Micpre of Graham
in [Design]
|
From: John Larkin on 22 Mar 2007 16:11 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. John
From: Fred Bartoli on 22 Mar 2007 17:53 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. -- Thanks, Fred.
From: John Larkin on 22 Mar 2007 18:21 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. 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. So Ic certainly won't. It seems to me that base current shouldn't either, but I'm not 100% sure about that. John
From: Eeyore on 22 Mar 2007 18:30 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. Graham
From: Fred Bartoli on 22 Mar 2007 18:43
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. > > Graham > 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. -- Thanks, Fred. |