EEVBlog #1116 - How to Remove Power Supply Ripple

hi welcome to another fundamentals building-block video today we're gonna take a look at the capacitance multiplier and it's name is a little bit weird but we'll get into why it's called that shortly and this is a bit of a follow-on to my previous video which are linking down below and at the end about the seven double six zero voltage inverter which we looked at which had we talked about the output ripple of such a voltage inverter so let's take a look at ripple pattern like typical power supplies and how to get rid of it because using a capacitance multiplier so what are the ways you can do it that's really effective it's better than a voltage regulator so let's take a look at a couple of scenarios where you might get ripple now you might have say a mains transformer like this you might go into a bridge rectifier like this of course you're familiar with this and then you may have an output capacitor like this of course and you're going to get some like ripple on there and this is very common for example if you're building an audio amplifier for example you want to generate you know positive and negative rails and you want them to be really clean especially for like class-a amplifiers and stuff like that so really you know you want to get rid of that sort of repoint you can increase the capacitance and to do that but really you might want to add some post regulation to that another one might be well you've just got a DC to DC converter like this so you've got just a positive voltage in and you might have either a higher if it's a boost converter or a lower voltage output like this so you know this is like V out and once again you may add some more capacitance onto that but you're gonna get some you know high frequency ripple out because these things typically might be you know tens of kilohertz hundreds of kilohertz even up into the megahertz region but you're going to get you know tens of millions might be typical or even hundreds of millivolts ripple or as per the previous video the classic seven double six Oh charge pump our converter which basically you know has are some charge which switches in capacitors in there and of course you have an output capacitance like this so you've got v+ going in and you've got V out like this but once again it's gonna have some ripple on it and it can be quite high commit tens of millions might be reasonably low for a charge pump converter like this for example or as we saw before it could even be a couple of hundred millivolts you know can really ruin your day especially if you're actually using this seven double 600 to go from say plus 5 volts to minus 5 volts you're inverting that rail so that you can power your op amp from plus 5 and minus 5 volts like that having a 5 volt rail is fine but if you've got a couple hundred millivolts or even tens and milli volts ripple it's not very good ripple but that's ripple on your rail then that can really ruin your day so you want to clean up ripple in you know any of these sorts of cases or you know other cases as well how do you do that well typically you might just go well that's easy Dave oh just whack in a regulator so you know you might have your minus 5 volts here and you might say have a minus 4 volt regulator there just a low dropout regulator that'll be super clean right under your problems ah as it turns out your voltage regulators which are used everywhere to give you what like a supposedly clean regulated hence their name regulated output voltage are actually quite poor at attenuating large amounts of input ripple yes they regulate well it'll give you your precise minus 4 volts or 3.3 volts or whatever voltage you actually are set your regulator to but if you've got tens or hundreds of millivolts of input ripple that you're trying to get rid of how do is can be a poor way to do it because the amount of attenuation of the ripple from input to output depends on not only the type of regulator it depends on the input to output or the dropout voltage of the regulator as you get lower it can potentially get worse here's a graph which can show you a typical of that and it depends on the amount of current on the output as well the high the output current the the less effective in the regulator's going to be and actually attenuating in the ripple if you don't believe me if you don't believe it's actually a problem let's go to the bench I'll show you let's have a look at a typical audio regulator in this case I've got a little sock 23 microchip MCP 1700 it's a 3.3 volt low dropout regulator I've just got an input filter cap and input an output filter cap and no load but let's actually see what happens if we add some ripple like a lot of ripple to the input here let's actually add a 500 millivolts of ripple right look what happens to the output this is 10 millivolts per division that ripple is coming through to the output like that but look what happens if we add a little load 270 ohm resister on there so it's about low and what's that about 13 milliamps or something like that look at the amount of ripple on here check that out it's absolutely terrible we're at 10 millivolts per division 10 20 30 40 50 60 millivolts peak to peak for 500 millivolts ripple input it's not doing a very good job is it and check it out if we AC couple both channels the regulator's still regulating by the way still given 3.3 volts out 50 millivolts per division input ripple here at three hundred and thirty-odd milli volts the output ripple in green is almost the same there's virtually no attenuation of that ripple at all at 10 kilohertz it's hopeless so as I demonstrate on the bench voltage regulators which are otherwise great for typical applications are really not very good at getting rid of high amounts of ripple especially at higher frequencies and high/low just saw it even it like you know the milli amp level tens of million player which is not a very big load if you take the example of the op amp that I gave before well they can take a couple of milliamps easily so just a generating a negative voltage rail for an op amp especially from a charge pump like this that can have you know tens or hundreds of millivolts of ripple and you really want a low amount of ripple on your rail for whatever particular reason it is and there's gonna be a whole dozens of different scenarios low ripple is good like ripples generally a good thing you generally don't want it but in some cases it's critical you want to get rid of it the voltage regulator just doesn't do it enter the capacitance multiplier so let's just consider all these are scenarios the same we've just got ripple and we want to get rid of it what's the easiest way to get rid of ripple well that's easy you have a resistor and you have a capacitor like this so this is in and this is out and depending on the value of the R and the C the larger you make the capacitance the more ripple rejection you're going to get it's going to attenuate your ripple even if you take the case of like a low value resistor low ish like a hundred ohms for example even with a small amount of current on the output 10 milliamps you kind of get a 1 volt drop across that resistor at 10 milliamps that's not terrific especially if you're you know you're generating a negative 5 volt rail and you need say a negative you know almost near negative 5 volt rail an output or even negative 4 you only get a lousy 10 milliamps there and even with that low value of resistor the lower value of resistor you go so as you decrease the resistor value you have to increase your capacitance value often to absurdly high values to get the ripple rejection that you actually want and of course you can add a second stage on here for example you could add another hundred R you can add another one and that works but you've doubled your amount of voltage drop across there for a given current and as you can see a typical RC filter even a multi-stage one is not very effective at all for anything but ridiculously low currents pretty much so one scenario where an RC filter like this or a two-stage RC filter is fine is if you've got say a pulse width modulator and your you want to actually generate a DC voltage from that well this is typically going to go into an op-amp over here like this and the input impedance the op-amp is very high so you're drawing no occurrence so it's not really a problem so you can use you know reasonable values of you know tens it you know 10 microfarad a couple of mic in there and you know hundreds of ohms or 1k resistor or something like that and you can filter out your pulse width modulator down to like bugger-all values that's fine but we want to actually do this for you know tens of millions hundreds of millions even like in the case of like several amps for a big audio amplifier for example so how do we do this yeah we can lower our resistor value down to one ohm or something like that but then the capacitor has to be so ridiculously high in value up to the like farad's range that when you are driving large output currents that it really becomes completely impractical so what if we could multiply the capacitance what if we could use a small capacitor value and somehow multiply it to make it appear bigger hmm we can do that let's take a look we can actually use the same trick you can use with voltage regulators when your regulator just doesn't have enough current capability you can put in what's called a series pass transistor on top of that I'm sure I've mentioned that in a video somewhere so we can do the same thing here we can actually go like this and go into a transistor NPN in this particular case and have it like that and bingo this can be our the out like this and we can get large amounts of current that bypass this resistor like this and we can use a smaller value of capacitor here and you might have noticed this as is a you classic transistor building block circuit the emitter follower because the output is on the emitter of the transistor like that so basically what that does is any voltage here is matched on the output here it's an emitter follower it just follows this value and because the input current of the transistor like this is relatively small because a transistor has current guy oK you've got a smaller smaller amount of current flowing through this resistor it's not zero because it's a BJT it's a bipolar Junction transistor it needs some base current but it has a gain that transistor has a multiplying gain that multiplies the base current to give you a higher collector current and that's where the multiplier comes in here as it turns out this simple configuration which is basically an RC filter with an emitter follower is what's called a capacitance multiplier and some people don't call it that it's just basically an RC filter which is a building block of its own combined with a series pass transistor like this which is again a building block topology circuit of its own so you combine those two and it in effect the capacitance value see here it gives you an effective capacitance value of not just the C but C times beta which is the gain of the transistor hence the name multiplier so the amount of ripple that you get on the output here is equivalent to a capacitance value which is the value you use it's let's say it's 1 microfarad times the gain of the transistor which might be a hundred so you have an equivalent of a hundred microfarads here that's not a good example because Haley you can just whack out a hundred microfarads isn't very big you could whack it in there but you can see that when you get to large amounts of current it can be a really huge benefit so if this capacitance multiplier you can use relatively large values of resistor like this you can use you know in the order of kilo ohms tens of kilo ohms and you know relatively reasonable values of capacitance and again you could actually put in a second stage there too if you are really you know a multiple stage one as well now of course you've only got the capacitance times the beta of the transistor if you use a single bulb bipolar Junction transistor they know particularly have high current gain so yes you guessed it you can actually use once again another classic building block which is the Darlington pair whoop like this there you go that's a Darlington transistor you could ever use like two separate transistors because you might have your favorite big high current pass transistor here for example and just a smaller signal one over here to feed in the base and your Darlington pair actually has a much higher gain so your capacitance multiplied effect is even bigger so you can effectively have you know many farad's of capacitance here easily like you know a Darlington pair might have a gain of a thousand or something like that you can really ramp things up in this sort of scenario so you can really reduce your ripple tip almost negligible levels like half Abby's dick but hey you still might not want to use a BJT because you don't have enough gain you know you really want a small value of capacitance here and really this resistor can't be too high otherwise it can now starve the base current even of a Darlington pair like this so you know if you want really small values of capacitance large values a resistor you guessed it you can get rid of that and you can use a MOSFET like this no workers whatsoever but using a MOSFET you might have like a larger voltage drop or that it depends on what part you choosing and things like that but it means because it's a MOSFET there is no gate in this particular case it's not a gate it's not a base it's a gate you've got no I get current here so this value of resistor can be as high as you want and that means you can use really seriously low values of capacitance to get your attenuation and just like a regular Darlington transistor you could replace it with a slick high pair here it's called which is a compound transistor and I won't go into the advantages and disadvantages between those maybe that could be another video but basically you can even use a single BJT a Darlington configuration BJT alaka'i pair or a mosfet configuration transistor but it works basically the same thing the capacitance value gets multiplied by the transistor gain and you can reduce your ripple to practically nothing it's awesome so I know what you're thinking well if this capacitance multiplier is so magic why don't they just build rank voltage regulators like this well you might notice here that there's no regulator element there's no feedback coming back there's no feedback loop which maintains a regulated voltage so this is not a regulator the output voltage will change with the input voltage and then I'll change with like temperature of the transistor and or you know sorts of things if you're dealing with high power and stuff like that basically it's only use if you want to get rid of Ripple it's not good for regulation so you could get could use this circuit to get rid of the ripple and then use a voltage regulator on the output of that that's a winner but this to use as a voltage regulator doesn't really work it's not the job of a capacitance multiplier so that's pretty cool let's go with some fun on the bench see what happens let's build up our capacitor multiplier we've got a BD one three seven power here fairly typical sort of you know old-school power transistor not particularly high gain anywhere from like 25 up to a hundred ish I've actually measured it at a hundred and we'll do that in a minute but there you go just an NPN power transistor a 1k resistor here for our and the C capacitance here is 470 micro farad and we've got our 270 ohm load so as before we've got about four point two ish faults for where three volts our DC in here with a 500 milli volt peak-to-peak 10 kilohertz signal superimposed on that or ripple so 10 kilohertz ripple at a fairly horrible 500 millivolts and here's our output it is supposed to be a green waveform but it's got cursors on it I'm so it looks yellow but there's our output nice and clean look at that and if we actually go over here and switch it to ACE and we go right down oh we have to 500 micro volts per division look at that it's still there but Wow it's attenuated a lot and that's just with a standard non Darlington transistor we know we're no chicken dinner but if we go back to our DC coupling we're getting about a 3.3 volts output there you can see that there's roughly about one volt drop due to the pass transistor there but as I mentioned it's not regulated so if we change your offset voltage like this look our output changes like that so it is not a regulator it's just to get rid of your ripple and the voltage drop here is going to be dependent upon your load that you've got it's going to be dependent upon your base resistor the type of transistor that you've got and the gain as well so it's you know it just happens to be around about a volt drop in this particular case and if we change our base resistor here or a filter resistor from 1k to 10k for example we'll find there you go we've now got a larger drop like that but of course our corresponding AC ripple should go right down like that but we're basically just down in the noise now yeah there's a lot of noise due to all sorts of crap but you can see that there's basically no ripple that we had there when we had our 1k resistor in there there you go there's our 1k resistor and you can see that so it's a trade off as you increase of the resistor R here it starves the transistor of base current therefore you get a larger voltage drop across the pass transistor but you increase the ripple attenuation due to the just the RC filter ratio and if we go down really wrote low to a hundred Hertz ripple here you see we're 2 millivolts per division it's still not much ripple but of course you can see it actually coming through so once again we're back at the 1k resistor there so if we really even wanted to knock out the 2 millivolts peak to peak ripple here a hundred Hertz then we could change our single transistor to a Darlington pair for example that would have higher gain and then we could use a larger value of resistor for a given capacitance and then filter it out that way or we could increase the capacitor value but we've already got a pretty large 470 micro farad in there so you wouldn't want to go much larger than that unless you had like a big audio amplifier you had plenty of room and all that sort of jazz but here's a little twist at the end let's actually confirm that we can actually get a capacitor multiplier in quote marks does it actually multiply this capacitance here C by the gain of this transistor which I'm going to say is a hundred and I've actually measured it as a hundred well let's have a look the the free the cutoff frequency the minus 3db frequencies should know it's one of the basic our formulas 1 over 2 pi RC that's for your RC filter so for 1k that we've got in circuit and I've changed the capacitor now down to a hundred nano farad here so for 1k and 100 nano ferrets our cutoff frequency should be one point five nine kilohertz so should be 3 DB down at that frequency but because we have a beta or gain of this transistor of a hundred so we should actually get a cutoff frequency of one kilohertz and equivalent to a hundred times that 10 nano farad or 10 microfarads our cutoff frequency should be fifteen point nine Hertz well what do we get let's actually turn it on look I've got my input signal here my input peak to peak ripple is for 70 millivolts I've got it at 1.59 kilohertz here so it should be way below that right because if it is actually a multiplier and it's equivalent to 10 microfarads our cutoff frequency should be fifteen point nine Hertz so we should get hardly any ripple at all what do we get turn it on what what what wha about 310 millivolts or around about that one point three and one point five nine kilohertz frequency our minus three P points so it's 470 times not 0.707 which is about 330 going to be near enough because we don't have much resolution in there so it's the end tolerance in the components of course the minus 3db frequency is not this expected molten capacitor multiplier it's exactly the same formula as the RC circuit why is it so well as it turns out this is why a lot of people don't like the name capacitor multiplier because it doesn't actually multiply this capacitance it's not really 10 microfarads in terms of filtering like this what it does is actually reduce the current through this resistor and hence the current that the capacitor has to smooth out by a hundred times so instead of having the the hole low that we got there about 1213 milliamps or whatever it is flowing through this resistor here we've got a hundred times less than that or about you know a couple of hundred micro amps flowing through this resistor but in terms of calculating your cutoff frequency the formula is actually the same as it is for a normal RC filter it's just that the currents are reduced the capacitor isn't actually multiplied but I guess it depends on how you want to look at it yeah but as far as calculating the frequency not it's exactly the same so capacitor multiplier near you either like that name or you don't so if we actually measure some of the voltages in here we can actually find the gain of this transistor let's just you know not be too precise but across our 270 ohm load resistor here we've got about three point four volts or so that's about you know twelve and a half millions through this load and that twelve and a half millions is coming through the series pass transistor here and if we measure across our 1k resistor there it was about 0.12 volts are about 100 20 millivolts or thereabouts so therefore our 12 milliamps divided by 120 that gives us a gain of about a hundred on our transistor here which would be fairly typical and of course if we put that into a Darlington pair we might get you know an order of magnitude increase in that gain so we might get a thousand times instead of a hundred times for example and of course this is all going to be dependent upon the actual components used and you know and the output load current as well it's going to vary any datasheet for a power transistor will tell you that the gain varies with your collector currents but the good thing is is that we can just demonstrate that we can really reduce the ripple to you know basically negligible levels using this capacitor multiplier circuit or an RC filter with a series pass transistor whatever you want to call it and just for completeness there is actually a variation in the capacitance multiplier that actually uses an op-amp instead of the series pass transistor and it basically works the same way but the thing with that is is that the op-amp can only drive a certain amount of current there might be more stability like type issues and also you're going to be a game bandwidth limited as well so it's not a terrific solution it's not designed for power applications like you get with a series pass transistor so I hope you found that video interesting if you did please give it a big thumbs up and as always you can discuss in the comments down below or over on eevblog comm and thanks to all my patrons over on patreon.com always linked in in the comments down below they often get to see videos early before everyone else thanks catch you next time you [Music]

Channel: EEVblog

Views: 486,065

Keywords: eevblog, video, capacitance multiplier, capacitance multiplier circuit, capacitance multiplier op amp, opamp circuit, tutorial, circuit tutorial, capacitro, capacitor multiplier, transistor circuit, emitter follower circuit, voltage regulator, ripple voltage, rc filter, building block circuit, emitter follower, bjt transistor, mosfet, powewr supply ripple, ripple rejection

Id: wopmEyZKnYo

Length: 27min 5sec (1625 seconds)

Published: Mon Aug 27 2018

Reddit Comments

[u/jfmoses]

This is a great video save for one aspect - he didn’t tell the whole story with respect to voltage regulators. If we look at the data sheet for the MCP1700 (page 8, Figure 2-15), we see that the power supply ripple rejection (PSRR) for the 2.8V version of the part is about -8dB - almost nothing, as he says in the video (and based on Figure 2-14, we have reason to believe the 3.3V version he uses is worse).

However, this is a pretty poor voltage regulator. If you ask the nuts at diyaudio.com to recommend a 3.3V regulator, they’re more likely to suggest something like the LT3045. Consulting its data sheet (page 9, bottom row), we see PSRR closer to -90dB for 10kHz ripple.

So using his example of 500mV input ripple at 10KHz, we should see something like 0.016mV ripple on the output - great! Better even than the 0.5mV he got from the capacitance multiplier.

On the other hand, an MCP1700 costs something like 60 cents, the capacitance multiplier could be had for a dollar or two and the LT3045 goes for something like $12.60. So as with anything, you get what you pay for. A commonly used regulator, the LM317, will get you something like -62dB PSRR at 10kHz, good enough for about 0.4mV output ripple in the above example for around the same price as a capacitance multiplier.

[u/pitchfork_economist]

Thank you for posting this. The concept, theory, and practice are all explained very well. This is an excellent video.