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Simulators >> Circuit Simulators >> a confusion about noise summary
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Message started by subgold on Sep 11^th, 2009, 1:29am

Title: a confusion about noise summary
Post by subgold on Sep 11^th, 2009, 1:29am

hi all,

i have a confusion about noise summary of spectre.

i have an opamp used in a closed-loop. when i change the feedback factor (the system is still absolutely stable), and resize some transistors (only in the output stage), the noise at the output node has increased (the white noise).

the noise summary shows that, now some current sources in the bias network become the main noise contributors, whose white noise has increased significantly. (before the change, the main noise contributors are the input transistor, which is as expected)

to me this noise summary is really confusing and should not be the case, because i checked the dc operating point of those transistors, and they remain exactly the same. (of course they should remain same because they are just in the bias network). also, this shouldn't have something to do with the change of the system bandwidth, because i only check the spot white noise at one certain frequency.

so ultimately what does this noise summary tell me? how can i reduce the noise then?

Title: Re: a confusion about noise summary
Post by Ken Kundert on Sep 11^th, 2009, 7:28am

A noise contribution is the product of two numbers, the noise produced by the device, and the gain from that device to the output. Could the latter portion of this product have increased as a result of your changes to the circuit?

-Ken

Title: Re: a confusion about noise summary
Post by Frank Wiedmann on Sep 11^th, 2009, 8:11am

In pnoise analysis, one can separate noise into sources and transfer functions by setting the parameter separatenoise=yes. Unfortunately, a similar parameter does not seem to exist for noise analysis.

Title: Re: a confusion about noise summary
Post by subgold on Sep 11^th, 2009, 8:36am

hi ken and frank, thank both of you for the reply.

i thought about the change of the gain/transfer function, but i just don't understand why.

for example, the noise increase appears mainly on transistors M1, M2, M3 (pls see the attachment.) imho, no matter how the transfer function looks, these three transistors should always have minor contribution to the output noise, and are never the biggest noise contributor, aren't they?

Title: Re: a confusion about noise summary
Post by raja.cedt on Sep 12^th, 2009, 7:06am

hi,
1.To reduce noise from bias circuit use less mirroring ratio (even though it increase's power dissipation it really helps some times)
2. I didn't understand why M1,2,3 don't contribute noise because their noise is directly dummped into signal path transistors

Could you please share you W/L ratio's of transistors.

Thanks,
rajasekhar.

Title: Re: a confusion about noise summary
Post by subgold on Sep 14^th, 2009, 1:17am

raja.cedt wrote on Sep 12^th, 2009, 7:06am:

hi

1. the mirror ratio is only one. the W/L is 10/30um.
2. i know these transistors contribute noise. what i meant was, they shouldn't be the biggest noise contributor of the this circuit, and dominate the total noise.

Title: Re: a confusion about noise summary
Post by raja.cedt on Sep 14^th, 2009, 1:30am

hi,
still i didn't understand why they are contributing...ok let me give last trail could you please increase their overdrive and check..if entire noise decreases means there are some nodes which are low impedance nodes. Then find Signal path node impedance and try to increase

Thanks,
Rajasekhar.