C-multiplier again
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From: Mike on 27 May 2010 12:42 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 27 May 2010 15:34:16 GMT, Mike <spam(a)me.not> wrote: [...] >>The AD587 supplies 10V. There are big electrolytics connected to both >>inputs of the AD797. In the event of a hard short on either input, the >>maximum current could be (10 - 0.7) / 49.9 = 0.186 Amp. >> >>This exceeds the rating by 7 times, so the 49.9 ohm resistor is not >>providing much protection against a hard failure, and it probably can >>be removed. > I'm sure it's there precisely because it does protect the diodes. Against a hard short? How? Since each input has a huge electrolytic to ground, and the voltage difference is close to zero, why not simply add back-to-back power schottkys across the inputs and let them take any surge. > Depletion fets or SSRs make nice current limiters and active > protectors. LND150 is nice but around 2k. Got any less than 50 ohms? > John Mike
From: Spehro Pefhany on 27 May 2010 13:11 On Thu, 27 May 2010 16:42:00 GMT, Mike <spam(a)me.not> wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 27 May 2010 15:34:16 GMT, Mike <spam(a)me.not> wrote: > >[...] > >>>The AD587 supplies 10V. There are big electrolytics connected to both >>>inputs of the AD797. In the event of a hard short on either input, the >>>maximum current could be (10 - 0.7) / 49.9 = 0.186 Amp. >>> >>>This exceeds the rating by 7 times, so the 49.9 ohm resistor is not >>>providing much protection against a hard failure, and it probably can >>>be removed. > >> I'm sure it's there precisely because it does protect the diodes. > >Against a hard short? How? > >Since each input has a huge electrolytic to ground, and the voltage >difference is close to zero, why not simply add back-to-back power >schottkys across the inputs and let them take any surge. > >> Depletion fets or SSRs make nice current limiters and active >> protectors. > >LND150 is nice but around 2k. Got any less than 50 ohms? DN3525 6 ohms at 0V, but your fault current is going to be something like an ampere...
From: John Larkin on 27 May 2010 13:48 On Wed, 26 May 2010 22:43:31 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >John Larkin wrote: >> On Wed, 26 May 2010 18:06:20 -0700 (PDT), MooseFET >> <kensmith(a)rahul.net> wrote: >> >>> On May 26, 7:02 am, John Larkin >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> On Wed, 26 May 2010 06:56:18 -0700 (PDT), dagmargoodb...(a)yahoo.com >>>> wrote: >>>> >>>> >>>> >>>>> On May 26, 8:26 am, Winfield Hill <Winfield_mem...(a)newsguy.com> >>>>> wrote: >>>>>> John Larkin wrote... >>>>> <snip> >>>>>>> How about an opamp powered from Vout, with a resistor from the opamp >>>>>>> output to ground? Let the opamp supply current fight the output >>>>>>> ripple. That's thermally stable, simple, high gain, and tunable. >>>>>>> (except I need regulation, too) >>>>>> +15V >--+--------+--/\/\--+-----> Vout 14.8v >>>>>> | | 4.7R | >>>>>> | R3 | >>>>>> | 2.7M | >>>>>> } | _| >>>>>> | C1 +------| \ >>>>>> '---||---+ | >--+---, >>>>>> 10uF | ,--|__/ | | >>>>>> | | | | | >>>>>> R7 '--- |----' R4 >>>>>> TBD 27k | 4.7R >>>>>> | | | >>>>>> --+--------+--------+---- >>>>>> I see your idea, not bad. It's a nice simplification of this, >>>>>> incorporating the current-sinking transistor into the opamp. >>>>>> +15V >--+--------+--------+----/\/\--+-----> Vout 14.8v >>>>>> | | | 4.7R | >>>>>> | R3 | | >>>>>> | 2.7M | | >>>>>> } | _| | >>>>>> | C1 +------| \ |/ >>>>>> '---||---+ | >------| >>>>>> 10uF | ,--|__/ |\V >>>>>> | | | | >>>>>> R7 '--- |----------+ >>>>>> TBD | | >>>>>> 27k | R4 >>>>>> | | 4.7R >>>>>> | | | >>>>>> --+--------+----------+---- >>>>>> This scheme is DC regulating as well. The class-A current >>>>>> is set by R3 and R7, so the dc voltage drop is fixed. >>>>> Both give line regulation, true. John's problem seems to be that he >>>>> needs(?) load regulation too. >>>> If there's no voltage reference, there's no regulation. >>>> >>>> >>>> >>>> >>>> >>>>> The multi-pole BJT C-mult looks great for feather-weight and constant >>>>> loads. If the ultra-clean part of the load is separable, I'd do that. >>>>> If John really needs low-dropout, 15mA, tight load regulation, and low >>>>> noise, my best shot so far is to bootstrap the op-amp's supplies on >>>>> the Gerber'd "filtered-reference feeding a R-R op-amp" thing he linked >>>>> to, to circumvent the op amp's CMRR / PSRR feeding thru. >>>>> Or, I guess, feed the op amp with a steady voltage, e.g., to make an >>>>> ultra-clean supply, start with an ultra-clean supply... >>>>> Or cascade a couple such op-amp stages, each feeding the next, each >>>>> stage improving PSRR by whatever it can muster. 50-60dB? (I don't >>>>> really trust op amps to have low noise and amazing PSRRs and CMRRs >>>>> over frequency, but then I've not looked at all the latest and >>>>> greatest.) >>>> All I want is a SOT-23 LDO regulator with 1 nv/rthz noise, 140 dB PSRR >>>> to 1 MHz, and not made by Maxim. >>> 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. >>> >>>> John >> >> 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 >> >> > >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. Hey, even better! Very low frequency noise will result from temperature fluctuations wiggling Vf, but luckily that's not a problem in our current situation. John
From: Mike on 27 May 2010 13:55 Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: > DN3525 6 ohms at 0V, but your fault current is going to be something > like an ampere... Actually that's an interesting part. 250V, 300mA, TO-243, -1.5V pinchoff. http://www.supertex.com/pdf/datasheets/DN3525.pdf I'll put that in my reference folder. Thanks, Mike
From: George Herold on 27 May 2010 14:10
On May 26, 4:53 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 26 May 2010 13:03:33 -0700 (PDT), dagmargoodb...(a)yahoo.com > wrote: > > > > > > >On May 26, 10:57 am, Mike <s...(a)me.not> wrote: > >> Winfield Hill <Winfield_mem...(a)newsguy.com> wrote: > > >> [...] > > >> > I see your idea, not bad. It's a nice simplification of this, > >> > incorporating the current-sinking transistor into the opamp. > > >> > +15V >--+--------+--------+----/\/\--+-----> Vout 14.8v > >> > | | | 4.7R | > >> > | R3 | | > >> > | 2.7M | | > >> > } | _| | > >> > | C1 +------| \ |/ > >> > '---||---+ | >------| > >> > 10uF | ,--|__/ |\V > >> > | | | | > >> > R7 '--- |----------+ > >> > TBD | | > >> > 27k | R4 > >> > | | 4.7R > >> > | | | > >> > --+--------+----------+---- > > >> > This scheme is DC regulating as well. The class-A current > >> > is set by R3 and R7, so the dc voltage drop is fixed. > > >> Cancellation schemes give a 6dB/octave drop to a notch frequency, then a > >> 6dB/octave rise. The depth of the notch is extremely sensitive to the > >> emitter resistance and probably the temperature of the transistor. Some > >> examples may show large amounts of second harmonic distortion on the > >> output. This does not appear on the frequency analysis plot. > > >> In this example, the notch frequency is about 2KHz with a depth of -92dB. > >> Try changing the emitter resistance to get an idea of how critical it is. > > >> I don't think you want to rely on this method for any more than a minor > >> amount of cancellation, say 20 dB or thereabouts. > > >> Mike > > ><snip LTSpice model> > > >20dB sounds about right. The advantages of this approach are low drop- > >out voltage and superior low-frequency noise cancellation (compared to > >practical passive equivalents). > > >A big part of the dynamic limitation is the f.f. network rolling off. > >If you change C1 to 100uF, and tack 100uF on the output to cover the > >high-end, overall performance is much improved--nearly as good as a > >passive version using 10,000uF caps, and a lot smaller. > > >For super massive attenuation of input noise and ripple, other > >approaches are better. > > >If John could knock down that 50mV switcher ripple with an LC at the > >input, that's a bonus. But he won't--The Brat would kill him. > > No, I survived. The Gerbered board had... > > Wall wart connector > > Polyfuse > > Transzorb > > 10 uF ceramic > > 47 uH inductor > > two 10 uF ceramics and one 120 uF polymer aluminum to make "+15 > volts." That's 12 dB/octave starting at about 2 KHz. > > Then the LM8261 low-noise LDO reg, which has its own 15 ohms + 2x10uF > + 120uF at its output. > > I also use two Hobbsonian c-multipliers in other supplies that don't > need LDO or regulation. > > Paranoia, groveling for nanovolts. > > But I really need to measure some actual c-multiplier circuits to see > what the Early slopes are like. Could be that LT Spice is grossly > pessimistic. I note here that everyone, including myself, would rather > sit in a swivel chair and simulate and theorize, than get up and > solder and measure. OK this is still a sim. However I spiced the C-multiplier (aka Hobbs filter at teachspin) I use to get down to ~1nV. I haven't every bumped into the Early effect (I don't do much on the transistor level.) But this shows attenuation down near 90dB at 100kHz. (Which is where the switcher I use lives.) George H. Version 4 SHEET 1 1140 1108 WIRE -608 -448 -688 -448 WIRE -448 -448 -528 -448 WIRE 80 -448 -448 -448 WIRE 112 -448 80 -448 WIRE -448 -432 -448 -448 WIRE 112 -400 112 -448 WIRE -448 -352 -512 -352 WIRE -400 -352 -448 -352 WIRE -304 -352 -320 -352 WIRE -224 -352 -304 -352 WIRE -112 -352 -224 -352 WIRE 48 -352 -32 -352 WIRE -688 -320 -688 -448 WIRE -512 -320 -512 -352 WIRE -224 -304 -224 -352 WIRE -448 -288 -448 -352 WIRE -304 -288 -304 -352 WIRE 112 -288 112 -304 WIRE 208 -288 112 -288 WIRE 256 -288 208 -288 WIRE 112 -256 112 -288 WIRE 208 -256 208 -288 WIRE -688 -224 -688 -240 WIRE -512 -224 -512 -256 WIRE -448 -224 -512 -224 WIRE -304 -224 -448 -224 WIRE -224 -224 -224 -240 WIRE -224 -224 -304 -224 WIRE -448 -208 -448 -224 WIRE 208 -176 208 -192 WIRE 208 -176 112 -176 WIRE 112 -160 112 -176 FLAG -448 -208 0 FLAG -688 -224 0 FLAG 112 -160 0 FLAG 80 -448 Vin FLAG 208 -288 Vout SYMBOL npn 48 -400 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL voltage -688 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -512 -448 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 Oh, and thanks again Mike for showing me how to post and read the LTspice stuff... I feel so empowered. > > John- Hide quoted text - > > - Show quoted text - |