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why CS amplifier at the last stage?

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firewind

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from somewhere I read, people recommended Common Source amplifier at the last stage. Could someone please explain why so? Why not Common Gate because you'll get an extra 20% gain boost due to body effect? CS has miller, putting it at the last stage would amplifier parasitic effect by the gain amount so wouldn't that be bad to add at the last stage? thanks
 

A.Anand Srinivasan

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there are many factors to consider when you are considering the usage of a particular amplifier.... for CS amp what do you think about the input impedance....
 

firewind

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could it because I recalled it wrong then? maybe they meant SF, those guys have low impedance and they can drive anything following it.
 

A.Anand Srinivasan

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by what u are mentioning i think they are talking about the output impedance.... and ya of course source follower has lower output impedance and can drive most loads easily...
 

iamxo

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the output stage usually supply drivability to the load, CS output stage can provide

large output swing and some drivability(not better than class AB). and it's simple.

maybe this is the reason most people choose it.
 

firewind

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iamxo, I think you're right. How do I give you credit for helped me? That was a very good answer. I think that might be it. Thanks.


So, CS has large swing output. Is that the largest one among all avaliable topologies: SF, CG, & Cascode. I know off hand cascode has the smallest swing. So how is the order goes from max to min output swing works?


CS > CG > SF > Cascode

Is that correct?
 

surianova

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firewind said:
iamxo, I think you're right. How do I give you credit for helped me? That was a very good answer. I think that might be it. Thanks.


So, CS has large swing output. Is that the largest one among all avaliable topologies: SF, CG, & Cascode. I know off hand cascode has the smallest swing. So how is the order goes from max to min output swing works?


CS > CG > SF > Cascode

Is that correct?


shd be right.. but to determine to use either CS or SF for the 2 stage, very important to consider the output driving resistive load or capasitive load.
 

firewind

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thanks guy, so what is the order from maximum to minimum output swing for CS CG SF an Cascode?

Is this right, CS > CG > SF > Cascode ?
 

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