integrating and feedback capacitors. The feedback capacitors
can be switched in order to change the gain between two
levels: 1, for high dynamic range in normal light conditions
and 64 for low noise at low light conditions. The openloop
gain is provided by an OTA. For the OTA design and
layout, the priority is given to the noise constraint. The
dynamic range is not critical since the voltage swing at
the output of the pixel is not higher than 1.5 V. A singleended
structure is used because it involves half the number of
noisy transistors compared to a differential one. Differential
amplifiers are certainly better at rejecting any common mode
noise (e.g. noise coming from the substrate and power supply)
but at the cost of more noise and more power. Indeed, in
the case of an operational transconductance amplifier (OTA)
such as the one used in this amplifier, the differential structure
requires to duplicate the circuit branch resulting in twice the
power consumption and twice the thermal noise excess factor
for achieving the same transconductance. In order to achieve
low noise and to stay within our power budget, we have
chosen a single ended implementation. The noise originating
from the power and bias sources is reduced on-bord by using
power filters. Regarding the area, it is manly set by the input
and feedback capacitors especially for high gains. Hence, a
differential topology would occupy about the same area but, as
explained above, with the penalty of double power and noise.
The situation would be even worse for a fully differential
amplifier (differential input differential output) since it would
generate twice as much noise, power and area compared to the
single ended implemented in this work. The high closed-loop
gain of 64 requires a large open-loop gain hard to achieve
with a simple single ended amplifier. It is known that cascode
structures provide much higher gain with a negligible noise
contribution of the cascode transistors. Hence, a fully cascoded
single-ended amplifier is used. In order to make the 1/f noise
contribution of the column-level amplifier negligible compared
to the one originating from the pixel, the transistors of the OTA
have gate areas more than 10 times larger than the SF. The
charge injection of the AZ switch can reduce considerably the
output voltage swing of the amplifier in the high column-level
gain mode. In order to reduce that charge injection, dummy
devices with a proper sizing are used in order to compensate
the charge injected by the main NMOS switch.