C-multiplier again

Yup, it's this one:

ftp://jjlarkin.lmi.net/P14_reg.gif

(hope you can see it; some people are reporting trouble accessing my FTP files)

The LM8261 is a great part. Pretty good RRIO opamp, 32 volts, 21 MHz, tons of current drive, stable into any capacitive load.

John

I thought you preferred 'Mary'? :)

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I have an interesting idea. How about a blue LED as the reference. It is a forward biased diode so it may be low noise.

Never had any problems. In fact, I like it that you store them in an economic and compact file format. It irks me when a schematic turns out to be some several megabyte download where a GIF or PNG containing the same information would have been 50k.

Just keep in mind that the output RR capability really falls off the cliff at around 20mA. So with your 15mA you are "dang close" :-)

So, is your circuit good enough now for its purpose?

Here's Walt's article:

Here's some data on leakage in electrolytics.

I = K * C * V ; leakage current

where I, C, and V are standard values.

Here's some values for K:

K = 0.002 ; low leakage electrolytic spec K = 0.02 ; typical electrolytic spec K = 1.89e-5 ; AVX Bestcap spec K = 3.86e-5 ; HiTeck supercap spec K = 5e-7 ; measured 1 Farad supercap K = 7e-7 ; measured supercap K = 8.5e-5 ; 470uf electrolytic caps measured by Win

Supercaps can be two orders of magnitude better than the best electrolytic. Pity the working voltage is so low.

Mike

equivalent

C-E

Decibels are always a power ratio. That formula with a 20 in it is a convenience for when you're measuring everything at the same impedance level, or when impedance isn't particularly relevant, e.g. at an op amp output.

Cheers

Phil Hobbs

I was playing with that. Still quite fiddly. If you don't get the balance exactly right you end up with two attenuation plateaus. It would be difficult to tell when it is adjusted correctly and to keep it there. See the asc file for example.

Any examples or links?

Mike

Version 4 SHEET 1 1140 1108 WIRE -1072 -432 -1088 -432 WIRE -944 -432 -992 -432 WIRE -896 -432 -944 -432 WIRE -832 -432 -896 -432 WIRE -720 -432 -832 -432 WIRE -656 -432 -720 -432 WIRE -496 -432 -576 -432 WIRE -480 -432 -496 -432 WIRE -416 -432 -480 -432 WIRE -832 -416 -832 -432 WIRE -416 -416 -416 -432 WIRE -944 -400 -944 -432 WIRE -1088 -352 -1088 -432 WIRE -416 -336 -416 -352 WIRE -480 -320 -480 -432 WIRE -944 -304 -944 -336 WIRE -832 -304 -832 -336 WIRE -832 -304 -944 -304 WIRE -720 -304 -720 -432 WIRE -752 -288 -768 -288 WIRE -624 -272 -688 -272 WIRE -544 -272 -624 -272 WIRE -1088 -256 -1088 -272 WIRE -832 -256 -832 -304 WIRE -800 -256 -832 -256 WIRE -752 -256 -800 -256 WIRE -832 -224 -832 -256 WIRE -720 -224 -720 -240 WIRE -768 -176 -768 -288 WIRE -736 -176 -768 -176 WIRE -480 -176 -480 -224 WIRE -480 -176 -736 -176 WIRE -480 -160 -480 -176 WIRE -832 -128 -832 -144 WIRE -480 -64 -480 -80 FLAG -1088 -256 0 FLAG -896 -432 Vin FLAG -496 -432 Vout FLAG -480 -64 0 FLAG -720 -224 0 FLAG -832 -128 0 FLAG -800 -256 U1P FLAG -736 -176 U1N FLAG -624 -272 U1O FLAG -416 -336 0 SYMBOL npn -544 -320 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL voltage -1088 -368 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -976 -432 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 WINDOW 3 -2 123 VRight 0 SYMATTR InstName V2 SYMATTR Value2 AC 1 SYMATTR Value SINE(0 0.1 2.111e3) SYMBOL res -496 -176 R0 SYMATTR InstName R1 SYMATTR Value 4.7 SYMBOL res -672 -448 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 4.7 SYMBOL opamps\\1pole -720 -272 R0 SYMATTR InstName U1 SYMBOL res -848 -432 R0 SYMATTR InstName R3 SYMATTR Value 2.7e6 SYMBOL res -848 -240 R0 SYMATTR InstName R4 SYMATTR Value 27k SYMBOL cap -960 -400 R0 SYMATTR InstName C1 SYMATTR Value 1000µf SYMBOL cap -432 -416 R0 SYMATTR InstName C2 SYMATTR Value 100µf TEXT -824 -528 Left 0 ;'Op Amp Ripple Cancellation TEXT -832 -488 Left 0 !.ac oct 100 0.1 4e6

That sounds familiar. Its dynamic impedance (hence Johnson noise) is low. I recently did the math to compare shot noise (which a diode has) to the Johnson noise. If I did it right, the shot noise current dumped into the dynamic impedance is somewhat less than the Johnson noise, so the sum isn't a lot higher than the Johnson noise alone.

I just used two diodes in series to make a low-noise -1.5 volt shunt-type supply. I could have used an LED, which would be cool - they light up! - but I didn't want any stray light inside our box.

John

The polymer aluminums are pretty spiffy. The 120 uF 16V one I'm using is 24 milliohms typ. And it's an affordable surface-mount thing.

John

The included asc file might be interesting. Part of the problem is the gain depends on the bjt Re, which changes with current, temperature, device, and phase of the moon.

If the bjt is replaced by a MOSFET, the gain appears to be much better controlled. Indeed, the correct source resistance for perfect match is now the same value as the VCC series resistor. So this approach might be ideal when two matched resistors in the same case are used.

Mike

Version 4 SHEET 1 1140 1108 WIRE -1072 -432 -1088 -432 WIRE -944 -432 -992 -432 WIRE -896 -432 -944 -432 WIRE -832 -432 -896 -432 WIRE -720 -432 -832 -432 WIRE -656 -432 -720 -432 WIRE -496 -432 -576 -432 WIRE -480 -432 -496 -432 WIRE -416 -432 -480 -432 WIRE -832 -416 -832 -432 WIRE -416 -416 -416 -432 WIRE -944 -400 -944 -432 WIRE -1088 -352 -1088 -432 WIRE -416 -336 -416 -352 WIRE -480 -320 -480 -432 WIRE -944 -304 -944 -336 WIRE -832 -304 -832 -336 WIRE -832 -304 -944 -304 WIRE -720 -304 -720 -432 WIRE -752 -288 -768 -288 WIRE -640 -272 -688 -272 WIRE -624 -272 -640 -272 WIRE -1088 -256 -1088 -272 WIRE -832 -256 -832 -304 WIRE -800 -256 -832 -256 WIRE -752 -256 -800 -256 WIRE -624 -240 -624 -272 WIRE -528 -240 -624 -240 WIRE -832 -224 -832 -256 WIRE -720 -224 -720 -240 WIRE -768 -176 -768 -288 WIRE -736 -176 -768 -176 WIRE -480 -176 -480 -224 WIRE -480 -176 -736 -176 WIRE -480 -160 -480 -176 WIRE -832 -128 -832 -144 WIRE -480 -64 -480 -80 FLAG -1088 -256 0 FLAG -896 -432 Vin FLAG -496 -432 Vout FLAG -480 -64 0 FLAG -720 -224 0 FLAG -832 -128 0 FLAG -800 -256 U1P FLAG -736 -176 U1N FLAG -640 -272 U1O FLAG -416 -336 0 SYMBOL voltage -1088 -368 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -976 -432 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 WINDOW 3 -2 123 VRight 0 SYMATTR InstName V2 SYMATTR Value2 AC 1 SYMATTR Value SINE(0 0.1 2.111e3) SYMBOL res -496 -176 R0 SYMATTR InstName R1 SYMATTR Value 4.7 SYMBOL res -672 -448 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 4.7 SYMBOL opamps\\1pole -720 -272 R0 SYMATTR InstName U1 SYMBOL res -848 -432 R0 SYMATTR InstName R3 SYMATTR Value 2.7e6 SYMBOL res -848 -240 R0 SYMATTR InstName R4 SYMATTR Value 22k SYMBOL cap -960 -400 R0 SYMATTR InstName C1 SYMATTR Value 1000µf SYMBOL nmos -528 -320 R0 SYMATTR InstName M1 SYMATTR Value IRF530 SYMBOL cap -432 -416 R0 SYMATTR InstName C2 SYMATTR Value 1000µf TEXT -888 -528 Left 0 ;'Op Amp MOSFET Ripple Cancellation TEXT -832 -488 Left 0 !.ac oct 100 0.1 4e6

If luck works instead of theory, I'm OK with that.

We'll know in a few weeks.

John

If the diode obeys the diode equation (i.e. low level injection assumed) the noise is exactly half what you'd calculate from applying the Johnson noise formula to the differential resistance. IOW, the junction has a noise temperature of T_J/2.

Cheers

Phil Hobbs

That's it! Thanks. I found in on my computer in a weird scan format, and was starting to ASCII it for the group...

Don't you love that bootstrap for the electrolytics? That's slick.

One thing I bet Walt didn't have--50mV of switcher ripple.

-- Cheers, James Arthur

Thanks, that's clear, but it does seem sort of a lame way to spec op- amps, where impedance is ill-defined, nearly infinite, or otherwise mysterious. Voltage is what we all care about.

John's right about dBV being absolute, as dBmV were also--that's what was haunting the dim cobwebs of ye olde noggin. We made and mixed measurements in dB, dBm, dBV, dBmV, etc., all to quantify the same things (i.e. to measure gain, noise, and power). Converting mixed measurements was annoying.

-- Cheers, James Arthur

Yes, I thought it was pretty neat. As far as switcher ripple, the article was written in 1993. The PC had been out for about a decade, and the switching noise was probably a lot worse than it is now.

Mike

Most people specify voltage gains as 20*log(voltage ratio) and ignore the impedances. That's not strictly correct, as dB are technically

Your use of dBV really points out that we need a way to specify that we are stating something as 20*log(voltage ratio). dBvr or something like that.

Oh well.

John

R3, with nearly a nV/rthz added noise, is unfortunate.

John

It's not clear why it's even in the circuit. I think we can simply use R1, R2 and C1, C2, and forget the rest.

Mike

On a sunny day (Wed, 26 May 2010 19:23:49 -0700) it happened John Larkin wrote in :

Stick it outside then :-)

Here used as power indicator, and 1.5V reference (old LED).