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Design >> Analog Design >> a question about bandgap reference
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Message started by subgold on Aug 26^th, 2009, 6:06am

Title: a question about bandgap reference
Post by subgold on Aug 26^th, 2009, 6:06am

let's say i have a typical bandgap voltage reference as shown in the attachment. assume the resistors and the amplifier are noiseless, only the bipolar transistors contribute noise, and their corresponding collector noise current are i1 and i2, for Q1 and Q2 respectively.

then what is the noise appear on the output Vout? what is the noise transfer function for the bipolar noise?

thanks for any help.

Title: Re: a question about bandgap reference
Post by andre75 on Aug 26^th, 2009, 3:31pm

Superposition: For each noise contributor, assume an equivalent noise source (eg. vn^2=4kTR for resistors) and calculate the transfer function while setting all other noise sources to zero (voltage zero, current open). Depends what you want to include (e.g. thermal noise of resistors and transistors) you can find a transfer function for each of the sources. The total noise at the output is simply the superposition (addition) of all individual noise sources.
In general I would use Spice, find the largest contributors and optimize those.

Title: Re: a question about bandgap reference
Post by subgold on Aug 27^th, 2009, 12:39am

andre75 wrote on Aug 26^th, 2009, 3:31pm:

hi andre,

thanks for the reply. however, it is actually the very question that i asked: how to find the transfer function from a certain noise contributor to the output? spice simulation doesn't tell me the transfer function, which i need for modelling before the real design.

i mean i can derive a transfer function, but i am not sure if it is correct. it would be great if you can show me an example.

Title: Re: a question about bandgap reference
Post by Forum Administrator on Aug 27^th, 2009, 9:00am

The xf analysis in Spectre will do this for you. Just identify the output and specify "stimuli=sources" as a parameter to the xf analysis. It will then compute the transfer function from every node to the output. The transfer function is computed assuming that a current is injected into the node.

-Ken

Title: Re: a question about bandgap reference
Post by subgold on Aug 29^th, 2009, 7:33am

Forum Administrator wrote on Aug 27^th, 2009, 9:00am:

thanks for the reply, sorry but i still dont get it.
you mean i can get an analytical (NOT nummerical) transfer function from the xf simulation?

Title: Re: a question about bandgap reference
Post by Forum Administrator on Aug 29^th, 2009, 6:15pm

No, it is numerical. It is similar to running an AC analysis.

-Ken