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Why two-stage OPamp has lower noise than single-stage?

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I read a paper which told me the two-stage opamp has better noise than
single stage? I'm wondering why? In terms of thermal noise, the input devices
contribute most, the second stage contributes noise but suppressed by the gain
of first stage. The situation is similar in single stage op. Why the paper told me that?

By the way, I saw an op based gain amplifier with resistor feedback in this paper.
One thing confusing me is the op doesn't include source follower buffer, this is
two stage op. I'm wondering also, why it doesn't worry the resistor load the gain
of this opamp?
 

Can you post the paper?
 

i think it is means the input referred noise is smaller,because the OPA's gain is larger.
 

Generally, the input referred noise depends mainly on the gm of the input stage. Perhaps using a two stage amplifier allows using more of the current budget in the input pair, thus improving their gm and hence reducing the input referred noise.

If you choose to have most of the current in the input pair and a small fraction in the output stage then you can have low noise performance ( high gm of input and smaller gm of output stage )

Another thing to consider is that two stage OTA has the smallest number of devices compared to single stage OTAs. this reduces the noise contributors and hence also reduces the input referred noise.
 
elbadry said:
Generally, the input referred noise depends mainly on the gm of the input stage. Perhaps using a two stage amplifier allows using more of the current budget in the input pair, thus improving their gm and hence reducing the input referred noise.

If you choose to have most of the current in the input pair and a small fraction in the output stage then you can have low noise performance ( high gm of input and smaller gm of output stage )

Another thing to consider is that two stage OTA has the smallest number of devices compared to single stage OTAs. this reduces the noise contributors and hence also reduces the input referred noise.
Click to expand...

Thanks for reply. But, I can't figure out why the two stage op has less device than single stage?
 

for little size deviecs, the noise is not all less than that of big size devices. thermal noise and 1/f noise are different
 

In the 2 stage OTA you have 6 devices thaa contribute to noise...

In single stage OTA's you will always have more than this. For example, the telescopic OTA will have 8 devices. the number increases for folded cascode or current-mirror type OTAs.

Regards
 

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