FWIW on one of the projects I compared noise performance of HC gate used as a comparator against real comparator of comparable speed (LM361). For the same conditions, the gate was about 3 times more noisy.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
Kinda like this. Particularly the right hand part, but maybe the left hand side is handy too.
The op-amps could be TL072, powered by the output rails (since you said you need bipolar anyway, so there will be a complement of this). Startup could be interesting.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
=A0 =A0 |
=A0 =A0 =A0|
=A0 =A0|
=A0|
=A0|
| >> =A0 =A0 =A0 =A0'---||---+----| =A0Q1 =A0 =A0 '--------+
=A0 =A0 =A0R4
=A0 =A0 4.7R
=A0 =A0 =A0|
=A0 =A0 |
=A0 =A0 =A0 |
=A0 =A0|
=A0 =A0|
=A0 =A0|
|. =A0 =A0 =A0 =A0|
|=A0 =A0 R4
=A04.7R
=A0 =A0 |
Yep, two-pole--that's the stuff I was fiddling with whilst you guys were posting...(ASCII takes time!)
Fig. 3 =3D=3D=3D=3D=3D=3D Q1 Q2 2n3904 2n3904
+15V >--+--------. .----+---. .--+---> +13.3V | \ ^ | \ ^ | R1 ----- R3 ----- | 100R | 100R | | | R2 | | | --- C4 +--/\/\/----+ +------' --- 100uF | 100R | | | C1 --- C2 --- --- C3 | 100uF --- 100uF--- --- 100uF | | | | | =3D=3D=3D =3D=3D=3D =3D=3D=3D =3D=3D=3D GND GND GND GNDOutput ripple is LT-Spice undetectable. Zout ~=3D 2ohms.
Fig. 4 =3D=3D=3D=3D=3D=3D Q1 2n3904
+15V >--+----------------+----. .----+-----> +13.3V | | \ ^ | R1 | ----- | 3.3k | | --- C3 | R2 |/ Q2 | --- 100uF +--/\/\/---+---| 2n3904 | | | 3.3K | |>. | | | | | | =3D=3D=3D C1 --- C2 --- +-------' GND 10uF --- 10uF --- | | | R3 =3D=3D=3D =3D=3D=3D 10k GND GND | =3D=3D=3D GNDBuffer Q2 eliminates loading on filter R1C1-R2C2, greatly improving transient response & recovery.
The output at Q2(e) is super-clean, but changes in load current modulate Re(Q1) and the drop across it, so output ripple is somewhat worse than the reference. Zout is the same as Fig. 3.
Early effect isn't as noticeable as Re, so far.
I did a version following Fig. 4 with a one-pole C-mult stage, biased off a divider from Q2(e), and a Sziklai PNP across the whole thing. That means the single-pole stage operates as a cascode and sees no d(Vce) to speak of. 1KHz ripple disappears, and Rout drops to about
0.25 ohms.This version is silly with parts. There needs to be an op amp in there somewhere to greatly simplify things, but it's time for me to turn in. Hopefully these musings will inspire John to continue the fight.
-- Cheers, James Arthur
This is what I have so far:
ftp://jjlarkin.lmi.net/P14_reg.gif
The wall wart is prefiltered by a C-L-C filter that should buy me about 40 dB at the switcher frequency. Then this thing should be good for maybe 100 more. Then I have some more RCs before the photodiodes and a couple of other critical things.
This regulates to 13.4 to allow some headroom here and there.
The LM8261 has about 10 nv/rthz noise, which is a whole nother story.
John
Phil, you mentioned earlier being able to reach 140 db in one stage:
That's a ratio of 10 million to one. How do you do it?
Thanks,
Mike
Oh, thanks, then I guess inverters don't look so good in this domain. No free lunch there :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
P.S.: N-channel depletion mode devices may be more practical here, less hairy during start-up and shutdown because of the +/-20V abs max on the gate and intrinsic diode. BSS159 etc.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Try a low noise audio opamp and a depletion mode n-channel.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Well, I can't spend the rest of my career working on this circuit!
It would be interesting to power an opamp from the output; then there's no PSRR problem. A depletion fet would add enough offset so the thing (might) start up and work. But the fet would be acting almost resistive at low voltage, so the PSRR thing returns with a vengence.
A bipolar PNP, with constant-current base drive (like Tim's circuit) is interesting, but still has Early feedthrough. A strategicly placed resistor could null out most of that.
An RRO opamp, like in my circuit, incorporates the error amp and the PNP, all in one SOT23. The guys who designed it cared about PSRR and had a zillion transistors to work with.
Any attempt to add regulation to a noise filter circuit must inject the noise of the regulator device into the loop; in my case, it's the
10 nv per of the LM8261. The loop comp parts can be tweaked to trade off noise injection versus transient load regulation.I *could* spend the rest of my career on this. Maybe I'll toss another
120 uF polymer cap after the first inductor, and move on with my life.If I keep changing this board, The Brat will take up patricide.
John
Yup, saw the thread too late, when you mentioned "Gerbers due on Monday".
If it works and is good enough, time to let it go. If not you could add a simple one-transistor cap multiplier into the positive supply of the opamp. Three parts, all tiny.
Aha, I thought that it's the brat pushing you on this project :-)
I think my dad would have had a hissy fit if I'd ever done that. He occasionally said "You electrical guys, you don't have a lot of patience, do ya?"
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
That's limited by the CMR of the op amp, though, which blows the whole thing out of the water--no? That's the point of using the BJT in the first place.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Like I said, it's basically C_CE/C_BFC. You pick a transistor with reasonable characteristics at frequencies you care about, drive its base from a really really filtered version of V_CC--with a resistor in series to make sure it doesn't oscillate and doesn't blow up if the input or output gets shorted--and put a BFC at the output. If the transistor has
10 pF C_CB and the BFC is 100 uF, that's 140 dB, provided you look after other stuff such as the Early voltage and the ESR of the output cap. Generally if your application needs more than 100 dB of ripple rejection, you have to be pretty careful.Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
That's assuming that your op amp is running off batteries? Because otherwise its PSR is going to dominate above ~10 kHz.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Opamp is running from the output. Autostart is no problem.
Even if it is, there is RC on the power rail anyway.
VLV
Yeah a bit of resistance is needed. I make these with 2N3904's.. and at certain voltage/ current levels they would sing at 300MHz or so. (Actually I now use 2N4401's and 4403's, these don't seem to sing.. but I've kept the base R. ) I've never measured the rejection ratio, but you can make a ~1nV/rtHz power supply able to deliver several hundered mA's of current, driven from a switcher.
George H.
--and put a BFC at the output. =A0If the transistor has
t -
Um, I thought power was 10*log(what ever / what ever else) and voltage was 20*log(what ever / what ever else) e.g. a reduction in ripple of 46 db = a factor of 200, no?
(I still like my idea of lm334 feeding LM431/tl431 - 4 small components
- 6 if you use 2 x 334)
Is that spiceable? I made a simple circuit with a voltage source driving the base and a cap on the emitter. I tried various transistors such as
2N2222 and 2N2369, and various ESR and ESL values for the cap.The capacitance had little effect on the attenuation floor, but mainly moved the low frequency corner. No reasonable combination of transistors or cap values got below -120dB. The base resistance had little effect. Here's the file if you'd like to show me how it should work:
Version 4 SHEET 1 1140 1108 WIRE -304 -448 -384 -448 WIRE -160 -448 -224 -448 WIRE -128 -448 -160 -448 WIRE -128 -400 -128 -448 WIRE -192 -352 -256 -352 WIRE -384 -320 -384 -448 WIRE -256 -320 -256 -352 WIRE -128 -288 -128 -304 WIRE -32 -288 -128 -288 WIRE 16 -288 -32 -288 WIRE -128 -256 -128 -288 WIRE -32 -256 -32 -288 WIRE -384 -224 -384 -240 WIRE -256 -224 -256 -240 WIRE -32 -176 -32 -192 WIRE -128 -160 -128 -176 WIRE -32 -80 -32 -96 WIRE -32 16 -32 0 FLAG -256 -224 0 FLAG -384 -224 0 FLAG -128 -160 0 FLAG -160 -448 Vin FLAG -32 -288 Vout FLAG -32 16 0 SYMBOL npn -192 -400 R0 SYMATTR InstName Q1 SYMATTR Value 2N2369 SYMBOL voltage -384 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -256 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 38 Left 0 SYMATTR SpiceLine Rser=1 SYMATTR InstName V2 SYMATTR Value 10 SYMBOL current -128 -256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 20ma SYMBOL voltage -208 -448 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value2 AC 1 SYMATTR Value "" SYMBOL cap -48 -256 R0 SYMATTR InstName C1 SYMATTR Value 1000µf SYMATTR SpiceLine Rser=1u Lser=1n SYMBOL res -48 -96 R0 SYMATTR InstName R1 SYMATTR Value 100m SYMBOL ind -48 -192 R0 SYMATTR InstName L1 SYMATTR Value 10n TEXT -216 -536 Left 0 ;'BJT Ripple Filter TEXT -224 -504 Left 0 !.ac oct 100 1 1e7
I'm used to dBV meaning 20*log(volts), namely 100 volts is +40 dBV. It's an absolute level thing like dBm or dBuw.
If a voltage regulator has 1 volt p-p ripple at its input and 10 mV at its output, its PSRR is 100:1 or 40 dB.
I want any switcher noise (or any other noise!) to be below 1 nV in any 1 Hz bandwidth referred to my input. The box is the size of a small sandwich and is full of stuff. A microvolt of switcher noise on a critical power rail would probably make a visible birdie in somebody's data spectrum. Like having roaches in the salad.
If my switcher has, say, 100 mV of ripple, 1e6:1 reduction, 120 dB, to
100 nV, might just do. 140 dB would be better.Milled aluminum blocks this time.
John
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)
John
A picosecond is a terrible thing to waste.
John
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.