Author Topic: EEVBlog #1116 - The Capacitance Multiplier  (Read 9559 times)

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Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #50 on: September 02, 2018, 10:44:56 am »
@T3sl4co1l,

I just did a diff on all the parameters (after removing NPN/PNP) and yes they are!  I normally don't use Qucs for these type of simulations (utilizing 3rd party contributed component models.  Guilty parties provided e-mail address).  After this embarrassment,  I probably won't again without checking much closer.  It is great software for many things, especially s-parameter simulations.  Again, sorry for wasting everyones time in this thread.  No good deed (attempted) goes unpunished... why is that almost always true?
 

Offline floobydust

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #51 on: September 02, 2018, 10:45:04 am »
My intuition squawked because of the difference in the two configurations, reasoning the PNP transistor does not contribute with pulsing DC on its emitter, so the Sziklai is using beta of only the NPN for multiplying capacitance.

I went down to 1kHz to see if speed is an issue knowing that both transistor configurations are relatively slow. Very rare to see a Darlington used as a SMPS switch for example.

Perhaps small signal AC analysis is better? If we assume the driver's base is an AC ground, it's acting as a low-pass filter and attenuation would show differences.

No need to apologize, I don't think electronics is about being correct or perfect.
For me the forum is about sharing and learning, I don't care if I'm wrong aside from the troll attack.

Similar transistor models would help in the comparison, I was just lazy and using the ST 2N3055 that comes in the library.

Rod Elliot from Elliot Sound compares the two configurations in audio and switching.
 
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Online T3sl4co1l

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #52 on: September 02, 2018, 10:56:09 am »
@T3sl4co1l,

I just did a diff on all the parameters (after removing NPN/PNP) and yes they are!  I normally don't use Qucs for these type of simulations (utilizing 3rd party contributed component models.  Guilty parties provided e-mail address).  After this embarrassment,  I probably won't again without checking much closer.  It is great software for many things, especially s-parameter simulations.  Again, sorry for wasting everyones time in this thread.  No good deed (attempted) goes unpunished... why is that almost always true?

No worries man, I appreciate taking the time to try. :)

Me, I'm just incessantly paranoid about everything I have to work with: models, simulator quirks, datasheets, tolerances...

Tim
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Offline digsys

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #53 on: September 02, 2018, 02:03:31 pm »
Quote from: radioactive
  ... Any requests from anyone for specific transistors to use? ....
Well, since you're taking orders :-) When I ever get a chance, I want to test these Power FETS AUIRFS-8409-7P at 10A (or something similar).
I can send you a couple (+) if you're interested. I use them in LT4320 bridges, up to 30A + and would love a simpler option for minimizing ripple.
Hello <tap> <tap> .. is this thing on?
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #54 on: September 04, 2018, 03:31:34 am »
Hi Dave,

I do aknowledge that the cap multiplier is a common and useful circuit, but to avoid frustration it is OK to show its limitations, too:

- This circuit is strictly for low frequency ripple removal only. Supression of fast switching spikes will be poor
- For the sake of efficiency, the transistors operate at rather low Vce voltages where there collector capacitance is very large.
  This capacitance feeds the unfiltered input right to the transistor base.
- As a safety precaution, the input of the darlington base should have a current limiting resistor attached. When your output is shorted
  and you have no such resistor, the filter cap will discharge thru the base of the first darlington transistor without a current limit,
  possibly killing it.
- The input resistor is a bit tricky to choose, because a proper value is dependent on transistor beta(s). If its too small, transistors
  almost go into saturation becoming excessively slow, if its too large the voltage drop increases too much. Transistor betas
  can vary a lot from device to device and over temperature, so a resistive divider is probably a better idea (or even a string
  of two diodes from the input, with a resistor going to ground, and then a series resistor plus the cap).


 

Offline jasonhanjk

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #55 on: September 07, 2018, 11:07:43 am »
I'm using this circuit actually. When my signal into the op amp increases, the noise from the output of the transistor goes up too. Otherwise it's actually a good few mV ripple supply.
 

Offline Leo Bodnar

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #56 on: September 09, 2018, 07:39:56 pm »
This is barely scraping into the topic of the discussion but I think it might be useful for practiotioners of ripple removal.

Instead of blocking the high frequency ripple you can shunt it out. Wenzel has an apponote here.

Cheers
Leo

 
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Offline Cerebus

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #57 on: September 09, 2018, 11:20:03 pm »
While some of the circuits in Wenzel's appnote might have fair to good performance the one illustrated above looks pretty mediocre. Surely the highest effective attenuation (with the shunt transistor fully on and theoretically a short circuit) would be that of the divider formed by the 15 ohm resistor and the ~1k to ground presented by the rest of the circuit - i.e. a gain of 0.985 or -0.13 dB. With that relatively huge 1k in the way of the shunt current I don't see how it could be effective under any circumstances. There must be some error in the presented resistor values.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #58 on: September 09, 2018, 11:49:08 pm »
Agreed. The 1K is probably 1Ohm in reality.
 

Offline JS

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #59 on: September 10, 2018, 12:39:14 am »
  With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.

  In this circuit it's not easy to apply superposition, and analyze bias and signal independently. Other way you can think it would be the 15Ω resistor as part of the collector resistor and picking frome where the noise is smaller, or summing cancelling out the noise using an inverted signal.

  The point is the noise rejection capability is given from the DC and the ratio of those resistors, not from the signal itself. Then it's a matter of tweaking the gain for best cancellation and bias point for best efficency, depending on the shape of the ripple.

JS

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Offline Cerebus

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #60 on: September 10, 2018, 01:42:12 am »
  With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.

Into an open circuit, yes. But with any kind of load? Remember, the LM317 is a 1 amp regulator. If it was drawn with a bijou regulator and a specified load, all well and good, but into an unspecified load with an implied maximum load of 1 amp and the shunt regulator capable of diverting at most 1mA/volt? The effectiveness is going to be very dependent on load.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline glarsson

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #61 on: September 10, 2018, 01:54:52 am »
The text under the figure says it us for low current loads. The resistor on the LM317 output also prevents high current.
 

Offline coppercone2

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #62 on: September 10, 2018, 03:18:10 pm »
how many of these problems does using an op-amp in the capacitance circuit eliminate?

how about using a full fledged gyrator with some fast op-amps? (though I had problems with this)

and he says its for instrumentation power supplies. Nothing in instrumentation is going to use 1 amps on the 15V rail unless its something really fast or there are a TON of chips.
« Last Edit: September 10, 2018, 03:40:59 pm by coppercone2 »
 

Offline coppercone2

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #63 on: September 10, 2018, 03:49:26 pm »
  With 1k the circuit is capable of removing about 150mVpp of garbage, as it sais, from a 12V rail, and it will change pretty linearly with the output voltage.

  In this circuit it's not easy to apply superposition, and analyze bias and signal independently. Other way you can think it would be the 15Ω resistor as part of the collector resistor and picking frome where the noise is smaller, or summing cancelling out the noise using an inverted signal.

  The point is the noise rejection capability is given from the DC and the ratio of those resistors, not from the signal itself. Then it's a matter of tweaking the gain for best cancellation and bias point for best efficency, depending on the shape of the ripple.

JS



Am I right in thinking that its biased by like 20uA in steady state w/10V rail?

I think its just a biased common emitter amplifier that uses the ac coupled noise as a input and uses the inverting output to cancel the signal? And its inverting output is current limited by 1k? and its output is isolated by 100k to the input so its not fighting itself
« Last Edit: September 10, 2018, 04:33:03 pm by coppercone2 »
 

Offline Leo Bodnar

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #64 on: September 10, 2018, 08:53:20 pm »
Wenzel circuit uses LM317 just as an example of noisy (and cheap) LDO. 
It's intended use is powering very low noise OCXOs.  The heater is powered from a separate regulator.
Leo

Here is higher current version from the same appnote

Offline b_force

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #65 on: September 10, 2018, 11:22:20 pm »
What I normally do with the standard circuit is using a extra RC filter.
That gives a steeper roll of (24dB/oct) with a Q of 0.5

Another way is actually forming a sallen-key filter.

With these methods you can even get away with smaller capacitors.
« Last Edit: September 10, 2018, 11:39:07 pm by b_force »
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Offline jasonhanjk

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #66 on: September 15, 2018, 06:07:20 pm »
MPC1700 has a ripple rejection of less than -10dB at the frequency of 10kHz. Should use LM317 for your video instead.

https://youtu.be/wopmEyZKnYo?t=352

 

Offline coppercone2

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #67 on: September 16, 2018, 12:34:54 pm »
the secret being unless you want to design for battery life don't use those. the mcp1700 has pretty much portable on 100% of its application suggestions
 

Offline Feynman

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #68 on: November 04, 2018, 07:13:53 pm »
Someone ever tried to tap the switching regulator's feedback signal behind a capacitance multiplier to regulate away its voltage drop? Is there any change the regulator is still stable (careful routing of the feedback signal assumed)?
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #69 on: November 04, 2018, 10:12:02 pm »
I would not be so much afraid of a routing induced problem, but a problem with loop stability.
Personally, I have never seen something like this in the wild.
Maybe some SPICE could help.
 

Online T3sl4co1l

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #70 on: November 05, 2018, 01:23:50 pm »
You don't want to do that within the same loop, the purpose of the C-mult is to have a very low frequency pole.  The controller won't be able to react to its own changes.

There are very few situations where you need perfectly stable output voltage AND low noise, anyway.  Better to address the failings of your circuit, than to nurse it with an ever-more-complicated power supply.

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #71 on: November 05, 2018, 05:16:39 pm »
A capacitance multiplier in the loop makes a lot less sense than a postregulator.

With this, you could
- kill as much ripple as a capacitance multiplier
- have a fast load response
- increase accuracy

There are some Jim Williams Appnotes describing such designs.
 

Offline MT

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #72 on: November 06, 2018, 08:50:24 am »
You don't want to do that within the same loop, the purpose of the C-mult is to have a very low frequency pole.
The controller won't be able to react to its own changes.
Quote
There are very few situations where you need perfectly stable output voltage AND low noise, anyway.

There are many situation where you need perfectly, very god output stability and low noise.
Quote
Better to address the failings of your circuit, than to nurse it with an ever-more-complicated power supply.
yes. Better to have a pre and post regulator and local circuitry regulators.
 

Offline VanitarNordic

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #73 on: January 25, 2019, 01:52:30 am »
How the circuit should be in case of a negative input to the capacitance multiplier?

for example from the 7660 negative voltage generator
 

Offline Wolfgang

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #74 on: January 25, 2019, 03:07:23 am »
Just turn the Zener around and use a PNP transistor.
 


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