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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> OTA replica bias https://designers-guide.org/forum/YaBB.pl?num=1208272841 Message started by Berti on Apr 15th, 2008, 8:20am |
Title: OTA replica bias Post by Berti on Apr 15th, 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 ;) (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 |
Title: Re: OTA replica bias Post by jiesteve on Apr 15th, 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. |
Title: Re: OTA replica bias Post by thechopper on Apr 15th, 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 |
Title: Re: OTA replica bias Post by Berti on Apr 15th, 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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