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Symmetrical Fully Differential Amplifier Layout

Junus2012

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Dear friends,

I am asking for your suggestion of symmetrical layout simulation or how much it is necessary, specilly I am talking here about the layout of the compensation capacitor CA and CB in the figures below, do I need (or must) make it like in the first figure or the second scheme is still acceptable, I would say the second scheme is easier.

Sheme (1)

schem1.PNG

Scheme (2)

schem2.PNG
 

sutapanaki

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If your amplifier is fully symmetrically laid out, that is it has a mirror symmetry, then obviously you should use the first scheme since it preserves the mirror symmetry.
 

Junus2012

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Dear Suta,

Thank sou for your reply

What about if I folooed the second scheme, I see it more easy to put all the capacitors in one side so I have more flexibility in routing the core amplifier as well as to the biasing circuit
 

sutapanaki

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You will break the symmetry this way, because one half of the fully symmetric amplifier will have longer connection to the cap.
 

lazybear

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well, in my opinion, what is better is depend on you floorplan and your criteria.
If you plan bias block a bit away from amplifier and cap, then you should go with scheme (1) for what so call perfect matching, considering non ideal effects could be.
If bias block is next to amplifier, then it's depend on your coupling spec. For lower coupling, you should go with scheme (2). If you only care about symmetry, both are fine.
For matching, you can route with "center/ symmetry" in mind.
 

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