Author Topic: EEVBlog #1116 - The Capacitance Multiplier  (Read 9561 times)

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Offline EEVblog

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EEVBlog #1116 - The Capacitance Multiplier
« on: August 28, 2018, 09:07:28 am »
Circuit building block time. The capacitance multiplier and how it gives almost negligible power supply ripple compared to a voltage regulator.
Whiteboard theory and then some bench demonstrations and experiments. Plus a twist at the end that proves that the "Capacitance multiplier" is perhaps one of the most mis-named circuits of all time.

 
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Offline chickenHeadKnob

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #1 on: August 28, 2018, 01:55:22 pm »
Good to see you doing this type of content again. I know you have expressed doubts about  whether the viewer-ship justifies the effort, I merely wanted to offer encouragement that's all.
 
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Offline boB

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #2 on: August 28, 2018, 04:06:34 pm »
TAPCO, a microphone mixer company in the 1970s that I used to work at used this same C multiplier circuit that drove the microphone preamp.

Audio preamps are one circuit that benefits highly from this type of thing.

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Offline sibeen

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #3 on: August 28, 2018, 04:06:42 pm »
Yeah, another shout out for Fundamentals Friday. Thoroughly enjoyed that one.
 

Online wilfred

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #4 on: August 28, 2018, 08:07:32 pm »
Good to see you doing this type of content again. I know you have expressed doubts about  whether the viewer-ship justifies the effort, I merely wanted to offer encouragement that's all.
I'll take quality content any day of the week. It justifies itself for that reason alone.
 

Offline fmiz

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #5 on: August 28, 2018, 09:54:16 pm »
This video gave me an idea...
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?
Adding an inductor in series with R to get a 2nd order filter for the base current... bad idea?
 

Offline digsys

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #6 on: August 28, 2018, 10:04:21 pm »
DAMN ! Now I have to try it on 5A+ linear P/Supplies. I've resorted to experimenting with switching in/out series caps / inductors with a CPU etc
I'm going to be pi$$ed off if it works !! :-)
Hello <tap> <tap> .. is this thing on?
 

Online cvanc

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #7 on: August 28, 2018, 10:08:37 pm »
Very interesting Dave, thank you.
 

Offline T3sl4co1l

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #8 on: August 28, 2018, 10:17:44 pm »
Dr. Phil Hobbs (of ElectroOptical Innovations) loves them for cleaning up dirty supplies (in the frequency range that takes the most space to filter otherwise -- say kHz to low MHz), say for low noise optical front-ends.  Particularly making multi-stage ones, where the base filter RC is cascaded, so each subsequent transistor only incurs a minimum Vce(sat) drop, rather than a full Vbe (if you were to chain stages naively, supplying the next base divider from the emitter of the proceeding stage).

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Offline David Hess

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #9 on: August 28, 2018, 11:25:33 pm »
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?

It does not have a higher high frequency line rejection than a linear regulator so no; it would only serve as a preregulator.

Better would be an LCR filter at the output of the switching regulator or before the linear regulator.

 

Offline T3sl4co1l

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #10 on: August 29, 2018, 12:20:09 am »
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?

It does not have a higher high frequency line rejection than a linear regulator so no; it would only serve as a preregulator.

Better would be an LCR filter at the output of the switching regulator or before the linear regulator.

Sort of.  Check the PSRR of the regulator in question.  LDOs are notoriously poor, while HDOs like the old fashioned LM7805 are okay (only "okay", I wouldn't call them "great").

All(?) active regulators have the problem that PSRR is proportional to loop gain, so PSRR is always dropping at high frequencies.  The C-mult. avoids this, being limited instead by Early effect and strays.

Related:

Some LDOs and references (usually low power kinds) have a bypass pin on them, which is the internal gain node of the voltage reference.  You attach a meaty bypass cap, to give much better noise rejection than would be possible with any on-chip solution.

But, because it's the reference, not the output driver, it's not technically a C-mult. circuit, and PSRR won't improve beyond what the output stage and control loop are capable of.

Tim
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Offline David Hess

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #11 on: August 29, 2018, 12:51:43 am »
Could this block be used to filter the output voltage of a switching power supply, then add a linear regulator to get the same noise level of a linear psu?

It does not have a higher high frequency line rejection than a linear regulator so no; it would only serve as a preregulator.

Better would be an LCR filter at the output of the switching regulator or before the linear regulator.

Sort of.  Check the PSRR of the regulator in question.  LDOs are notoriously poor, while HDOs like the old fashioned LM7805 are okay (only "okay", I wouldn't call them "great").

All(?) active regulators have the problem that PSRR is proportional to loop gain, so PSRR is always dropping at high frequencies.  The C-mult. avoids this, being limited instead by Early effect and strays.

Output impedance of the error amplifier driving the pass element increases with frequency for the reason you identify but this does not have to be the case.  A parallel capacitor could be added to ground and appropriate frequency compensation adjustments made if necessary (1) producing a "regulated" capacitance multiplier.  In the primitive form, this could be the old ubiquitous zener series regulator in the form of a capacitance multiplier with one added zener diode.

The reason this is rarely done (2) is that it can only equal the performance of a capacitance multiplier and if better high frequency rejection is required, there are better ways like adding LCR input decoupling to remove high frequency ripple so each circuit element is applied where it works best.  Now you have a regulator which may approach the performance of a capacitance multiplier at high frequencies but does not need to.

More interesting I think are the capacitance multipliers which used a shunt element instead of a series element.  The audio guys like these but I am unconvinced.

(1) If the capacitance is large enough, then this becomes dominant pole compensation and the circuit becomes a capacitance multiplier with low frequency feedback to produce better load regulation.

(2) Some rail splitters operate this way.  If you squint a little at their schematic, it shows a capacitance multiplier with a class-ab output stage driving an output capacitor.
 

Offline chriswebb

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #12 on: August 29, 2018, 01:47:27 am »
Thank you so much for this video, Dave! Always happy to see more fundamentals videos, since I am still so new to all of this. Felt good being able to completely understand one for once!
Always learning. The greatest part of life is that there will always be more to learn.
 

Online luismh

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #13 on: August 29, 2018, 04:04:16 am »
Thank you Dave!
I was just dealing with this noisy power supply. Impossible to use a bigger capacitor due to the lack of space.
I will try this immediately!
Thanks!
 

Offline T3sl4co1l

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #14 on: August 29, 2018, 04:21:25 am »
Output impedance of the error amplifier driving the pass element increases with frequency for the reason you identify but this does not have to be the case.  A parallel capacitor could be added to ground and appropriate frequency compensation adjustments made if necessary (1) producing a "regulated" capacitance multiplier.  In the primitive form, this could be the old ubiquitous zener series regulator in the form of a capacitance multiplier with one added zener diode.

Yup.  Of course, as history has shown, designers are loathe to touch anything that needs extra capacitors, let alone capacitors that have confusing, complicated requirements, like... "compensation".

I would say the other reason this is rare, is more because you need a big stinking cap to do it, in semiconductor terms.  So you'd be asking for another pin (ain't gonna happen on a TO-220 device, say), plus inviting that opportunity for the user to get it wrong.  But yeah, not having all that much benefit is another drawback.

On that note, there are a few LDOs out there, targeted for RF applications, with impressive PSRR up to and beyond 1MHz (where LC filtering is quite economical to take over at).  But they aren't cheap.


Quote
More interesting I think are the capacitance multipliers which used a shunt element instead of a series element.  The audio guys like these but I am unconvinced.

Mmmh, shunt is fundamentally limited by the gain of the device.  Say you do a BJT shunt amp, collector to supply, emitter to ground, with some base bias, and a coupling cap from supply to base.  This has exactly the impedance 1/Gm.  So, for 10mA bias, say, only 2.5 ohms.  A small electrolytic is better, and by a MHz say, a modest ceramic cap more than dominates.

Whereas a pass device is limited by Early effect and Ccb vs. Cce, which can probably give a much higher ratio.

I suppose you could combine both, so that the capacitor being multiplied, is multiplied twice, in a sense.  You could supply the whole thing from a gyrator or something, so that the source impedance is higher than an equivalent resistor, giving more stopband width and depth for the same bias current.  Doing this for just the base RC filter network, you wouldn't be improving the main pass transistor at all, but you could improve its filtering with respect to bias current, economizing it a little.

Tim
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Bringing a project to life?  Send me a message!
 

Offline radioactive

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #15 on: August 29, 2018, 06:05:14 am »
Dave,  Thanks for the great video on the capacitance multiplier.  I was not aware of this circuit before.  After watching the video, I decided to try and simulate with Qucs.  It seems to work very well in simulation.  Looking forward to trying it in a real design. 

[edit]  removed link to simulation due to suspect component models
« Last Edit: September 02, 2018, 10:47:31 am by radioactive »
 

Offline exit_failure

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #16 on: August 29, 2018, 06:29:08 am »
First of all, thank you a thousand times for that video. This is something that I hadn't seen before but actually very nicely solves a problem I have with my current project.

Not being a native speaker of English, one thing I would appreciate very much is if you would wrote down some of the more unusual or not as easy to understand terminology. For instance, it took me a bit of googling until I found out that the one configuration you mentioned was spelled "Sziklai pair".

Anyway, thanks a bunch again.
 
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Offline AviZiv

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #17 on: August 29, 2018, 12:26:58 pm »
Hi Dave - I was so excited to see this video! About a year ago one phono channel of my NAD stereo preamp stopped working. I found the problem to be a dead transistor on the power supply path and could not understand what it was doing there. I even tested bypassing the transistor and observed that the preamp still worked. NOW I UNDERSTAND :) I'm linking to a partial scan of the schematics and you can see the exact configuration that you showed on the 26V supply going to the MM (moving magnet) module. It always bugged me that I made a repair without understanding it completely. Thank you!

https://flic.kr/p/2axJ7ka

Avi
 

Offline EEVblog

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #18 on: August 29, 2018, 01:13:39 pm »
Not being a native speaker of English, one thing I would appreciate very much is if you would wrote down some of the more unusual or not as easy to understand terminology. For instance, it took me a bit of googling until I found out that the one configuration you mentioned was spelled "Sziklai pair".

Yep, noted, I'll write down such thing in the future.
 

Offline EEVblog

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #19 on: August 29, 2018, 01:17:39 pm »
DAMN ! Now I have to try it on 5A+ linear P/Supplies. I've resorted to experimenting with switching in/out series caps / inductors with a CPU etc
I'm going to be pi$$ed off if it works !! :-)

Be sure to report back!
 

Online ali_asadzadeh

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #20 on: August 29, 2018, 03:57:43 pm »
Thanks dave for sharing ;)

I would suspect that what happens if we just put the cap Multiplier circuit between the R1/R2 that would set the regulators output voltage, does this solve the regulation problem too?
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Online Dubbie

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #22 on: August 29, 2018, 08:07:37 pm »
I really enjoyed this video a lot as well. I hadn’t come across this concept before, and it’s good to hear a nice rundown of the technique.

These fundamental themes are by far my favourite of your videos. Please keep them coming! There are probably hundreds of nifty tricks hidden in application notes or in your brain (behind the boxes of couch feet) that you could turn into episodes.

Good job.
 

Offline AviZiv

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #23 on: August 29, 2018, 10:51:05 pm »
https://flic.kr/p/2axJ7ka
Whats up with C422?

If you are asking why does C422 not go to ground, but rather to the output of the transistor, my own little research suggests that this is a bit more than a simple RC filter. It looks to me like the topology of a Sellen-Key filter like this: https://en.wikipedia.org/wiki/Sallen–Key_topology

Do I have it right? I can't explain exactly how it works though. Need to study this.

It was interesting to note that the board designers placed this filter right up against the module that used that power - probably to clean up the power line as close to the target as possible.

Avi
« Last Edit: August 29, 2018, 10:53:48 pm by AviZiv »
 
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Offline P_Doped

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Re: EEVBlog #1116 - The Capacitance Multiplier
« Reply #24 on: August 30, 2018, 01:56:55 am »
I think a better name for this circuit topology would be resistance multiplier instead of capacitance multiplier.

The reason is that the presence of the current gain element allows the use of a resistor in the R-C filter larger by the current gain and still get the same drop across the R-C network.

Dave finally got to this aspect towards the end when he pointed out the R-C network itself isn't doing anything magical, the f_-3dB is still what you expect by looking at the R-C values, but the current going through the R is reduced by the current gain.

In a similar vein, this brings to mind the approach of using a resistance divider in series with another resistor (call it a compound resistor) as the feedback resistor in an op amp gain circuit.  Dave could talk about that, pros and cons.
 


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