c-multiplier, real life
in [Design]
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From: John Larkin on 3 Jun 2010 09:43 On Thu, 03 Jun 2010 06:41:47 GMT, Mike <spam(a)me.not> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >[...] > >> 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. > >He needs low dc drift. His entire circuit is AC coupled. John
From: John Larkin on 3 Jun 2010 09:52 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. 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
From: John Larkin on 3 Jun 2010 09:53 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
From: Mike on 3 Jun 2010 10:50 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
From: Mike on 3 Jun 2010 10:56
John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 03 Jun 2010 06:41:47 GMT, Mike <spam(a)me.not> wrote: > >>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>[...] >> >>> 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. >> >>He needs low dc drift. > > His entire circuit is AC coupled. > > John He is trying to measure very low amplitudes at low frequencies. A single- ended stage would have tens or hundreds of microvolts per degree C shift in the dc level and make it virtually impossible to keep the signal on screen. Differential inputs can give less than a microvolt without chopping. All the low-noise op amps have differential inputs. They still can give less noise than a 50 ohm resistor. Mike |