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what is the difference between the following two amp

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reg_ic

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i have two choice of the amp ,but i cant make understand what is the different between them ! pls give me some advice and thx advance!
from the view of amp gain i think they are the same, but if you meet these two amps ,which will you choose and why ?
 

transbrother

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You are better off having the second configuration whereby both PMOS paths have a current gain of 1:k. and the nmos has a gain of 1:1. So, the 1:k current gain is more "matched" since it is only pmos dependent.

On the first configuration you rely on 1:k gain of the pmos path and 1:k gain on the nmos path. And, hence you are relying that the 1:k gain of the nmos path "matching" the 1:k gain of the pmos. The problem is nmos and pmos matching are quite uncorrelated.
 

reg_ic

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from the view of "matching" ,i do think amp1 is better ,but in the low power low dissipation design ,amp2 can save some power dissipation! and how should i make the trade-off between "precision" and "power dissipation"?
 

Fahmy

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The current consumption of the second circuit circuit is larger .
 

hr_rezaee

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Hi
in addition to power, I think the second configuration has better freq. response and PM.
regards
 

transbrother

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Hello reg_ic,

When i talked about matching, it didnt mean just precision. I meant that that your symmetry of your ckt is better preserved, which leads to better PSRR, distortion, random & static offset. If you are convinced that power is the only criteria to choose from, then you have to tradeoff.

hr_razaee,

I do not understand how the the freq. resp of one is better than the other. First order, your BW is determined by gm6 and the output cap that is present on the drain of M6 and M7. And, the gm of M6 is the same in config1 and config2. Could you please explain to me and tell me what I am missing?
 

reg_ic

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better PSRR

to transbrother:thanks for your advice firstly , could you like tell me in detail how this ckt improves the psrr performence , thx advance!
 

engronger

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i think amp1 can have a good match, so if you use it as a amplifier, you'd better use amp1, but amp2 have less dissipation, when you want to design a comparator, i am afraid it is a better choice

reg_ic said:
i have two choice of the amp ,but i cant make understand what is the different between them ! pls give me some advice and thx advance!
from the view of amp gain i think they are the same, but if you meet these two amps ,which will you choose and why ?
 

safwatonline

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i think #1 is better in PM due to the lower parasitic caps
 

sahn001

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I think,

For Amp1 :
M6, M7, M8,M9 all have identical current flow.
since M8 and M6 has k-tims gate poly Length,
It has lower Vds compare to M7 and M9.
All are in good current matching.

For AMP2:
Since M9 has K-times longer gate poly campare to M7, the M7 will have K-times less current flow compare to M9.
But, M6 will try to push K-times bigger current since it's gate is connected to M4.
Due to this miss-match, the current flow ratio on M1, M2 will be dustorted.
 

hr_rezaee

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transbrother said:
Hello reg_ic,

hr_razaee,

I do not understand how the the freq. resp of one is better than the other. First order, your BW is determined by gm6 and the output cap that is present on the drain of M6 and M7. And, the gm of M6 is the same in config1 and config2. Could you please explain to me and tell me what I am missing?
Hi
As I know pole and zero of current mirror are proportional to gm .
in the second configuration, gm is greater so the non dominant pole is bigger.
regards
 

transbrother

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reg_ic,

If Vss moves by some amount, the current injected from Vss into the ckt is asymmetric because the Nmos has a 1:k ratio. That is, in the first config (amp2), Vgs of M9 and M7 are equal; however, if their source voltage goes down by some amount their drain currents change by 1:k and not 1:1 as is the case with the second config (amp2). This asymmetric injection of Vss current will give rise to a input referred offset if your current gain 'k' of nmos and k of pmos side are not the same, which could be very likely because. That is, DC PSRR- will be degraded. In other words, usually it is a good idea to make your design so that you rely on matching between nmos and another nmos; not between nmos and pmos. You can use the same argument for AC PSRR- because there is a CGS of M9 and M7 that is present that will change their Vgs and will follow changes in Vss. On top of that, CGS of the M8, M6 are different; and you can find similar reasoning as to why PSRR+ will be worse.

hr_rezaee,

Thanks for the explanation. I was assuming a resonably low freq. application. That is why I didnt take into account mirror poles and zeros. However, since we do not know the application of this amp., you are correct.
 

jjsnail

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symmetry is very important on cancelling the noise from power line ! you will find it in many papers!
 

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