I've been playing around with photodiodes and op-amps lately, and thought it might be fun to construct a really crude camera with a dozen pixels. I was going to model it on an active-pixel CMOS sensor, which seems conceptually easier than a CCD: each pixel amplifies it's own signal and the pixels are multiplexed to the ADC.
At first, I assumed this would be setup using a transimpedance op-amp at each pixel, because all the literature I can see about photodiode amplification talk about op-amps (in either photoconductive or photovoltaic mode):
But when reading more about how CMOS sensors are actually constructed, it seems that they use a three-transistor configuration (3T) or some derivation. In this setup, a common drain amplifier is used instead of an op-amp (plus associated circuitry for the reset and multiplexing):
Which seems to work in photovoltaic mode too: (from "Towards Self-powered Cameras"):
I thought perhaps the reason 3T is chosen is because op-amp are "instantaneous", in that the 3T configurations naturally integrate over the exposure time but op-amps require constant polling. But I realized that switched op-amp integrator would do the same integration job:
So now I'm just confused. It appears that both 3T and op-amp setups work in both reverse- and zero-biased configurations. I don't see why you would choose one vs the other
- Is there a specific reason why CMOS sensors use the 3T setup instead of op-amps?
- Is it due to physical size (3 transistors is easier to fabricate on an IC than an op-amp)
- Is one preferable to the other for noise considerations? E.g. When using discrete photodiodes, you have space for op-amps which have better noise characteristics so the literature only talks about those? Whereas ICs dont have luxury of low-noise op-amps?
- Are common-drain amplifiers somehow better for longer integration times? I always see op-amps in reference to high bandwidth applications, which imaging definitely isn't.
- If an op-amp could magically be shrunk to a size that can fit in a 3T setup, would that be better?
I guess I'm just trying to figure out why the photodiode literature exclusively talks about op-amps, while it seems every CMOS imaging sensor has some derivation of the 3T configuration.
EDIT: Perhaps related to my confusion, I'm not quite sure how common drain amplifiers actually amplify since they are just buffering the voltage? So maybe the 3T configurations aren't doing what I think they are?