c-multiplier, real life
in [Design]
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From: John Larkin on 2 Jun 2010 20:02 I think I did all this right... ftp://jjlarkin.lmi.net/C-mult_bb.JPG ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG John
From: Joel Koltner on 2 Jun 2010 20:21 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:tjrd06hpdgiafmof0hq05devdqebcecd06(a)4ax.com... > I think I did all this right... > ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG I think the results line second to the bottom is supposed to be 170uV rather than 170mV. :-)
From: John Larkin on 2 Jun 2010 20:34 On Wed, 2 Jun 2010 17:21:37 -0700, "Joel Koltner" <zapwireDASHgroups(a)yahoo.com> wrote: >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:tjrd06hpdgiafmof0hq05devdqebcecd06(a)4ax.com... >> I think I did all this right... >> ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG > >I think the results line second to the bottom is supposed to be 170uV rather >than 170mV. :-) Right. Typo. The dB rejection is right. The numbers here aren't far from the 2N3904 LT Spice sim I posted a week or two ago. The actual BCX70 is a bit worse than the 2N3904 sim. There must be a way to convert these numbers into Early voltage, but my brain is used up for the day. I barely have the energy left to stagger home and stir up a rum+coke. The Tek AM502 and a digital scope, signal averaging, has a nice signal-recovery capability. This would easily resolve a 1 nV signal. John
From: Mike on 2 Jun 2010 20:56 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > I think I did all this right... > > ftp://jjlarkin.lmi.net/C-mult_bb.JPG > > ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG > > > John 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. This means the transistor collector-emitter can be modeled as a resistor in parallel with a capacitor. In order to get substantial ripple reduction, hang a large electrolytic from emitter to ground. Phil uses a 10uF ceramic. I recommend using the 10uF in parallel with a 3300uf low ESR cap. The scope probe will not be sufficient to measure the ripple. The ground lead has enough inductance to pick up all kinds of noise radiated from the equipment and coax cables. This measurement will need a coax connector the same as the ones you are using, with a very short coax to the preamp. The AM 502 has 25uV noise. If you are planning on measuring 25nV signals, it will require (25e-6/25e-9)^2 averages, or one million. Since you want to find ripple much lower than that, it will take correspondingly greater averaging. The liklihood of drift during the averaging is very high, which will wipe out the results. So your equipment will limit you to a minimum detectable signal level, perhaps in the region of 250nV. I find the leads in your layout are quite long. These will radiate signals and act as antenna. Also, soldering the coax connectors along the edge of the pcb means they will pick up the noise currents that are forced to flow along the edge of the pcb due to skin effect. This is surprisingly effective even at fairly low frequencies, say in the tens of KHz. A better arrangement would be to solder the connectors directly to the copper near the signal. You might be able to bend the legs on the existing ones enough to tilt them up so the coax can be screwed on. Failing that, there are coax connectors with legs that can be soldered vertically to the copper. Or drill a hole and use a bulkhead connector. When you start reaching decent ripple attenuation, radiation from the coax shields will start limiting the results. You will need better coax cables with 100% shielding. Or go to hardline. Another problem is the reference voltage driving the base. When you finish making the ripple measurements, you need to find a way to supply the base with well-filtered voltage from the same supply as the collector. This will give an indication of the overall performance of the ripple filter. The filter in the base circuit will require farily large series resistance, which will give additional voltage drop that is dependant on load current, beta, temperature, and the phase of the moon. This is probably why Phil went with a MPSA14 darlington. What you are trying to do is not trivial. Most people end up with a shielded box, low noise preamplifiers, and battery operation. Anyway, good luck. Mike
From: Joerg on 2 Jun 2010 21:32
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 > > > John > Yay, you re-invented the capacitance multiplier :-) What's a fun generator? I want one of those ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |