Micpre of Graham
in [Design]
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From: Fred Bartoli on 24 Mar 2007 05:05 Winfield Hill a �crit : > John Larkin wrote: >> Kevin Aylward >>> The shot noise is >>> icn = sqrt(2.q.Icdc) acrross the emitter juction transfering >>> through the collecter ibn = sqrt(2.q.Ibdc) across the emitter >>> junction transferring through the base. That's it. Nothing >>> you can do will eliminate this shot noise. Why don't you try >>> it in spice? Hint, it doesn't model have a shot noise in >>> resisters! >> Which is reasonable. >> >>>> So Ic certainly won't. It seems to me that base current >>>> shouldn't either, but I'm not 100% sure about that. >>> Ic and Ib are both inherent. End of story. >> It is *not* the end of story. >> >> If the metal-film resistor jams zero shot noise current into >> the emitter, how can full shot noise emerge from the collector? > > Right, thanks the gods. Kevin needs to take some measurements. > >> I'm not sure about the base current in the case where Ie has >> no shot noise. > > Surely the base-current shot noise is reduced. Worth checking. > I'd have expected the opposite (full base shot noise), so I just took time to check it. Added 1K at the emitter for obvious stability issues (about 150p+1M load). 15V >---. | .-. Rc | | | |1K '-' | | || Adjust +---||--> SA | || for Ve=5V | ___ |/ 10.7V >----|___|--+-----| 2N3904 | |> Rb --- | --- | || ___ | +---||-|___|-> SA SA <---' | || 1K | .-. Re | | | |1K '-' | | === GND Rb measured to 157kR, which is 31.8uA Measurements taken at 10kHz at the emitter: 250nV/rtHz which seemed first wrong but when you realize that with identical voltage drop in Rb and Re, you have Rb = beta.Re that makes a nice by two current divider for the base noise current. So, we have inb = 2*250.10^-9/157000 = 3.18pA/rtHz shot noise current, which translates to 31.65uA Ib. That's wonderfully close to the measured 31.8uA, so the base current see full shot noise. Then, just for sanity check, I measured the base noise voltage (with a jfet buffer stage): 250nV/rtHz as expected. -- Thanks, Fred.
From: Fred Bartoli on 24 Mar 2007 05:30 Fred Bartoli a �crit : > John Larkin a �crit : >> On Thu, 22 Mar 2007 18:30:04 GMT, "Kevin Aylward" >> <kevin_aylward(a)ntlworld.com> wrote: >> >>> John Larkin wrote: >>>> On Wed, 21 Mar 2007 19:44:43 GMT, "Kevin Aylward" >>>> <kevin_aylward(a)ntlworld.com> wrote: >>>> >>>>> Ban wrote: >>>>>> Phil Allison wrote: >>>>>>> "Ban" >>>>>>>> I have attached a link to a commercial product, just to show that >>>>>>>> the art of making a good preamp is not *that* simple. >>>>>>>> http://rapidshare.com/files/21831341/mic_pre_02.png >>>>>>>> >>>>>>> >>>>>>> ** Good pre-amp ? >>>>>>> >>>>>>> With a pair of 2.2 uF film caps ( = 1.1 uF ) in series with the >>>>>>> mic input ? >>>>>>> >>>>>>> Not too good for the noise figure at or below 1kHz. >>>>>>> >>>>>> Why do you think a reactive element increases the noise? >>>>> >>>>> Input shot noise droped accross the capacitor. Say Ic=2ma, hfe=200, >>>>> In=sqrt(2.Ic/hfe.q) = 2pa/rthz. At 1k, Xc = 160 ohms, giving >>>>> 0.3nv/rthz. >>>> Does Ib have full shot noise? If Ie is set by the voltage drop across >>>> a metal-film resistor, it has no shot noise. So I'd expect Ib, some >>>> fraction of Ie, to be shot-noise free as well. >>> I am not sure what you are saying here. Base current and collector >>> current shot noise are modelled as inherent current sources across >>> the respective junctions. External components can't change these >>> values. However, external components, might modify the effect of the >>> inherent shot noise. >> >> OK, simplest case first, an emitter follower: >> >> >> +10 >> | >> | >> | >> c >> +5----------------b >> e >> | >> | >> 1K metal film >> | >> | >> | >> GND >> >> >> Does Ib or Ic have full shot noise? Ie sure doesn't. >> > > Actually that's pretty easy to check with an LF SA (1M input) > > 15V >---. > | > .-. > Rc | | > | |1K > '-' > | > | || > Adjust +---||--> SA > | || > for Ve=5V | > ___ |/ > 10.7V >----|___|--+-----| > | |> > Rb --- | > --- | || > | +---||--> SA > SA <---' | || > | > .-. > Re | | > | |1K > '-' > | > | > === > GND > > > For shot noise to dominate you need: > 2q.I.R > 4kT or R.I > 52mV > That's easily satisfied. > > > Now your SA has about 10nv/rtHz noise floor so you want the measurement > to be higher. > > Shot noise across R will be en^2=R^2.2q.I = 2q.R.RI > or en= sqrt(R) sqrt(2q.R_drop). > > In the above test, we have 5V across each resistor. > Full shot noise should then develop: > en=sqrt(R)x1.27nV/rtHz > > which is 40nv/rtHz for a full collector shot noise. > For base shot noise if you take beta=100, then you'll have 400nV/rtHz, > 800nV for beta=400. > > A small mistake was made there. Since we have identical voltage drop across Re and Rb, we have Rb=beta.Re This makes a current divider for the base noise current. Then we'll see half that predicted noise voltage at the base point, and half too at the emitter point (and obviously too at the collector). -- Thanks, Fred.
From: Kevin Aylward on 24 Mar 2007 07:47 John Larkin wrote: > On Fri, 23 Mar 2007 19:08:28 GMT, "Kevin Aylward" > <kevin_aylward(a)ntlworld.com> wrote: > > >> The shot noise is >> >> icn = sqrt(2.q.Icdc) acrross the emitter juction transfering through >> the collecter >> ibn = sqrt(2.q.Ibdc) acrross the emitter juction transfering through >> the base. >> >> Thats it. Nothing you can do will eliminate this shot noise. >> >> Why dont you try it in spice? Hint, it dosnt model have a shot noise >> in resisters! > > Which is reasonable. > >> >>> So Ic certainly won't. It seems to me that base current shouldn't >>> either, but I'm not 100% sure about that. >> >> Ic and Ib are both inherent. End of story. > > It is *not* the end of story. It is as far as internal shot noise goes. > > If the metal-film resistor jams zero shot noise current into the > emitter, This statement makes absolutely no sense. Internal shot noise has absolutely nothing to do with any ability "to jam" noise free current in from an external source. The simplified model of shot noise assumes from the outset that external sources are noise free. So, its simply irrelevant whether this is via a metal film resister or teletubbies. There is a shot noise generator associated with an ideal, otherwise noise free, DC current source feeding the emitter, irrespective of whence the current originates. This generator is due to the statistical nature of charge carriers crossing a junction. This noise generator is directly across the dynamic impedance re. This drops a voltage right at the base-emitter junction. Well, actually, rather than stating that there is a collector noise dropped across re as one simplified convenient model, it is better to use the output noise equivalent model, that is: icn_shotout = (icn_in.re).gm = icn_in That is, the collecter shot noise is directly at the internal ce nodes. This internal noise current is a constant set by ICDC, irespective of any external circuitary. Now..if there is an emitter resister, a simple calculation on the equivelent circuit will show that the noise actually generated into the external collecter load is: in_load = icn_shotout/(1+Re/re), because the controlled source swips some of the current away. So, in an actual circuit, the output noise current, dropped across the load resister, is reduced by Re feedback. However, again, this noise is not a function of the nature of the driving source or resistance, in addition this reduced output noise does not imply a better S/N ratio, as the signal also gets reduced by Re. Of course, if Re generates additional noise, then this will have to be added into the calculation. > how can full shot noise emerge from the collector? I hope the above explains what Re does, but draw the equivalent circuit and do the algebra by all means. -- Kevin Aylward ka(a)anasoftEXTRACT.co.uk www.anasoft.co.uk SuperSpice
From: Kevin Aylward on 24 Mar 2007 07:51 Winfield Hill wrote: > John Larkin wrote: >> Kevin Aylward >>> >>> The shot noise is >>> icn = sqrt(2.q.Icdc) acrross the emitter juction transfering >>> through the collecter ibn = sqrt(2.q.Ibdc) across the emitter >>> junction transferring through the base. That's it. Nothing >>> you can do will eliminate this shot noise. Why don't you try >>> it in spice? Hint, it doesn't model have a shot noise in >>> resisters! >> >> Which is reasonable. >> >>>> So Ic certainly won't. It seems to me that base current >>>> shouldn't either, but I'm not 100% sure about that. >> >>> Ic and Ib are both inherent. End of story. >> >> It is *not* the end of story. >> >> If the metal-film resistor jams zero shot noise current into >> the emitter, how can full shot noise emerge from the collector? > > Right, thanks the gods. Kevin needs to take some measurements. Why? > >> I'm not sure about the base current in the case where Ie has >> no shot noise. > > Surely the base-current shot noise is reduced. Worth checking. I can only assume here Win, that you haven't actually read what I have wrote and that you are making statements not related to inherent transistor, junction caused, shot noise. To wit, internal base-current shot (and collector) noise cannot be reduced by external components. This is trivially obvious. The effect of such noise may well be modified by the external circuit, as I stated in my original post, and show in my later one to John. -- Kevin Aylward ka(a)anasoftEXTRACT.co.uk www.anasoft.co.uk SuperSpice
From: John Larkin on 24 Mar 2007 13:20
On Sat, 24 Mar 2007 08:36:11 +0100, Fred Bartoli <fred._canxxxel_this_bartoli(a)RemoveThatAlso_free.fr_AndThisToo> wrote: >John Larkin a �crit : >> On Fri, 23 Mar 2007 09:01:11 +0100, Fred Bartoli >> <fred._canxxxel_this_bartoli(a)RemoveThatAlso_free.fr_AndThisToo> wrote: >> >>> Eeyore a �crit : >>>> Fred Bartoli wrote: >>>> >>>>> Eeyore a �crit : >>>>>> John Larkin wrote: >>>>>> >>>>>>> But metal film resistors don't have shot noise current. >>>>>> You mean they're not supposed to surely ? I thought only bulk metal types can be >>>>>> considered to be truly 'noiseless' like that. >>>>> LOL! >>>>> Resistors don't have shot noise. Shot noise comes from the electrons >>>>> getting through a potential barrier, which resistors haven't. >>>>> >>>>> You're probably confusing with excess noise, which shows as 1/F noise >>>>> and is proportional to the current flowing through the resistor since >>>>> it's basically resistor fluctuation. >>>> Yes. They sound similar though. >>>> >>>> Graham >>>> >>> Not quite. >>> Shot noise has a strict flat PSD and is proportional to current, while >>> excess noise PSD is almost 1/F and is proportional to current^2. >> >> Do you mean noise power? >> >> Shot noise current is proportional to the square root of DC current. >> > >Isn't PSD *power* spectral density? I'll take that for a "yes." John |