c-multiplier, real life
in [Design]
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From: John Larkin on 3 Jun 2010 14:12 On Thu, 03 Jun 2010 16:51:00 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >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. It's the +14 power rail for the front end of a photodiode and the following jfet+opamp gain stages. It's not very sensitive to very low-frequency noise on the power rail, but I do want DC regulation and very little noise above, say, 1 KHz. My ultimate noise level at the jfet input should be around 1 nv/rthz, so the power rail noise shouldn't be much worse than, say, 10 nv/rthz. My ultimate power supply is 15 volts from a switchmode wall-wart, figure a couple of hundred millivolts of crud maybe. So I need maybe 1e7:1 noise rejection from the 15 volts to this rail. The c-multiplier doesn't regulate of course, so its lf rejection primarily depends on the base RCRC thing. At intermediate frequencies, the Early feedthrough dominates, -40 to -60 dB depending on Vce. Once the output lowpass kicks in, at around 1 KHz maybe, we have maybe 2 ohms of Re driving the output caps and their ESR. This is not a simple circuit! There are maybe four pole-zero things in the graph of dB rejection vs frequency. I'm actually using a different circuit for the +14, and Vbe multipliers on other supplies that don't need tight regulation. Some of this is new to me so I'm still learning; there are noise sneak paths everywhere you look. John
From: Jan Panteltje on 3 Jun 2010 14:50 On a sunny day (Thu, 03 Jun 2010 11:12:39 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <p0rf06lll7ev4mbo1rlh5tunl3o89hdjim(a)4ax.com>: >shouldn't be much worse than, say, 10 nv/rthz. My ultimate power >supply is 15 volts from a switch mode wall-wart, figure a couple of >hundred millivolts of crud maybe. So I need maybe 1e7:1 noise >rejection from the 15 volts to this rail. Bought a very nice Chinese switch mode, 12 V 3A, 15 Euro. This looks good enough to supply with professional equipment: ftp://panteltje.com/pub/chinese_12V_3A_switcher_top_img_1985.jpg ftp://panteltje.com/pub/chinese_12V_3A_switcher_bottom_img_1990.jpg Note the Euro mains connector, nice! this one is for LED strips, 60 RGB LEDs per meter, testing now. And the other miracle is that the DC connector fitted my other devices... Actually, to reduce the number or 'wallwarts' I made a multi way cable for the DC side... Should patent that, it frees up mains sockets :-) I dunno yet if this one is short circuit or overload proof, will find out one day.
From: Phil Hobbs on 3 Jun 2010 16:04 On 6/3/2010 10:50 AM, Mike wrote: > George Herold<gherold(a)teachspin.com> wrote: > >> When I tried a Darlington in a Cap-multiplier I found that it reduced >> the DC impedance of the filter. (As one would expect) but that there >> was more noise on the output. Something like 4nV/rtHz versus 1nV/rtHz >> with a 2N3904. I'm not sure what darlington I used... perhaps the >> MPSA14 or BC517. (Those are in my darlington parts drawer.) >> >> George H. > > I'd expect the darlington to be worse, but not by that much! > > Can you explain what you mean by "reduced the DC impedance of the filter"? > > How did you measure the noise? > > Mike Darlingtons are noisy because the driver stage runs at really low emitter current, and hence has a lot of shot noise. You get rid of that with a moderately big bypass cap on the output, which rolls it off at frequencies you care about. Going from 4 nV to 1 nV is the easy part. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Phil Hobbs on 3 Jun 2010 16:08 On 6/3/2010 2:37 AM, Mike wrote: > John Larkin<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 03 Jun 2010 01:52:17 GMT, Mike<spam(a)me.not> wrote: >> >>> Mike<spam(a)me.not> wrote: >>> >>>> 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. >>> >>> Sorry, this is not very clear. The collector-emitter capacitance may >>> be quite low, perhaps 400nF. This means the feedthrough will be small >>> until you get up to 10KHz or so. >>> >>> [..] >>> >>>> What you are trying to do is not trivial. Most people end up with a >>>> shielded box, low noise preamplifiers, and battery operation. >>> >>> Please see what it took Jim Williams to do a 775 Nanovolt Noise >>> Measurement in AN124: >>> >>> http://cds.linear.com/docs/Application%20Note/an124f.pdf >>> >>> Shielded box, low noise preamplifiers, and battery operation. >> >> Mostly overkill for what's almost a microvolt! I wonder why he used >> differential jfets. That's just throwing away 3 dB of noise >> performance. Battery operation is a crutch. That's what good layout and cap multipliers are for! Twenty years ago, everybody thought you had to have linear regulators for sensitive analogue stuff. Not any more. >> >>> >>> And that's only 775nV. I believe you wanted to see down to several nV. >> >> I don't need nV for the c-multiplier here. But nanovolts are easy if >> you do can do narrowband tuning or synchronous detection. JW was >> trying to measure noise. > > Nanovolts are never easy. Again, your goals seem to have changed, but you > do not mention the new ones. > > Narrowband tuning or synchronous detection constrains you to sine waves. <snip> Nonsense. A two-phase synchronous detector is exactly equivalent to a narrow bandpass filter followed by a frequency mixer. The envelope is preserved. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Jamie on 3 Jun 2010 18:37
Jan Panteltje 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? > Head room ? |