c-multiplier, real life
in [Design]
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From: Mike on 2 Jun 2010 21:52 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. And that's only 775nV. I believe you wanted to see down to several nV. > Anyway, good luck. > > Mike
From: John Larkin on 2 Jun 2010 22:12 On Thu, 03 Jun 2010 00:56:16 GMT, Mike <spam(a)me.not> wrote: >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. I know that the c-mult works at high frequencies. What I wanted to measure is how it works at low frequencies. Re is pretty low, so the output pole is in the KHz range if you use a reasonable cap. Plus Re and the ESR make a divider. 3300 uF wouldn't fit inside my current product. I'm using a 120 uF polymer aluminum cap and some ceramics. > >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. The scope probe is measuring the input ripple, 200 mv p-p. Collector lead. It works fine for that. And I'm signal averaging 64:1 on both scope channels anyhow. > >This measurement will need a coax connector the same as the ones you are >using, with a very short coax to the preamp. The emitter output is via coax, to the AM502. > >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. As you can see, the signals are pretty big. They don't even need averaging... it just makes them prettier. > >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. Wild overkill at 400 Hz! > >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. Not at 400 Hz! > >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. That's calculable. It's the transistor I'm measuring here. I wanted to see if the LT SPice models were in the ballpark. Looks like they probably are. There was some conjecture in a previous thread that teit Early voltages were unrealistically low. > >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. A volt or three of Vce seems to improve rejection. So a resistor from base to ground is good, if you can waste the voltage. > >What you are trying to do is not trivial. Most people end up with a >shielded box, low noise preamplifiers, and battery operation. I think these numbers are good; 140 dB would be tricky, but 66 ain't. But if anybody wants to reproduce them, I'd be delighted. If I have any Darlingtons around, I'll try one of them. Any predictions? John
From: John Larkin on 2 Jun 2010 23:47 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. > >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. This Rigol scope will do digital bandpass filtering, signal averaging, and FFTs. I wonder if you could turn them all on at the same time. I suppose ADC overload would limit how far you could push that. At low source impedances, like this situation, there are opamps like the Lt1028 that have noise below 1 nv/rthz. John
From: Mike on 3 Jun 2010 02:22 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. But you did not mention your goals had changed for this measurement. Mike
From: Mike on 3 Jun 2010 02:37
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. > >> >>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. Even then, you need good low-level preamplifiers. Your AM502 won't be much good down to nanovolt levels. > This Rigol scope will do digital bandpass filtering, signal averaging, > and FFTs. I wonder if you could turn them all on at the same time. I > suppose ADC overload would limit how far you could push that. What is the input noise level? That pretty much determines everything else. When you get into the millions or hundreds of millions of averages, the system will likely drift during the measurement. This will render the results unusable. So you will probably need a good low-noise preamp to boost the signal into the scope. > At low source impedances, like this situation, there are opamps like > the Lt1028 that have noise below 1 nv/rthz. I posted this list earlier. "nV" stands for nV/sqrt(Hz). The maximum differential input voltage is shown, along with any minimum gain or maximum input current if applicable. AD797 : 0.90nV +/-0.7V 25 mA AD8099 : 0.95nV +/-1.8V +/-10mA G >=2 ADA4898 : 0.90nV +/-1.5V LMH6624 : 0.92nV +/-1.2V LT1128 : 0.85nV +/-1.8V +/-25mA LT6200 : 0.95nV +/-0.7V +/-40mA OPA687 : 0.95nV +/-1.2V G >= 12 OPA847 : 0.85nV +/-1.2V G >= 12 > John Mike |