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'[EE]: Noise in spice'
2008\01\01@144718 by Sean Breheny

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Hi all,

One of my favorite programs (which seems to disprove TANSTAAFL) is
LTSpice (aka Switcher CAD from Linear Tech). However, it has me a bit
frustrated lately.

I'm trying to use it to help me design a simple, low-noise audio
amplifier using discrete transistors. It seems that it can do some
noise modeling, but deciphering what it does and does not model is
difficult. It seems that in a normal time or frequency based sim run,
it doesn't include noise at all. When you run the "noise" simulation,
it shows you only noise. It also seems that the only noise it takes
into account is Johnson noise. It doesn't seem to model shot noise at
all (strange as this is fundamental to current flow) and none of the
diode or transistor models included seem to have any flicker noise
parameters (although the sim claims to be able to sim that).

Has anyone here tried to model noise in LTSpice (or any other version
of spice for that matter)?

Thanks,

Sean

2008\01\02@102559 by Paul Hutchinson

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> -----Original Message-----
> From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu On Behalf Of Sean Breheny
> Sent: Tuesday, January 01, 2008 2:47 PM
>
> Hi all,
>
> One of my favorite programs (which seems to disprove TANSTAAFL) is
> LTSpice (aka Switcher CAD from Linear Tech). However, it has me a bit
<snip>
>
> Has anyone here tried to model noise in LTSpice (or any other version
> of spice for that matter)?

IIRC, there was a discussion about this on the LTspice mail list recently.
http://tech.groups.yahoo.com/group/LTspice/

Paul Hutch

>
> Thanks,
>
> Sean

2008\01\02@111404 by Vasile Surducan

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Even you'll be able to simulate the schematic noise, BE SURE the
simulation will not be
equivalent with the reality. On audio amplifiers there are bigger
problems than transistor noise (like ground loop or ground supplying
point on the ground plane relative to the output/input of the
amplifier). You may stop an annoing 50Hz uV noise just by moving 2cm
away the ground wire...
So what the hell are you simulating?

Vasile

On 1/1/08, Sean Breheny <.....shb7KILLspamspam@spam@cornell.edu> wrote:
{Quote hidden}

> -

2008\01\02@194205 by Sean Breheny

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Hi Paul,

Thanks very much! I did find a few messages there relevant to my
question and I will probably post an additional question there.

Sean


On Jan 2, 2008 10:27 AM, Paul Hutchinson <paullhutchinsonspamKILLspamyahoo.com> wrote:
> > {Original Message removed}

2008\01\02@194739 by Sean Breheny

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Hi Vasile,

I'm not trying to get an exact replica of reality. I'm trying to get
an idea for two things:

1) What order of magnitude the noise will be for a particular topology

2) How factors such as transistor bias current and source impedance
will affect noise. These are things which I have a textbook for, but
it would be nice to verify that I understand what the book is saying
by checking my answers in the sim.

Basically, I have a pretty good audio amplifier which I built for a
direct-conversion receiver (general coverage from 0.1 to 30MHz). As it
stands now, it has a differential audio input, 5uV input referred
noise in its 6kHz bandwidth, has a voltage gain of about 10^4, and can
drive headphones. It works well. However, I'm toying with the idea of
either changing the op-amp which forms its input stage or adding a
transistor-based preamp to it, to get the noise below 1uV.

I am very aware of the ways in which interfering signals, ground
loops, power supply noise, etc. can completely swamp thermal noise in
a sensitive audio amp. I've already gotten down to the thermal noise
(5uV is what I would expect from the sum of the resistor Johnson noise
plus the op-amp's spec'd noise), and I'd like to push it a bit
further.

Thanks,

Sean


On Jan 2, 2008 11:13 AM, Vasile Surducan <.....piclist9KILLspamspam.....gmail.com> wrote:
{Quote hidden}

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