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Berti

Community Fellow

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Posts: 356

OTA replica bias
Apr 15^th, 2008, 8:20am

Hi all,

Today I discovered that I can save a lot of power when I scale down the current in the replica-bias
of my power-hungry OTA by 1000 Wink

(see a very simplified figure in the attachment).
However, where I work it is good practice to scale the current only by 8 to 12 and further current multiplication
is done in a seperate bias circuit.
Why should I not scale this current directly by 1000 (assuming it wouldn't be a problem in the layout)????

Cheers

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ota_001.jpg

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AnalogDE Senior Member Offline Posts: 137	Re: OTA replica bias Reply #1 - Apr 15^th, 2008, 9:03am I can think of 2 issues: 1. �The matching will probably not be good with the high M factor (1:1000) 2. �The current mirror will be very, very slow. �Any sort of noise that couples into the gate node of the current mirror will take a long time to settle, which could impact the overall settling of the amp.
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HdrChopper

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Posts: 493

Re: OTA replica bias
Reply #2 - Apr 15^th, 2008, 6:57pm

Hi Berti,

I agree with what jiesteve stated. In addition, regarding #2, not only the current mirror will be very slow, but also if you happen to copy such bias current from a bandgap reference circuit you might be generating potential stability problems (due to the large load cap) for such circuit if the mirror is directly copied from such reference.

Regards
Tosei

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Keep it simple

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Berti Community Fellow Offline Posts: 356	Re: OTA replica bias Reply #3 - Apr 15^th, 2008, 10:30pm Hi Tosei and Steve, Thank you for the answers. Matching is probably not so important, but I agree that the slow speed might be a problem. Also thanks for the remark concerning the bandgap stability. Regards
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