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xz781122
Joined: 19 Nov 2004
Posts: 40
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 06 Dec 2004 15:03 about rail to rail constant gm op-amp |
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| i am doing some research about rail to rail constant gm op-amp.But i don't know what is the "constant gm" meaning.Is "constant gm" input stage'gm constant or the whole amp's gm constant?i have read some books.They offer a equation such as sqrt(Ia)+sqrt(Ib)=const.But I don't know why?who can do me a favour.
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sunking
Joined: 25 May 2004
Posts: 914
Helped: 46
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| 07 Dec 2004 7:08 Re: about rail to rail constant gm op-amp |
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it means when the common mode voltage change, the gm is constant.
For the mosfet,gm=KI**0.5, if keep Kn=Kp, then sqrt(Ia)+sqrt(Ib)=const
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ArchAngel
Joined: 01 Jul 2004
Posts: 49
Helped: 14
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| 07 Dec 2004 14:52 about rail to rail constant gm op-amp |
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This question arise from rail to rail input, commonly implemented using two diferential pairs. If ur bias point not choosen properly, there exists a situition that two pairs both operate, results a doubled gm.
search IEEE JSSC for rail to rail input opamp for suggestions.
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gevy
Joined: 17 Nov 2004
Posts: 377
Helped: 41
Location: Russia
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xwcwc1234
Joined: 21 Jul 2001
Posts: 313
Helped: 5
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| 08 Dec 2004 10:18 Re: about rail to rail constant gm op-amp |
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| Many books existed for rail-to-rail opamp design such as "Low voltage CMOS Operational Amplifiers----Theory , Design and Implementation" ----by Satoshi Sakurai and M.Ismail
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DaBiz
Joined: 09 Dec 2004
Posts: 4
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| 09 Dec 2004 22:42 Re: about rail to rail constant gm op-amp |
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| Be carefull with constant Gm Rail to Rail inputs cause input referred offset voltage and input referred errr in general will change dependant on common mode voltage
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