Linear-logarithmic pixel sensors and gain control circuits therefor

Abstract

A system and method are disclosed to enlarge the sub-threshold current coefficient 'α' of a reset transistor connected to a photodiode in an L-L (Linear Logarithmic) pixel sensor without modifying any semiconductor process parameters. In one embodiment, a coupling capacitor is introduced between the gate and source terminals of the reset transistor. The gate node of the reset transistor is kept floating and the change in its source voltage Vs is coupled to its gate voltage Vg by a certain rate with the help of the coupling capacitor. Thus, an effective change in (Vg-Vs) is made small, which is equivalent to enlarging the sub-threshold coefficient 'α' In this manner, the signal gain in the logarithmic region of operation of the pixel sensor can be controlled by changing the coupling capacitance between the source and gate terminals of the reset transistor connected to the photodiode. The signal conversion gain in the logarithmic region is increased, but the gain in the linear region is unchanged. In one embodiment, the stray capacitance (which also affects the value of the enlarged “α”) present at the gate terminal of the reset transistor is externally controlled to achieve adaptive gain control of the output signal in the logarithmic region so as to obtain large output photo voltage swings even under varying light conditions. Because of the rules governing abstracts, this abstract should not be relied upon in construing the claims.