Low voltage drop capacitance multiplier

Right, an LDO is not suitable for you.

You have two choices, neither one very attractive. You can use a PNP or PMOS pass element with an active-regulator circuit that tracks the average value of Vin, but that's a painful circuit to make and leaves you with a brute-force compensated output. Or you can modify your circuit above to drive the NPN base from a voltage that's 400mV or so above Vin. To get this voltage you'll need to add a simple dc-dc converter with its output stacked on Vin, and appropriately regulate its output. For myself, often in this very situation, I settle for the modest drop across the pass element, and design the follow-on circuitry appropriately.

One way to improve the attenuation-vs-voltage-drop tradeoff is to make the filter active by splitting the base resistor and bypassing it from the output. The resulting 12dB/octave cutoff slope allows you to use lower-value resistors (less base-current voltage drop) and still get improved 120Hz and high-frequency noise attenuation.

. ,---||----------, . | | . Vin ---+------ | ---- C E ---+------ out . | | B . | | | . '-/\\/\\--+--/\\/\\--+---||--- gnd

An issue not always considered in these circuits is, what happens to the transistor's dissipation in the event of a short circuit? Unless Vin current limits at a fairly low current, the transistor may be exposed to a damaging power-dissipation level. I deal with this issue by adding a collector resistor, like a small 3W WW type power resistor. The resistor is chosen for less than 400mV drop at the maximum operating current. The tradeoffs in selecting this protection resistor reveal one more problem to solve in any attempt to design such a circuit with a voltage drop under about 700mV.

One last comment. If this type of filter is used directly after the 60Hz rectifier storage filter capacitor, where the ripple may be 500mV or more, the 0.5 Vbe average drop won't be high enough to allow proper filtering. In such a case, using a logic-level power MOSFET pass element not only gets you the extra voltage you need, but allows using much higher filter-resistor values, and eliminates the need for an awkward current-limit resistor, because the MOSFET delivers high output currents without excessive base-resistor drop.

Following a MOSFET filter stage with a three-terminal regulator is one way to get a very quiet high-current regulated power supply, with micro-volt ripple levels.

(One awkward issue you'll encounter is understanding the MOSFET's subthreshold region of operation to predict the Vgs value. Sadly, this issue won't be dealt with in the FET's datasheet. Moreover, available Spice models won't show the correct Vgs value either. But we do discuss the theory and give you guidance in AoE.)

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 Thanks,
    - Win
Reply to
Winfield Hill
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How about using an inductor?

John

Reply to
John Larkin

That's sometimes used in high-voltage power supplies to get low ripple, where active components wouldn't be practical. But the amp can't be powered directly from the rail it's filtering; it can be powered by a secondary rail that's RC filtered from the main rail!

Since this is a virtual capacitor, it must be able to store enough energy to accomplist the filtering job. That requirement limits what such tricks can accomplish.

John

Reply to
John Larkin

I've used that scheme before, it's useful. But having thought of it myself, I had no idea it had been patented, US 4,710,861. Now I'm trying to remember if I used this before 1986. Ahem, when did the US patent life change from 17 to 20 years?

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 Thanks,
    - Win
Reply to
Winfield Hill

Yeah, but they still rely on loop gain to suppress ripple, which is slow, whereas the cap multiplier relies on low C_ce and high Early voltage, both of which are fast. You can easily get > 100 dB ripple suppression in the tens of kilohertz with a two-pole cap multiplier made from an MMBT3904, which is really tough to do with feedback. For lower dropouts, it's possible to do transformer tricks, or to use a power op amp--google for the Kanner Kap. Still not as good as a cap multiplier.

If by chance you have a negative supply, you can also capacitance-multiply ground instead of Vin, and adjust the offset voltage anywhere you like. The disadvantage is that this spoils the commonality of ground, but it's sometimes worthwhile.

Alternatively, if you can get at a higher (unregulated) supply voltage, you can filter that separately and add a current source from there to the base of the pass transistor to pull it up a bit--maybe you can get Vin-Vout down to 300 mV this way without losing all your beta, at least if the load current is smallish.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

. Vin ---+------------- C E ------- out . | B . | | . '-/\\/\\--+--/\\/\\--+---||--- gnd . | . '----||--- gnd

The optimum configuration to use would appear to depend on the goal. A spice trial with two 430-ohm resistors, two 22uF caps, a 2n4401, and a 500-ohm load (32mA at 16V) shows the two in a dead heat 35dB down at 120Hz, the Sallen-Key form winning at 360Hz (66dB compared to 54dB), and the both-caps-to-ground form winning above 500Hz (52dB compared to 70dB at high frequencies - the latter assumes the cap's esr is less than 1 ohm above say 3kHz). Using a 5k, 3mA load erased the Sallen-Key advantage over two-caps-to-ground above 170Hz (the Sallen-Key was still better by 13dB at 120Hz, where it enjoyed a resonant dip). Above 10kHz the two-caps-to-ground form won by an impressive 35 dB.

In summary, the Sallen-Key appears superior for hum reduction tasks if properly designed, and two-caps-to-ground appears superior for high-frequency noise reduction. And either form dramatically beats using the same total capacitance with one twice-value resistor, typically by 20dB in the critical line-frequency-harmonics region.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Hi,

The "capacitance multiplier" circuit can be used for protecting a sensitive circuit from noise on the supply rail:

Vin _______ _________ Vout | \\_^ R | '-------| C | --------------------

This circuit will drop 1V or so. Is there something with lower voltage drop?

A normal LDO voltage regulator requires a specific Vin value. This is not ideal since if Vin gets too low, it will go out of regulation and (presumably) start to let noise through.

What I want is a circuit that will track Vin, with only a couple of hundred mV drop, suppressing anything above a kHz or so.

Perhaps there is a way to hack a normal LDO to do this?

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John Devereux
Reply to
John Devereux

The newer LDOs from National are unconditionally stable with arbitrarily large output capacitors and any ESR and 10's mV I/O differential, check them out.

Reply to
Fred Bloggs

The Sallen-Key trick gives you a much sharper knee, but because of the sneak path through the cap to the output, it limits your ultimate attenuation. It's usually better just to split R, as you did, but return both caps to ground.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Substituting a MOSFET for the BJT (admittedly with a poor model), using the same values (430 ohms and 22uF), the region of Sallen-Key superiority shrinks, and doesn't occur at a useful frequency unless the values are carefully chosen. The two-caps-to-ground form wins by 35 to 45dB above 1kHz -- a nice result. But of course, when using a MOSFET one wouldn't use small resistors and big electrolytics. If the values are scaled by 10x the Sallen-Key form doesn't look so bad.

If the values are scaled by ~200x to 220k and 0.1uF (makes sense to me), the filter's performance is significantly improved, e.g. -50dB at 120Hz. Surprisingly, the Sallen-Key and two-caps-to-ground forms are nearly identical with a 500-ohm load (both have nice 86dB dip at 360Hz - can we believe that?), and are better than -67dB above 1kHz. With a light 5k load the two-caps-to-ground form pulls ahead by a modest 6dB, above 2kHz. But with the 67 to 86 attenuation results shown by the Spice analysis, I'd want to spend some time fixing the 2n7000 FET's subthreshold model before taking them too seriously.

. 2n7000 . Vin ---+------------- D S ----- out . | G * use a 10V gate-source . | 220k 220k | 0.1 zener if Vin > 20 volts . '-/\\/\\--+--/\\/\\--+---||--- gnd . | 0.1 . '----||--- gnd

Before leaving this investigation, in which I used resistive loads, the issue of capacitive loading should be taken into account. For example, if a three-terminal regulator follows the noise filter, a separate input capacitor is good for the regulator's stability. Or if other "ordinary" linear circuitry follows, that'll certainly need bypass caps.

Note, if a BJT is used, as shown below, a small base resistor or ferrite bead might be wise to dampen RF oscillation, viz,

. ___ . Vin ---+-------------- C E ---+-------|___|--- out . | B | | . | | '----||---+-- gnd . '-/\\/-+-/\\/-+-/\\/-' . | '----||--- gnd . '--||-- gnd

A quick spice look with the MOSFET: adding load capacitors didn't show much difference, more HF attenuation (78dB), and both plots coming together above 20kHz. However, in these high-attenuation regions, poor capacitor esr modeling can create significant errors.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Dropout means loop is saturated so there is no control and all you have left for filtering is RDS,ON x C. But if ESR is milli-ohm and RDS,on is ohms, then something like 40dB attenuation of KHz ripple should be obtainable.

Reply to
Fred Bloggs

Eh- that will only work for the PFET types, the NFET ones charge-pump the gate drive to achieve ultra-low RDS,ON.

Reply to
Fred Bloggs

Right, use brute force. E.g, check out this heavy beast: cat no 193Q, 10H, 500mA, 53ohms, 1kV, 21 pounds.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Hi Fred,

I'll take a look.

But, do they maintain line regulation during dropout (for kHz+ frequencies), in the way that the "capacitance multiplier" does?

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John Devereux
Reply to
John Devereux

REAL MEN don't count on followers having unity gain ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Tube guys don't care...

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Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams

Yes, that's more or less what I've found too. I'm usually much more worried about tens-of-kilohertz crap from SMPS ripple.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

That is one weird circuit. It's certainly low dropout! . . . In --------------------------- Out . | | | . | |\\ | . '-CAP---| --CAP-' . |/ . | Gain -A . 0V ---------------------------- .

I cannot quite get my head around that, I will have to fire up the simulator.

No negative supply I'm afraid.

I can't really do this either.

The man himself... Of course it was your book I got the basic circuit from in the first place.

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John Devereux
Reply to
John Devereux

It would need to be too big, I think. Similarly with just using a BFC.

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John Devereux
Reply to
John Devereux

OK, Thanks. For me also, it does look like it will be simpler to design the follow on circuitry to accept the lower voltage.

I saw that version in Phils book, too! It still leaves you with about a 0.8V drop though.

What do you think about that "Kanner Kap" circuit!?

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John Devereux
Reply to
John Devereux

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