Amplifier adapted for CMOS imaging sensors
A column readout amplifier and imaging array using the same method are disclosed. The column readout amplifier includes a signal amplifier having an amplifier signal output, a first filter capacitor, a buffer amplifier having a buffer amplifier input and a buffer amplifier output, and a switching network. The switching network connects the amplifier signal output to the buffer amplifier input and the buffer amplifier output to the first filter capacitor during a first time period, and connects the amplifier signal output directly to the first filter capacitor during a second time period. The time periods can be of fixed duration or determined by the difference in potential between the input and output of the buffer amplifier. The column readout amplifier can be used in an imaging array to readout columns of pixels.
1. An apparatus comprising:
a signal amplifier haying an amplifier signal output;
a first filter capacitor;
a buffer amplifier having a buffer amplifier input and a buffer amplifier output; and
a switching network,
wherein said first filter capacitor has first and second terminals, said second terminal being connected to ground, and said switching network connects said amplifier signal output to said buffer amplifier input by a first direct current path and said buffer amplifier output to said first terminal of said first filter capacitor by a second direct current path during a first time period, and connects said amplifier signal output directly to said first terminal of said first filter capacitor by a third direct current path during a second time period.
2. The apparatus of claim 1 wherein said signal amplifier charges said first filter capacitor with a first time constant when directly connected to said first filter capacitor and wherein said first time period is less than five times said first time constant.
3. The apparatus of claim 1 wherein said buffer amplifier has a gain substantially equal to one.
4. The apparatus of claim 1 , wherein said apparatus further comprises a controller that measures a difference between said buffer amplifier input and buffer amplifier output, said controller causing said apparatus to switch from said first time period to said second time period when said difference is less than a predetermined threshold.
5. The apparatus of claim 1 wherein said first time period and said second time period are fixed.
6. The apparatus of claim 1 wherein said buffer amplifier output is disconnected from said first filter capacitor during said second time period.
7. The apparatus of claim 1 wherein said signal amplifier comprises a capacitive transimpedance amplifier with a gain greater than one.
8. The apparatus of claim 1 wherein said first filter capacitor is characterized by a first filter capacitor capacitance and said signal amplifier comprises a second filter capacitor, characterized by a second filter capacitor capacitance, said second filter capacitor being connected to said amplifier signal output, said second filter capacitor capacitance being less than five times said first filter capacitor capacitance.
9. An imaging array comprising:
a bit line conductor;
a plurality of pixel sensors, each pixel sensor being coupled to said bit line conductor in response to a word select signal;
a column amplifier connected to said bit line conductor, said column amplifier comprising:
a signal amplifier having an amplifier signal output;
a first filter capacitor;
a buffer amplifier having a buffer amplifier input and a buffer amplifier output; and
a switching network,
wherein said first filter capacitor has first and second terminals, said second terminal being connected to ground, and said switching network connects said amplifier signal output to said buffer amplifier input by a first direct current path and said buffer amplifier output to said first terminal of said first filter capacitor by a second direct current path during a first time period, and connects said amplifier signal output directly to said first terminal of said first filter capacitor by a third direct current path during a second time period.
10. The imaging array of claim 9 wherein said first amplifier charges said first filter capacitor with a first time constant when directly connected to said first filter capacitor and wherein said first time period is less than five times said first time constant.
11. The imaging array of claim 9 wherein said buffer amplifier output is disconnected from said first filter capacitor during said second time period.
12. The imaging array of claim 9 wherein said signal amplifier comprises a capacitive transimpedance amplifier with a gain greater than one.
13. The imaging array of claim 9 wherein said first filter capacitor is characterized by a first filter capacitor capacitance and said signal amplifier comprises a second filter capacitor, characterized by a second filter capacitor capacitance, said second filter capacitor being connected to said signal amplifier output, said second filter capacitor capacitance being less than five times said first filter capacitor capacitance.
14. The imaging array of claim 9 , wherein said first filter capacitor is characterized by a first filter capacitor capacitance and wherein said imaging array further comprises a sample and hold circuit comprising a sample switch and a hold capacitor, said sample switch connecting said first filter capacitor to said hold capacitor in response to a control signal, said hold capacitor being characterized by a capacitance less than five times said first filter capacitor capacitance.