Sample and hold based temporal contrast vision sensor

1. A pixel circuit comprising:

a photo-sensor stage comprising a photodiode and having an output, said photo-sensor stage configured for delivering a photoreceptor current dependent on a light intensity of an exposure of said photodiode, and

a comparison stage configured for detecting a change in a signal voltage derived from said photoreceptor current, characterised in that the pixel circuit comprises a sample-and-hold circuit having an input, an output, and a control terminal, the output of said sample-and-hold circuit connected to an input of the comparison stage, and the comparison stage is configured to output an input signal for the input of the sample-and-hold circuit,

wherein the control terminal of the sample-and-hold circuit is connected to the comparison stage, said comparison stage configured for emitting a sampling signal to the control terminal of the sample-and-hold circuit when the change is detected in the signal voltage.

2. The pixel circuit according to claim 1 , wherein the sample-and-hold circuit is configured for sampling the input signal at the input of said sample-and-hold circuit when the comparison stage emits the sampling signal to the control terminal of the sample-and-hold circuit, and for holding a hold voltage when the comparison stage does not emit the sampling signal to the control terminal of the sample-and-hold circuit.

3. The pixel circuit according to claim 2 , wherein the sample-and-hold circuit is configured so that the hold voltage at the output of said sample-and-hold circuit follows the input signal at the input of said sample-and-hold circuit when the sampling signal is received at the control terminal of the sample-and-hold circuit.

4. The pixel circuit according claim 1 , wherein the comparison stage is configured for comparing the signal voltage against at least one threshold voltage, and the comparison stage is configured for emitting the sampling signal to the control terminal of the sample-and-hold circuit on the basis of the comparison between the signal voltage and said at least one threshold voltage.

5. The pixel circuit of claim 4 , wherein the comparison stage is configured for comparing the signal voltage against a first threshold voltage and a second threshold voltage, said first threshold voltage being greater than said second threshold voltage, and for outputting a first signal event signal when the signal voltage exceeds the first threshold voltage and for outputting a second event signal when the signal voltage is inferior to the second threshold voltage.

6. The pixel circuit of claim 1 , wherein the comparison stage comprises a differential amplifier configured to compare the signal voltage to a reference voltage and to output hg input signal for the input of the sample-and-hold circuit on the basis of a comparison.

7. The pixel circuit of claim 1 , wherein the comparison stage comprises a differential comparator with multiple shifted outputs, said differential comparator having two inputs for inputting two differently amplified values of the signal voltage, and being configured for outputting the input signal for the input of the sample-and-hold circuit and for comparing the difference of the two inputs with at least one threshold voltage.

8. The pixel circuit of claim 7 , wherein a first amplified signal voltage is applied to an inverting input of the differential comparator, a second amplified signal voltage is applied to a non-inverting input of the differential comparator, and the differential comparator has at least three outputs:

a first output that transitions when the second amplified signal voltage and the first amplified signal voltage differ by a positive threshold voltage,

a second output that transitions when the second amplified signal voltage and the first amplified signal voltage differ by a negative threshold voltage, and

a third output is the input signal for the input of the sample-and-hold circuit and transitions when the first amplified signal voltage and the second amplified signal voltage are equal.

9. The pixel circuit according to claim 1 , wherein the comparison stage comprises at least one series of diode-connected transistors, each series of diode-connected transistors having a gate of a first diode-connected transistor as an input and a drain of said first diode-connected transistor as an output.

10. The pixel circuit according to claim 9 , wherein the at least one series of diode-connected transistors is arranged between a first terminal configured to apply a first biasing voltage and a second terminal configured to apply a second biasing voltage, and wherein the pixel circuit comprises a biasing circuit for generating at an output terminal at least one biasing voltage among the first biasing voltage and the second biasing voltage, said biasing circuit comprising:

a current source connected to the output terminal, and

a series of diode-connected transistors connected to the current source and the output terminal.

11. The pixel circuit according to claim 10 , wherein the series of diode-connected transistors is arranged between the first terminal and the second terminal, and the current source is connected to the second terminal, and the biasing circuit further comprises a differential amplifier having a inverting input, a non-inverting input and an output, said inverting input connected to a drain of a transistor of said series of diode-connected transistors, a reference voltage applied to said non-inverting input and the output connected to the first terminal.

12. The pixel circuit according to claim 1 , wherein the comparison stage comprises at least one first series of diode-connected transistors and a second series of diode-connected transistors, each series of diode-connected transistors having a gate of a first diode-connected transistor as an input and a drain of said first diode-connected transistor as an output, and

wherein the comparison stage comprises a differential comparator with multiple shifted outputs, a first input of the differential amplifier being the output of the first transistor of a series of diode-connected transistors and a second input for the differential amplifier being the output of the first transistor of another series of diode-connected transistors.

13. The pixel circuit according to claim 9 , wherein the output of the sample-and-hold circuit is connected to a gate of a transistor of the first series of diode-connected transistors.

14. The pixel circuit according to claim 9 , wherein the output of the sample-and-hold circuit is connected to the source of a transistor arranged at an end of the first series of diode-connected transistors, and wherein the output of the photo-sensor stage is connected to the gate of said transistor.

15. The pixel circuit according to claim 1 , wherein the pixel circuit comprises a converting stage having at least an input connected to the output of the photo-sensor stage and a first terminal, said converting stage configured for delivering on the first terminal the signal voltage derived from said photoreceptor current, and the converting stage has at least a second terminal and the output of said sample-and-hold circuit is connected to the second terminal of the converting stage, said converting stage being an input for the comparison stage.

16. The pixel circuit according to claim 15 , wherein the second terminal of the converting stage is configured for controlling a voltage shift at the first terminal of the converting stage through a hold voltage applied by the sample-and-hold circuit at the output of said sample-and-hold circuit.

17. The pixel circuit according to claim 15 , wherein the photo-sensor stage comprises a current mirror and the converting stage comprises at least one diode-connected transistor connected to said current mirror.

18. The pixel circuit according to claim 15 , wherein the converting stage comprises a series of diode-connected transistors between the output of the photo-sensor stage and the output of the sample-and-hold circuit.

19. An image sensor comprising a plurality of pixel circuits according to claim 1 .

20. The pixel circuit according to claim 12 , wherein the output of the sample-and-hold circuit is connected to a gate of a transistor of the first series of diode-connected transistors, or is connected to the source of a transistor arranged at an end of the first series of diode-connected transistors and the output of the photo-sensor stage is connected to the gate of said transistor.