SAMPLE AND HOLD CIRCUITS FOR CMOS IMAGERS
A CMOS image sensor includes a photodiode for receiving incident light that is converted to a charge signal; a transfer mechanism for transferring the charge to a sensing node that converts the charge signal to an image voltage signal; and a sample-and-hold circuit. The sample-and-hold circuit includes a capacitor that receives the image voltage signal; and only one buffer amplifier that passes the image voltage signal to a differential amplifier. The buffer amplifier also receives a reset signal that is passed to the differential amplifier.
1 . An active image sensor comprising:
(a) a photodiode for receiving incident light that is converted to a charge signal;
(b) a transfer mechanism for transferring the charge to a sensing node that converts the charge signal to an image voltage signal; and
(c) a sample and hold circuit comprising:
(i) only one capacitor that receives the image voltage signal; and
(ii) only one buffer amplifier that passes the image voltage signal to a differential amplifier;
wherein the buffer amplifier also receives a reset signal that is passed to the differential amplifier.
2 . The active image sensor as in claim 1 , wherein the buffer amplifier is a unity gain amplifier.
3 . The active image sensor as in claim 1 further comprising two operational switches connected to the capacitor for respectively transferring the image voltage signal and the reset signal.
4 . The active image sensor as in claim 1 , wherein the image sensor is CMOS.
5 . A digital camera comprising:
a CMOS image sensor comprising:
(a) a photodiode for receiving incident light that is converted to a charge signal;
(b) a transfer mechanism for transferring the charge to a sensing node that converts the charge signal to an image voltage signal; and
(c) a sample and hold circuit comprising:
(i) only one capacitor that receives the image voltage signal; and
(ii) only one buffer amplifier that passes the image voltage signal to a differential amplifier;
wherein the buffer amplifier also receives a reset signal that is passed to the differential amplifier.
6 . The digital camera as in claim 5 , wherein the buffer amplifier is a unity gain amplifier.
7 . The digital camera as in claim 5 further comprising two operational switches connected to the capacitor for respectively transferring the image voltage signal and the reset signal.
8 . The digital camera as in claim 5 , wherein the image sensor is CMOS.