c-multiplier, real life
in [Design]
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From: Mike on 3 Jun 2010 11:13 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 03 Jun 2010 06:22:29 GMT, Mike <spam(a)me.not> wrote: > >>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Thu, 03 Jun 2010 00:56:16 GMT, Mike <spam(a)me.not> wrote: >>> >>>>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>[...] >> >>> Wild overkill at 400 Hz! >> >>[...] >> >>> John >> >>Sorry, your previous posts said you needed nanovolt-level noise levels >>for the new circuit, and that it would be easy to do averaging and get >>down to 1 nV. > > You keep saying that. What I said on the subject is exactly > > "I don't need nV for the c-multiplier here" > >> >>But you did not mention your goals had changed for this measurement. > > My goal here was to characterize the low-frequency performance of a > transistor used as a c-multiplier. And I did it, in real life, not > simulated. What you have is only part of the picture. You need to get the total attenuation with your 140uF and several ceramics bypassing the emitter to ground. You might be able to play some tricks by increasing the input signal to the max, reducing the bandwidth as much as possible, and averaging. But it is doubtful the AM502 will let you go down far enough to see the resulting noise at the emitter. Then you need to look at the result with the base filter connected to the collector to get the overall performance. Your earlier statement "The Tek AM502 and a digital scope, signal averaging, has a nice signal- recovery capability. This would easily resolve a 1 nV signal" is probably not attainable. > As far as the product we're designing goes, we do need nV front-end > sensitivity. That will measure itself. > I am considering doing an AM502-like box, but much lower noise. 0.5 > nV/rthz might be good. > John That is doable. Multiple low-noise devices in parallel, maybe 4. Get the signal up high enough, then sum the outputs and continue from there. Looks like the source impedance will have to be 15 ohms or less, so I'm not sure what you would measure with it. Mike
From: Winfield Hill on 3 Jun 2010 11:22 Mike wrote... > > John Larkin wrote: > >> I think I did all this right... >> ftp://jjlarkin.lmi.net/C-mult_bb.JPG >> ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG I'll comment on these in a later post. John, what's the 33-ohm resistor for? > Not exactly. The depletion width modulation from the Early effect acts as > a conductance from collector to emitter. The base current and voltage are > not altered, and the shielding provided by the base region has no effect. That's not correct. One can easily manipulate the BJT equations to show this is the case. In fact, in Early's original 1952 IEEE paper, "Effects of space-charge layer widening in junction transistors," he starts by evaluating small changes in collector current vs collector voltage, or a change in conductance if you wish, but immediately he proceeds to analyzing his new base-voltage feedback factor, mu_ec, to get a base-voltage change mu_ec * Vc. Here mu is independent of current, and describes the primary effect. mu has a value of 10^-4 in his measured transistors. mu is supposed to be independent of collector voltage, although Early observed it increasing at very low Vce, and he saw a small increase in mu with increasing collector current. (There's also a small change in rb' base-resistance effect that he spends a page evaluating, but dismisses.) In his 1952 paper Early didn't use the term Early effect, of course, and as it happens he also didn't use the concept of Early voltage, which came later and is used by spice to model bipolar transistors. These are related V_A = V_T / mu_ec, where V_T = kt/q = 25mV, etc. That would mean his 1952 transistors had V_A = 250V, which must be pretty good for old low-voltage germanium parts. Further commenting, you can measure the Early effect through change in current vs Vce, with constant base current (change in beta = Hfe), or as the change in collector current with constant base voltage, which is more Ebers-Moll friendly. They extrapolate back to nearly the same value for V_A on the Vce plot. The theory Early details is based on the base voltage rather than the current gain, however, and I suppose the Early-effect spice modeling must be the same. > This means the transistor collector-emitter can be modeled as a resistor > in parallel with a capacitor. That's correct, but as stated your other assertion is not. It's not very useful to simply model it as a resistance, because the the value r_o changes with collector current, whereas V_A does not. You can calculate r_o = VA/Ic, which works for any current. -- Thanks, - Win
From: John Larkin on 3 Jun 2010 12:24 On 3 Jun 2010 08:22:01 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >Mike wrote... >> >> John Larkin wrote: >> >>> I think I did all this right... >>> ftp://jjlarkin.lmi.net/C-mult_bb.JPG >>> ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG > > I'll comment on these in a later post. > John, what's the 33-ohm resistor for? To keep the NPN from RF oscillating. It may not be needed if the emitter were hard bypassed to ground. Being burned many times, I always add a base resistor to emitter followers, sort of like garlic and vampires. > >> Not exactly. The depletion width modulation from the Early effect acts as >> a conductance from collector to emitter. The base current and voltage are >> not altered, and the shielding provided by the base region has no effect. > > That's not correct. One can easily manipulate the BJT equations to > show this is the case. In fact, in Early's original 1952 IEEE paper, > "Effects of space-charge layer widening in junction transistors," he > starts by evaluating small changes in collector current vs collector > voltage, or a change in conductance if you wish, but immediately he > proceeds to analyzing his new base-voltage feedback factor, mu_ec, > to get a base-voltage change mu_ec * Vc. Here mu is independent of > current, and describes the primary effect. mu has a value of 10^-4 > in his measured transistors. mu is supposed to be independent of > collector voltage, although Early observed it increasing at very low > Vce, and he saw a small increase in mu with increasing collector > current. (There's also a small change in rb' base-resistance effect > that he spends a page evaluating, but dismisses.) > > In his 1952 paper Early didn't use the term Early effect, of course, > and as it happens he also didn't use the concept of Early voltage, > which came later and is used by spice to model bipolar transistors. > These are related V_A = V_T / mu_ec, where V_T = kt/q = 25mV, etc. > That would mean his 1952 transistors had V_A = 250V, which must be > pretty good for old low-voltage germanium parts. > > Further commenting, you can measure the Early effect through change > in current vs Vce, with constant base current (change in beta = Hfe), > or as the change in collector current with constant base voltage, > which is more Ebers-Moll friendly. They extrapolate back to nearly > the same value for V_A on the Vce plot. The theory Early details is > based on the base voltage rather than the current gain, however, and > I suppose the Early-effect spice modeling must be the same. > >> This means the transistor collector-emitter can be modeled as a resistor >> in parallel with a capacitor. > > That's correct, but as stated your other assertion is not. It's > not very useful to simply model it as a resistance, because the > the value r_o changes with collector current, whereas V_A does not. > You can calculate r_o = VA/Ic, which works for any current. In olden days, "Transistor Manuals" had a lot of transistor theory, and datasheets had a lot more data. Lately, the datasheets are skimpy and stuff like Early effect has to be measured experimentally. Spice is nice but I like to verify. John
From: Joerg on 3 Jun 2010 12:42 John Larkin wrote: > On Thu, 03 Jun 2010 10:59:51 GMT, Jan Panteltje > <pNaonStpealmtje(a)yahoo.com> wrote: > >> On a sunny day (Wed, 02 Jun 2010 17:02:45 -0700) it happened John Larkin >> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >> <tjrd06hpdgiafmof0hq05devdqebcecd06(a)4ax.com>: >> >>> I think I did all this right... >>> >>> ftp://jjlarkin.lmi.net/C-mult_bb.JPG >>> >>> ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG >>> >>> >>> John >>> >> LM317 has > 60 Db ripple rejection? >> Why bother with all this? > > It has gobs of output noise. > With a cap on the ADJ pin it isn't too bad but the BCX70K is certainly the component of choice here, nice low 1/f knee. I have used them in cap multipliers as well with laserdiodes. Then, the client increased the power requirement about 8-fold and we had to say good-bye to the BCX70K :-( -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jan Panteltje on 3 Jun 2010 12:51
On a sunny day (Thu, 03 Jun 2010 06:53:27 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <jtcf061tdo76qp51lfrol4vc3327j3113n(a)4ax.com>: >On Thu, 03 Jun 2010 10:59:51 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>On a sunny day (Wed, 02 Jun 2010 17:02:45 -0700) it happened John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >><tjrd06hpdgiafmof0hq05devdqebcecd06(a)4ax.com>: >> >>> >>>I think I did all this right... >>> >>>ftp://jjlarkin.lmi.net/C-mult_bb.JPG >>> >>>ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG >>> >>> >>>John >>> >> >>LM317 has > 60 Db ripple rejection? >>Why bother with all this? > >It has gobs of output noise. > >John Well, your minimum measured noise is about .1 mV I think LM317 with a decent cap on the output can match that. But I have not tried. But from your other posting I see you need nano Volts, so curious what you measure there, the Vbe drop is hugely temp dependent too in your diagram. So, temp changes, if these happen fast, will wipe out your results. |