detection circuit in an exposure state and a voltage read from the light detection circuit in a reset state. The differential circuit includes a second amplifier transistor that outputs the determined difference, a first transistor connecting a gate electrode and a drain electrode of the second amplifier transistor, and a second transistor connecting the drain electrode of the second amplifier transistor and a first voltage.
1 . An optical sensing circuit, comprising:
a light detection circuit including a photodiode with a cathode electrode and a first amplifier transistor with a gate electrode connected to the cathode electrode, the light detection circuit being configured to output a voltage corresponding to a voltage at the gate electrode, the light detection circuit being operable in an exposure readout state in which the voltage at the gate electrode changes based on an amount of light received by the photodiode or in a reset readout state in which the voltage at the gate electrode is a constant predetermined voltage;
a differential circuit configured to determine a difference between an exposure voltage read from the light detection circuit in the exposure readout state and a reset voltage read from the light detection circuit in the reset readout state, the differential circuit including a second amplifier transistor configured to output the determined difference, a first transistor connecting a gate electrode of the second amplifier transistor and a drain electrode of the second amplifier transistor, and a second transistor connecting the drain electrode of the second amplifier transistor and a first voltage; and
a controller configured to control the light detection circuit and the differential circuit.
2 . The optical sensing circuit according to claim 1 , wherein
the controller causes the first transistor to be conductive and the second transistor to be nonconductive when the exposure voltage is read from the light detection circuit.
3 . The optical sensing circuit according to claim 1 , wherein
the controller causes the first transistor and the second transistor to be nonconductive when the reset voltage is read from the light detection circuit.
4 . The optical sensing circuit according to claim 1 , wherein
the controller causes the first transistor to be nonconductive and the second transistor to be conductive when the differential circuit outputs the difference between the exposure voltage and the reset voltage.
5 . The optical sensing circuit according to claim 1 , wherein
the differential circuit includes a capacitor including an electrode connected to a source electrode of the first amplifier transistor and another electrode connected to the gate electrode of the second amplifier transistor.
6 . A method for driving an optical sensing circuit, the optical sensing circuit including a light detection circuit and a differential circuit, the light detection circuit including a photodiode with a cathode electrode and a first amplifier transistor with a gate electrode connected to the cathode electrode, the light detection circuit being configured to output a voltage corresponding to a voltage at the gate electrode, the light detection circuit being operable in an exposure readout state in which the voltage at the gate electrode changes based on an amount of light received by the photodiode or in a reset readout state in which the voltage at the gate electrode is a constant predetermined voltage, the differential circuit being configured to determine a difference between an exposure voltage read from the light detection circuit in the exposure readout state and a reset voltage read from the light detection circuit in the reset readout state, the differential circuit including a second amplifier transistor configured to output the determined difference, a first transistor connecting a gate electrode of the second amplifier transistor and a drain electrode of the second amplifier transistor, and a second transistor connecting the drain electrode of the second amplifier transistor and a first voltage, the method comprising:
causing the first transistor to be conductive and the second transistor to be nonconductive when the exposure voltage is read from the light detection circuit;
causing the first transistor and the second transistor to be nonconductive when the reset voltage is read from the light detection circuit; and
causing the first transistor to be nonconductive and the second transistor to be conductive when the differential circuit outputs the difference between the exposure voltage and the reset voltage.
7 . The method according to claim 6 , further comprising:
causing the first transistor to be nonconductive and the second transistor to be conductive when the differential circuit outputs the difference between the exposure voltage and the reset voltage.