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tia_design
Joined: 22 Feb 2005
Posts: 105
Helped: 3
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 26 Feb 2005 17:46 Biasing for low noise Op Amp |
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| I designed a low noise Op Amp, how can I design biasing network to avoid adding too much noise into the circuit? Should I design low noise bandgap for each biasing point?
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steer
Joined: 14 Jan 2005
Posts: 122
Helped: 11
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| 26 Feb 2005 21:46 Re: Biasing for low noise Op Amp |
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A correctly designed low-noise op-amp is not very sensitive to bias noise.
Not to say you can adopt a completely careless approach to biasing, but bias noise qualities are less important.
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rfsystem
Joined: 25 Feb 2002
Posts: 811
Helped: 88
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| 28 Feb 2005 10:31 Re: Biasing for low noise Op Amp |
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An ideal symmetric opamp surpress the common mode noise components from the bias. If the opamp is used in circuits which have to reject some big signals the common mode noises get modulated by the big signal and distort the wanted small signal. So in addition to AC small signal noise optimisation run a period noise analysis with a blocking signal.
Other optimisations are:
1. Maximize gm/I which lowers also noise.
2. Use voltage gain in the first stage (5-10) so noise is only from the diffpair.
3. Scale bias network current not to low in comparison to signal (20-50%)
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