Micpre of Graham
in [Design]
|
From: Kevin Aylward on 23 Mar 2007 15:08 John Larkin wrote: > On Thu, 22 Mar 2007 22:53:24 +0100, Fred Bartoli > <fred._canxxxel_this_bartoli(a)RemoveThatAlso_free.fr_AndThisToo> wrote: > >> 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. > > > But metal film resistors don't have shot noise current. Oh dear... >So, if a > transistor's bias current is mostly set by the voltage drop across an > emitter resistor, emitter current won't have much shot noise either. Ahmmm. The voltage being set across a noiseless resister makes no difference whatsoever to the internal shot noise of a transistor. Shot noise is the statistical effect of carriers across the junctions. 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! > 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. -- Kevin Aylward ka(a)anasoftEXTRACT.co.uk www.anasoft.co.uk SuperSpice
From: John Larkin on 23 Mar 2007 17:12 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. If the metal-film resistor jams zero shot noise current into the emitter, how can full shot noise emerge from the collector? I'm not sure about the base current in the case where Ie has no shot noise. John
From: Winfield Hill on 23 Mar 2007 21:45 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.
From: John Larkin on 23 Mar 2007 23:10 On 23 Mar 2007 18:45:05 -0700, "Winfield Hill" <hill(a)rowland.org> 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. > >> 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 checked the datasheet input current noise of a few "simple" opamps (without bias current cancellation) as compared to input bias current, but they all had much more than shot noise, so that was inconclusive. Besides, I'd guess that internal to a linear IC, it's hard or impossible to make a sub-shot-noise current source. I've been interested for some time in the shot noise reduction in metal resistors. I'd expect things like carbon comps, with a short mean free electron path, to be bad, but I can't find decent references and my measurements indicated well below full-shot. Some day when things are slow, I'll have to do better experiments. John
From: Fred Bartoli on 24 Mar 2007 03:36
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? -- Thanks, Fred. |