why simulation don't show the noise from bias circuits?

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wateror

Joined: 26 Aug 2007
Posts: 81

28 Nov 2008 9:40 why simulation don't show the noise from bias circuits?

when I simulated the opamp noise, I found none of them came from the bias circuits. is it right?

ict_eda

Joined: 29 Nov 2008
Posts: 8
Helped: 1

30 Nov 2008 5:13 Re: why simulation don't show the noise from bias circuits?

Here are two reasons:
1. Noise models not correct (probably not the case).
2. The bias circuit noise is irrelevant for the output of interest. Could be like common-mode signal for a differential output. To further test this hypothesis, run an AC sim with the input being the bias current and the output as your output of interest.

Keep looking,

ict

FvM

Joined: 22 Jan 2008
Posts: 5154
Helped: 766
Location: Bochum, Germany

30 Nov 2008 9:24 why simulation don't show the noise from bias circuits?

Depends on your bias circuit model. An ideal current source is noise free, you have to use real transistor models to model current source noise. Apart from this issue, the noise contribution from bias circuit in a good designed opamp should be small (but non-zero).

surianova

Joined: 01 Sep 2004
Posts: 410
Helped: 21
Location: ASIA

30 Nov 2008 10:48 Re: why simulation don't show the noise from bias circuits?

wateror wrote:

when I simulated the opamp noise, I found none of them came from the bias circuits. is it right?

if u use Cadence, u need to turn on the Transient Noise function, if not. u can't see the noise.

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wateror

Joined: 26 Aug 2007
Posts: 81

01 Dec 2008 15:42 why simulation don't show the noise from bias circuits?

thanks. I think it is because of fully differential circuits.

rfsystem

Joined: 25 Feb 2002
Posts: 858
Helped: 96

02 Dec 2008 0:29 Re: why simulation don't show the noise from bias circuits?

Could be the case.

If the circuit is symmetric to the biasing there is no noise from the bias generating circuit. That could lead to crazy noise effects. For instance if you have a differential signal the noise amplitude will be modulated with the signal. That is likely if you have a bias current generating source which has, mostly because of supply current efficiency reasons, a much smaller current. The noise is current is proportional instead of square-root.