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Old 18th February 2005, 03:15 AM   #1
Mr Evil is offline Mr Evil  United Kingdom
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Default Improving the capacitance multiplier

In my continuing quest to do things slightly differently to everyone else, I've come up with some ideas to improve on the capacitance multiplier. I'm developing it to use with the horn subwoofer I'm planning, which being very efficient will want the lowest possible ripple to keep hum down. I've rejected the idea of a normal voltage regulator on the grounds that they are boring.

The improvements were inspired by R.G's thread on amp output protection where he mentioned using P-channel MOSFETs on the positive rail as switches. This made me realize that the same idea could be applied to a capacitance multiplier to lower the dropout voltage (NPN BJTs need a couple of volts; N-channel MOSFETs need even more), further improving what is already one of the main benefits of capacitance multipliers.

The extra drive circuitry necessary to accomplish this also has the benefit of increasing the effective multiplier due to high input impedance. This allows the use of a smaller 'base' capacitor (what was the base capactor anyway - it's not connected to a base now); small enough to make a film cap possible here. Base capacitors can of course still be large for ridiculously low ripple, but it will then take quite a while for the output to ramp up to full potential.

Using low on-resistance MOSFETs, such as IRF5305/IRLZ34N, dropout voltage can be really very low. As low as 0.1V is possible, but more is needed if very high currents are required. This means vanishingly small power disipation, which is nice.

With appropriate component choice this circuit should be able to provide tens of amps with only mV of ripple and very relaxed heatsink requirements. Regulation is not good, but that's because it's not a regulator. It's a bit more complicated than the standard design, but still has a reasonable component count.
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Old 19th February 2005, 08:41 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
have you tested this circuit yet?
How about an extra CR filter on the supplies to x1 & x4 gates to reduce ripple even further?
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Old 19th February 2005, 05:02 PM   #3
jcx is offline jcx  United States
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Default sometimes boring is better

why not just add voltage references and call your circuit a discrete low drop out series voltage regulator?

in a subwoofer amp you really can't hope for much dynamic headroom from the power supply when the major freq components at the output are much less than the rectified line freq - so a cap multiplier really isn't suited to this app at all
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Old 19th February 2005, 05:42 PM   #4
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Default Re: sometimes boring is better

Quote:
Originally posted by jcx
[B]why not just add voltage references and call your circuit a discrete low drop out series voltage regulator?
Because...that would be......... BORING!!!!!!1


It always mystifies me how people always want to do roundabout things and totally trash performance for the sake of boredom.

Tim
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Old 19th February 2005, 06:28 PM   #5
Mr Evil is offline Mr Evil  United Kingdom
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Default Re: Re: sometimes boring is better

Quote:
Originally posted by AndrewT
Hi,
have you tested this circuit yet?
How about an extra CR filter on the supplies to x1 & x4 gates to reduce ripple even further?
It's only simulated at the moment since I don't have any spare MOSFETs. Come to think of it, I don't think I've ever used small-signal MOSFETs for anything before.

Filtering the multiplier's supply doesn't help much since the output is determined by the voltage across X1/X4 gates and ground. Thus ripple rejection is more effected by putting current sources in the tails instead of a resistor, but it still has only a very small effect.

There are improvements to be had by bypassing R9/R13 with large capacitors, and splitting the input filter in two to make it 2nd order, as seen in the ESP project.



Quote:
Originally posted by jcx
why not just add voltage references and call your circuit a discrete low drop out series voltage regulator?

in a subwoofer amp you really can't hope for much dynamic headroom from the power supply when the major freq components at the output are much less than the rectified line freq - so a cap multiplier really isn't suited to this app at all
Well you're right that there is no advantage for dynamic headroom, but there are multiple other benefits (and disadvantages) for a capacitance multiplier vs. a regulator. For instance:
  • Controlled turn-on: it takes a few seconds for the output voltage to ramp up, which reduces current inrush and may help reduce thumps from the amp.
  • Reduced power disippation: Allows use of relatively low-power components and small heatsinks, which reduces cost.
  • More benign ripple spectrum: Regulators tend to reduce the magnitude of all harmonics evenly, resulting in the output ripple being the same (sawtooth) shape as the input, whereas capacitance multipliers attenuate high frequencies more, resulting in a smooth output ripple.
Also, there is no particular advantage to maintaining a constant supply voltage to most power amp designs, so that's one less benefit a regulator would have.


Quote:
Originally posted by Sch3mat1c



Because...that would be......... BORING!!!!!!1


It always mystifies me how people always want to do roundabout things and totally trash performance for the sake of boredom.

Tim
Yay!

Hey, wait a minute! It's hardly totally trashing performance!


P.S. I feel I should point out that the transistors in the top current mirror are upside down.
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Old 22nd February 2005, 07:22 PM   #6
Mr Evil is offline Mr Evil  United Kingdom
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Since it's going to be a while before I can order those MOSFETs, I made a BJT version just for testing, one side of which is shown in the attached schematic.

Resistor values decreased a little to allow for lower input impedance, and capacitors increased to balance. Removed the current mirror because the extra gain makes no difference.

Not much intersting to say about its performance. As the sims predicted, ripple remains very low (just a few mV) when pulling several amps through it, as long as the dropout voltage is high enough. Required dropout is much higher for the BJT version.
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Old 17th September 2011, 05:24 AM   #7
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Any prototypes made with this?
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Old 17th September 2011, 11:00 AM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally Posted by 454Casull View Post
Any prototypes made with this?
I made a thread when I built this: Finished capacitance multiplier. I suppose I should have put a link to it in here when I did that. I've been using it for 6 years now, and it has served me well.
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Old 14th August 2013, 07:56 AM   #9
jayadev is offline jayadev  India
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Quote:
Originally Posted by Mr Evil View Post
I made a thread when I built this: Finished capacitance multiplier. I suppose I should have put a link to it in here when I did that. I've been using it for 6 years now, and it has served me well.
Mr Evil,
I dont get it.what is a multiplier ?
Why should we call it capacitance multipler ?

I also use somewhat similar circuit in my 12v bench power supply the schematics is a 2n3773 npn transistor biased by a 7805 regulator .the ground pin of 7805 has an 2.2v led and a 5.6 volt zenner doide making the 7805 output 13.1volts.
The good thing is power supply remains hygienically clean of transformer harmonics and impedance and voltage remains dead stable.
Can we call it a capacitance multiplier-How ?
Can it store the charge longer -How ?
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Old 14th August 2013, 08:28 AM   #10
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally Posted by jayadev View Post
Mr Evil,
I dont get it.what is a multiplier ?
Why should we call it capacitance multipler ?

I also use somewhat similar circuit in my 12v bench power supply the schematics is a 2n3773 npn transistor biased by a 7805 regulator .the ground pin of 7805 has an 2.2v led and a 5.6 volt zenner doide making the 7805 output 13.1volts.
The good thing is power supply remains hygienically clean of transformer harmonics and impedance and voltage remains dead stable.
Can we call it a capacitance multiplier-How ?
Can it store the charge longer -How ?
It's called a capacitance multiplier because that's a literal description of what it does. In essence, it consists of an RC filter with the voltage across the C used as the input to an amplifier. The behaviour of the resulting circuit is similar to a real capacitor with a size equal to C x the current gain, hence the name "capacitance multiplier".

Yes, normal voltage regulators do exactly the same thing, but with a voltage reference in place of the RC filter. So I suppose using the same logic, a regulator could be called a "voltage reference multiplier".

Capacitance multipliers don't behave exactly like real capacitors. The big difference is that they can't store energy (a bit like how a Gyrator looks like an inductor, but can't store energy).
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