CCD IMAGE SENSORS WITH VARIABLE OUTPUT GAINS IN AN OUTPUT CIRCUIT
An output circuit in a charge-coupled device (CCD) image sensor includes a charge-to-voltage conversion region, a gain control transistor connected to the charge-to-voltage conversion region and a reset transistor connected in series with the gain control transistor. One or more additional gain control transistors can be connected between the reset transistor and the gain control transistor. The one or more gain control transistors are used to set a capacitance of the charge-to-voltage conversion region to two or more difference capacitance levels. For each capacitance level, a reset voltage and a signal voltage are measured from the charge-to-voltage conversion region. A signal processing device computes multiple signal values for a single charge packet using the measured reset and signal voltages. The signal processing device selects one of the multiple signal values to be the signal value for the pixel.
1 . A method for producing a signal value for a pixel included in a charge-coupled device (CCD) image sensor having an output circuit that comprises a charge-to-voltage conversion region, a gain control transistor connected to the charge-to-voltage conversion region, and a reset transistor connected in series with the gain control transistor, the method comprising:
(a) setting a capacitance of the charge-to-voltage conversion region in the output circuit to a first capacitance level;
(b) resetting the charge-to-voltage conversion region;
(c) measuring a first reset voltage of the charge-to-voltage conversion region in the output circuit while the capacitance of the charge-to-voltage conversion region is at the first capacitance level;
(d) setting the capacitance of the charge-to-voltage conversion region in the output circuit to a second capacitance level;
(e) measuring a second reset voltage of the charge-to-voltage conversion region while the capacitance of the charge-to-voltage conversion region is at the second capacitance level;
(f) transferring a single charge packet accumulated by the pixel to the charge-to-voltage conversion region in the output circuit while the capacitance of the charge-to-voltage conversion region is at the second capacitance level;
(g) measuring a first signal voltage of the charge-to-voltage conversion region while the capacitance of the charge-to-voltage conversion region is at the second capacitance level;
(h) setting the capacitance of the charge-to-voltage conversion region in the output circuit to the first capacitance level; and
(i) measuring a second signal voltage of the charge to voltage conversion region while the capacitance of the charge-to-voltage conversion region is at the first capacitance level.
2 . The method of claim 1 , further comprising repeating (b) through (i) for each pixel readout of the CCD image sensor.
3 . The method of claim 1 , further comprising:
(j) subtracting the first reset voltage from the second signal voltage to generate a first gain signal; and
(k) subtracting the second reset voltage from the first signal voltage to generate a second gain signal.
4 . The method of claim 3 , further comprising (l) for each pixel, selecting a signal value from either the first gain signal or the second gain signal.
5 . The method of claim 4 , further comprising (m) multiplying the selected signal value by a gain ratio for at least a portion of the pixels.
6 . A method for producing a signal value for a pixel in a charge-coupled-device (CCD) image sensor having an output circuit that receives charge packets from a CCD shift register, wherein the output circuit includes a charge-to-voltage conversion region, a gain control transistor connected to the charge-to-voltage conversion region, and a reset transistor connected in series with the gain control transistor, the method comprising:
(a) setting a capacitance of the charge-to-voltage conversion region in the output circuit to a first capacitance level by receiving a gain control signal having a first signal value on a gate of the gain control transistor;
(b) resetting the charge-to-voltage conversion region;
(c) measuring a first reset voltage of the charge-to-voltage conversion region in the output circuit while the capacitance of the charge-to-voltage conversion region is at the first capacitance level;
(d) setting the capacitance of the charge-to-voltage conversion region in the output circuit to a second capacitance level by receiving the gain control signal having a second signal value on a gate of the gain control transistor;
(e) measuring a second reset voltage of the charge-to-voltage conversion region while the capacitance of the charge-to-voltage conversion region is at the second capacitance level;
(f) transferring a single charge packet accumulated by the pixel to the charge-to-voltage conversion region in the output circuit while the capacitance of the charge-to-voltage conversion region is at the second capacitance level;
(g) measuring a first signal voltage of the charge-to-voltage conversion region while the capacitance of the charge-to-voltage conversion region is at the second capacitance level;
(h) setting the capacitance of the charge-to-voltage conversion region in the output circuit to the first capacitance level;
(i) measuring a second signal voltage of the charge to voltage conversion region while the capacitance of the charge-to-voltage conversion region is at the first capacitance level;
(j) subtracting the first reset voltage from the second signal voltage to generate a first gain signal; and
(k) subtracting the second reset voltage from the first signal voltage to generate a second gain signal; and
(l) selecting a signal value from either the first gain signal or the second gain signal.
7 . The method of claim 6 , further comprising repeating (a) through (l) for each pixel readout of the CCD image sensor.
8 . The method of claim 7 , further comprising (m) multiplying the selected signal value by a gain ratio for at least a portion of the pixels.