EEVblog #600 - OpAmps Tutorial - What is an Operational Amplifier?

hi welcome to fundamentals Friday today we're going to take a look at the operational amplifier or better known as the op amp really important building block absolutely essential that you understand how they work now there are two ways to learn about op amps one is this way the hard way we don't want to do it that way that sucks so let's get rid of this and let's do it the easy way so what is an op amp or an operational amplifier well the name operational amplifier comes from the fact that when I first developed they were developed to do mathematical operations hence the name operational amplifier and back then we didn't have digital computers they did they use these four hell log computers so analog mathematical operations addition subtraction integration differentiation stuff like that even that real hard calculus stuff op amps could actually do these operations in hardware now this digital software rubbish so that's where they came from so although we don't have analog computers today we still use them for those mathematical operations you can turn an op-amp into an integrator for example you can turn it into a summer which is just an adder and things like that so they're really useful circuit building blocks but the main thing we're going to look at is the operational amplifier as an actual amplifier because that's what they're most commonly used for and probably what you'll mostly use them for as well so an op-amp is essentially just an amplifier yes it can be used for those mathematical operations but essentially what it comes down to is this is a differential amplifier and what that means is that it's got two inputs over here which we'll talk about and an output and it's got some gain in there because amplifiers have again and what it does is it takes the difference between these two input signals amplifies it by its internal gain or what's called open loop gain and gives you an output voltage but op amps really can't be used as differential amplifiers on their own even though that's what they are rather confusing but any aspect you should understand so why can't this be used as just a differential amplifier input signal here output signal with some gain in there well the answer is they're not designed to be used as differential amplifiers as strange as that may seem because they are essentially differential amplifiers that was that was that hard circuit you saw over here before was actually the internal circuitry of an op-amp showing it as a differential amplifier but hey let's forget about differential amplifiers I shouldn't even mention it but it is important to understand the operation of how an op-amp actually works now the reason they don't work is differential amplifiers because the op amp the gain of the natural gain the internal natural gain of the op-amp is enormous and that's the first thing you need to know about op amps is it's almost and not quite infinite but you can think of it as infinitely large it's like millions of times and well the datasheet won't even tell you so if we just tried to use an op amp like this with no external circuitry and just fed you know like one milli volt on the input here the gain is so large that the output voltage is going to be so huge that it's just not a practical device at all so that's why you never see an op amp without any external circuitry or what's called negative feedback so that brings us to our first practical application for the op amp which is a comparator but before we look at that we will look at the symbol here now and op amp is typically drawn as a triangle like this it's got two inputs over here and one input here sometimes it might be flipped depending on the ease of drawing your circuit and the way the signal flows but it's exactly the same thing now these two inputs here one is the positive input is called the non-inverting input easy to remember because it's positive the inverting input is likewise easy to remember because it's negative negative inverts something so that's the terminology you should be using when referring to op amps very important to get the terminology right otherwise just sound like a bit of a deal now there's an output pin here easy and there's two power supply pins a positive and negative one which we'll talk about as well so I mentioned that the gain of an op-amp naturally inside is designed to be enormous almost infinite so what happens if you just feed a voltage on the input here well let's assume that we have one volt on our non-inverting input here and we have 1.01 volts or slightly above 10 millivolts or even 1 millivolt above this one here well the amplifier will actually amplify the difference or attempt to amplify the difference between these two inputs so the output here will be this huge gain like a million times that one millivolt so it'll try an output hundreds and hundreds or thousands of volts and well it can't do it because where your circuits only you know five ten fifteen volts something like that so your output is going to saturate so if you've got one volt here and let's say one point double-o one volts here then your output is going to go boom right up to v+ it's just going to saturate right up at the positive voltage so we've got ourselves a comparator and likewise if you switch switch those voltages around so that the non-inverting input is bigger than the inverting input even by a tiny amount bingo your output is then going to go from positive and it's going to slam right down to the negative rail down here so you can see that it's just used as a comparator it's going to be a very crude comparator and you can use an op-amp as a comparator in a pinch but they aren't quite as good as a proper comparator that you can actually buy they're designed to be comparators but hey you can't actually use op set op amps as comparators but that's what happens if you connect an op amp with no feedback at all and what that's called is the open loop configuration because there is no loop there's no loop the loop is open and we'll close the loop in a minute but with an open loop configuration like that an op amp is just a comparator so now that we've got that little non sequitur the way the key the oddball configuration of the comparator for the op-amp let's have a look at what where op amps come really useful and that's as proper amplifiers now to do that as I said we need to go from the open-loop configuration with no feedback to add in what's called negative feedback and hence the t-shirt negative feedback and once you do that I've had to become incredibly useful and powerful devices now there are two rules with op amps that's all you have to remember it's fantastic this is how easy op amps are if you know these two rules if you remember these two rules you can analyze practically any op amp circuit you can't get into the real nitty-gritty details of the performance of it perhaps but you can look at a schematic you can understand how it works and the two rules are very simple rule number one no current flows in or out of these inputs so there's nothing flowing in or out of these two input pins ever that's it nothing nothing flows in or out regardless of how you connect this circuit up whether it was the open-loop comparator configuration we saw before or whether or not it's a closed loop configuration and inverting or non inverting amplifiers we're going to look at nothing flows in or out rule number two now this rule only applies when you have a closed loop like this it doesn't apply at all to the open loop one we just saw it with the comparator that's why I did the comparator first even though might have been a little bit confusing to start that way most people start op amp explanations with these two rules but I wanted to show you that comparator first because to highlight that rule number two does not apply or only applies to closed loop configurations with negative feedback now rule number two is the op amp does whatever it can internally right now internal circuitry which we won't go into but it does whatever it can to keep these two input voltages the same now the op-amp can't actually change its input voltage it these are inputs it has no way to actually drive a voltage out and keep them the same but it can do it with feedback and that's why this rule only applies to closed loop configurations so the op-amp only has control over its output but if you have feedback it will change this output voltage to make sure this input equals this input here and that's a very powerful rule of op amps and if you see a closed loop configuration like this you can be pretty sure that rule is going to apply so using these two rules let's look at the simplest confirm configuration possible and it's not this it actually has no external components so what it has is the output tied back to the inverting input like this and you feed your signal or your voltage into the non-inverting positive input like that and this is called an op-amp buffer so using our two rules very easy to analyze this op-amp buffer circuit let's say we let's just do DC because op amps the other thing is op amps are DC coupled amplifiers they can amplify DC as well as AC signals very important property so but let's do the DC case we're feeding 1 volt into our non-inverting input here what do we get on the output of our op amp well look rule number two always applies when you're when you've got feedback in a circuit in op amp circuit the op amp tries to keep these two input voltages identical so because of the rule this inverting input here is going to be equal to this pin up here the the op amp will ensure that by driving this output to get this input to match this once I forgot one volt here then we've got one volt here and because it's just connected by a bit of wire we're going to get 1 volt out here that's why it's called a buffer it's not an amplifier it because there is no gain 1 volt in one vote out minus one volume minus 1 volt out whatever the voltage is with in within the limits of the power supply voltages here what use is that well rule number one no current flows in or out of the inputs so nothing no current flows in so if you've got a load over here I don't know it could be some sort of sensor or whatever it could be a low-pass filter for example like you're feeding a pulse with modulated signal from your microcontroller or something like that and then you want to buffer that voltage off there because no current flows into the input this op-amp does not disturb your sensor or your circuit that you're actually trying to do is eat what's called a very high impedance input essentially open circuit so it doesn't disturb anything you hook up to it but the op-amp has a what's called a low impedance output so we can drive in a reasonable amount of current you know milliamps tens of millions that sort of thing some can go as high as a couple of hundred milliamps for your power op amps but it can drive a reasonable amount of current so that's why it's buffering the signal a high impedance signal and giving you a low impedance output just allows you to drive things with a sensitive input like that pretty easy very useful configuration the op amp buffer now the next configuration we're going to take a look at is what's called the non-inverting amplifier and this is where we tame our op amp beast that huge unwieldy gain that changes everywhere with temperature and it's horrible anyway it's got this massive unusable gain in there as a differential amplifier but as a single-ended amplifier that's what single-ended means you feed the input here and it's always referenced to ground we can use this as a single-ended amplifier and we can tame that gain by adding negative feedback on it and I won't explain negative and positive feedback in the mechanisms and how it works because well that's for a more advanced topic but anyway we feed in a feedback resistor here just like we did before it was short it out but we put a resistor in there and we put a resistor back down to ground so what it's doing now is this input the inverting input is in a small portion area this feedback resistor will call RF is always bigger than our one here so we're fence so we've just got a voltage divider here that feeds back a smaller part of the input and that's essentially with negative feedback is you're taking a part of the output and you're feeding it back to the input and there's a very simple formula you need to remember for this non-inverting amplifier configuration and I won't try and derive it but the gain of this amplifier or what's called a V that's the actual terminology used a V is just gain you can use gain gain equals RF the feedback resistor divided by r1 which goes down to ground here plus 1 you've got to add that plus 1 on there so easy if we've got a 9k feedback resistor and a 1k resistor down to ground here our game is 9 K on 1k or 9 plus 1 our gain is equal to 10 so if we feed 1 volt into the input here we'll get 10 volts on the output easy and because we've got positive and negative rails which we'll get into we can feed AC or DC signals into here about ground and so we can feed negative 1 volunteer and we'll get negative 10 volts out so there you go that is the basic configuration of a non-inverting amplifier and you might see weird configurations there might be a capacitor across here or something like that which we won't get into in this one but you know the configuration is the same if you see your input being fed into the non-inverting input and the feedback going back to the inverting input you know that's a non-inverting amplifier and this formula here applies and from this formula you can also see why our buffer amplifier had a gain of 1 before because our feedback resistor is zero with zero ohms so zero on our 1 here which was infinite so zero on over infinity or a very large value is zero plus 1 so I'll gain is 1 that's why our buffer had a gain of 1 Izzie the math doesn't lie so now we get on to the second of our two major configurations we've already looked at the first one which was the non-inverting amplifier the buffer was just a variation of that now we have instead of the non-inverting amplifier we have the inverting amplifier how can you tell it's an inverting amplifier well just like before we could tell it was a non-inverting one by the signal going into the positive input here the non-inverting input hence the name non-inverting amplifier our signal now goes into our inverting amplifier pin so hence it's called an inverting amplifier and you'll notice that I've switched the two symbols around here the positive is now on the bottom now op amp hasn't changed I've just done that visually to you know to make it a bit easier here and that's what you'll commonly find in schematics and CAD packages and all sorts of stuff you might find them flipped around upside down back to front whoopee-doo or going all around the place some pointing down for various feedback paths and all sorts of things it's exactly the same op-amp it's just visually different you can draw it any way you want now our inverting amplifier that this one is we have the same as before we have our feedback resistor we have our negative feedback going to in this case L inverting amplifier pin instead of our non-inverting one so now we are feeding our input into through the resistor here so it's a different configuration our signal is not going directly into the non-inverting pin and this brings up our next really important concept with op amps I really need to understand and here's where rule number one really comes into play in trying to analyze this thing it's called virtual ground stick with me so once again how do we analyze this always go back to your two rules what's our second rule here the op amp tries to keep the input voltage is the same in fact it will if you've got this non inverting configuration and you haven't hit the rails yet so if the amplifiers working normally within normal bounds of your power supply rail these two inputs will always be the same so we're actually connected our non-inverting input down to ground here it's connecting to ground we're for stirred around it's never gonna change so what is the inverting input here going to do well of course rule number two it's going to be identical it's going to be the same so this point is also going to be ground or 0 volts so this seems like almost like a pointless circuit because look at rule number one no current flows in or out so there's no current flowing in or out of that pin and it's ground we've got both pins grounded and no current flows in or out it's almost is that what's the point of having an op-amp it's a very confusing concept but once you grasp it you go oh that's easy and it's quite brilliant so the op-amp you remember does whatever it needs to on the output drives it to whatever voltage positive and negative in order to make sure that this inverting pin here is equal to the non-inverting pin down here makes in the same wear force this pin so I can't change this pin all it can do is change the voltage V and the nature of the feedback resistor here to make this zero and trust me will do a practical measurement of this in a minute and this node here will actually be zero volts this confuses the heck out of a lot of beginners they build up their up an circuit they start probing around and they've got their input signal here you know it's a 1 kilohertz 1 volt sine wave for example and let's say they measure this side of the resistor and the signals there they measure this side of the resistor and it's ground the signals vanished where's it gone Oh strange but true so let's follow this through and use our rules and see if we can analyze this circuit once again the DC case to make it make it easy we've got one volt on the input here positive 1 volt with respect to ground of course now we've said before that trust me we'll measure it later but this pin is going to be ground it is going to be zero volts there always so all we've got is 1 volt across our one here which is one case are we going to have one milliamp flowing through there where does it flow well it doesn't flow down here to ground how can it because no current rule number one no current flows into or out of the input pins so it can't flow through the ground here it has to flow it's going through here it's going somewhere there's one volt across that 1k resistor Ohm's law always it must be obeyed so that current is flowing trust me it can't flow into the input pin when we know it's high impedance so it must be flowing up here like this through this 10k resistor and it's been sourced from the output remember this op-amp has internal circuitry it's got an output buffer so it can actually drive currents into and out of the various suppliers back into there and that is where it's syncing the current to and that's the sneaky part about this our current has now been forced up this node here and is flowing through in this case our our feedback resistor RF which is 10k I've made it ten times larger you'll see why in a minute then it must be flowing through there so we must have a voltage drop across that resistor once again Ohm's law always must be obeyed so if we got that one milliamp flowing through our 10k there we're going to have 10 volt drop across this resistor with positive here and negative here AHA negative this these voltages are with respect to the ground here now here's where it gets a little bit tricky this positive voltage here is we are going to get the +10 volts across that resistor there but because this pin is positive but we're forced we know this pin is zero okay we know it's zero because we're forced it by way of the op-amp action and rule number two here in what's called a virtual ground which I talked about in a minute then we have that means if this is ground this is positive then we've got minus 10 volts coming out of here bingo there's our inverting amplifier one volt in - 10 volts out so our gain our formula AV gain equals our F on our one there is no plus one with the inverting amplifier the plus one only applies to the other non inverting configuration so by way of op amp action we'll call it and negative feedback here this point this node here at the non it at the inverting pin is what's called a virtual ground because typically in this configuration it is actually grounded because we've grounded this pin it doesn't have to be we can feed other voltages into this pin and offset and do all sorts of other stuff but it's still called even if you do feed another pin in here it's still called virtual ground because it's virtual it's not real it's not hard tied if it was hard tied to ground if we actually tied that pin to ground this thing wouldn't work because all of our current would flow through here through this resistor down to ground and around like that and then this output here well it wouldn't know what to do the output would be zero because there'd be zero volts difference in here remember it's still a differential amplifier as such so we've got zero volts difference here we're going to use zero out we'd have no current flowing through here and whatever zero volts out so you can see that it doesn't work unless if you talk tied that hard ground but when it becomes a virtual ground by via nature of the op amp action it all magically works I hope that makes sense because once you get it it's really easy so functionality-wise it's pretty much exactly like the non-inverting amplifier except it inverts and that's it and the gain formula is slightly different but apart from that pretty much works exactly the same but that magic virtual ground is at play in this configuration and of course as with op amps their DC couples that works with DC signals you can just feed in a fixed DC voltage as I said one volt DC n would give minus 10 volts out in this case these value resistors or we can feed in a one volt peak-to-peak or rms sine wave for example about ground so it's centered on ground like this this is the blue waveform here let's just say that's 1 volts not quite the scale but you'll get the idea and then our output will be the inverse of that so when the input Rises the output goes negative because it's an inverting amplifier now of course one of the disadvantages of the inverting amplifier compared to the non-inverting we saw before is that as you can see there is input current coming from your load here so you don't want to use this where you have a high impedance load because then it can change the gain equation and MUX everything up that's where you want a non-inverting amplifier or at least a buffer some people will actually for will put a buffer on the input here and then drive than Verdean amplifier but usually in that sort of case you'd probably use a non-inverting amplifier now we have to go deeper into this and talk about the power supplies and split rails and all this sort of stuff and single supply op amps I'll try and keep it as brief as possible but you saw in this configuration the op amp only has 2 power pins okay it's usually called v+ + v - now V - you can actually connect that to ground there is nothing regardless of what the data sheets tell you there's nothing inherent in op amps that make them really a single supply op amp so you can take an op amp that is v+ + v - and connect this down to ground like that there's nothing to stop you as long as you meet the minimum voltage specification and don't exceed the maximum etc so what happens if we did that in this case our input is or our non-inverting input is also grounded here well now it becomes a problem you get into the practical limitations of op amps we've been talking about what's called an ideal op amp up until this point these rules here aren't strictly true I lied then but there's still a fantastic way even professionals used to analyze these circuits as a first order as a first pass no current flows in or out well if you've been watching my videos you'll know I've done a previous video on this talking about input bias currents a little itty-bitty teeny-weenie currents can flow into and out of these pins depending on what type of op-amp you're actually got and that's a real practical limitation of these things and the other one is that I talked about in previous video which I'll link in down below if you haven't seen it the inputs cannot necessarily go right to the rails be it whether it's positive negative reference to ground or whatever so you can get what's called it rail to rail op amps or rail to rail input op amps in this case if you had a rail to rail input op amp then yeah you might be able to get away with this and have the invert and have the non-inverting input tied down to ground like this part hang on what's the point of that if you've only got ground this is an inverting amplifier it inverts your signal so if you feed one volt in you're going to try a the op amp is going to try and give you 1 or minus 10 volts out but how does it do that when your supply is negative like that it doesn't work so you have to it's got no room to do it so your op amp has to always be powered in the configuration that you expect your input signals to be reference to so if we were to use and the inverting op amp configuration like this with a single supply rail like this and we wanted to amplify AC signals well the signals can't go negative like this so your negative on the input but you're never going to get that negative voltage on the output but you still want to amplify your signal clearly like this but what we need to do is this zero point needs to go right down the bottom here like this so we need to offset so that's zero volts we need to offset our input wave our input and output reference by a certain amount of voltage how much well tip Hathi a supply rail to maximize your Headroom how do we do that I hinted at it before you feed in if this is V Plus you would go V Plus on two you would feed that voltage half rail in there he'd usually do that simply by putting a resistor like that going to V plus and a resistor down there going down to ground and bingo voltage divider there's your half rail so we're offsetting our voltage here our virtual ground remember this is still called a virtual ground even though it's not going to be so the voltage here is going to be equal to the voltage here due to our second op-amp rule so if our power supply is 20 volts for example this point here would be half that if we make these you know exactly the same value of course make them the same very half rail so we're going to have an offset voltage here at this point and that shifts they'll waveform up and we'll see that in the practical experiments to follow now as I said some time back you might see some other components around here like some capacitors and things like that around the circuit that is to change the bandwidth of the circuit effectively because we're not going to go into it I have to do a second part of this video that goes into op amp bandwidth and things like that I have done one on costing out cascading op amp bandwidth switch I'll link it down below but suffice it to say that an ideal op amp that we've been looking at has an infinite bandwidth it's infinite frequencies and signals but in practice no of course not your practical op amp might have a 1 megahertz bandwidth or 100 kilohertz bandwidth or something like that you know it could be a nice fast hundred megahertz but it's always going to have a bandwidth which changes with your gain or gain bandwidth product and I've done a separate video I'll link it in but sometimes you might see a little bypass cap in there it might be in a 10 path for a hundred puffs or something like that and that's just a rolling off the frequency response of that and likewise you might see a little cap across something like this for example if you have if you are offsetting this thing using a single supply like this you know I go wait go into the details but basically any noise on this point here will be amplified and picked up on that virtual ground so you'll get noise on your output signal so you might stick a big ass you know one or 10 microfarad cap across here for example and really make that virtual ground really noise free but hey that's that's beyond the basics what little mistake I noticed oops my formula here for the inverting amplifier it needs a negative in front of it because the gain is actually native so it's so the gain is not in this case is not 10 K is not 10 it's minus 10 oops so just back to this voltage rail thing briefly because it is something that is rather confusing because there is no ground pen on an op amp there's only the positive and negative so well where does your reference go well the reference is part of the external circuit in this case back to our non-inverting amplifier configuration here's our ground reference here and then our positive and negative supply is here like this o plus 15 volts and minus 15 volts if we want to feed in a signal that goes both positive and negative if we're only feeding in a signal that is positive above ground then this here could be tired down to here like this and then it has to be above that the output cannot magically go negative it can only go negative to your ground reference if you have that minus 15 volt rail in there clear as mud and just like the inverting configuration if we wanted to power this from a split supply we could have this grounded like this and then we can add a bias voltage in here like this to actually offset the voltage and then you can get into all sorts of weird and wonderful things with AC coupling these amplifiers all of the op amp configurations we looked at have been DC coupled but you can actually AC couple them so that's why you start might seeing capacitors on the inputs and outputs to the op amps now here's a tricky configuration which I'll briefly touch on that come binds the two different configurations we've seen before and a couple of the things we've looked at its the differential amplifier you know how I said op amps are essentially a differential amplifier that's how they work but you have to but they do that in the open loop configuration so they're hopeless they're useless for that but if you combine the knot of the inverting amplifier configuration that we just saw so we've got the feedback going here our signal going in that's a standard inverting configuration and we have exactly those two resistors that we saw before to bias that voltage up but instead of going to the supply rail we make that our other differential input and bingo it becomes a differential amplifier I'll let you go through the actual calculation yourself to find out but basically the difference that we're feeding in if we're feeding in one volt into here and one point one volts into here we have a difference of naught point one volts and the gain of this amplifier exactly like the inverting configuration negative r2 on our one we used RF before I'll call it R 2 e so R 2 on our one 10k on one K we have a gain and you're going to add negative in there so it's a gain of minus 10 but because our bias voltage is not fixed it's actually the differential input signal uh-huh look what happens we got one volt here we've got our divider here r1 these two values are the same r1 is equal to r1 here r2 is equal to r2 here they must match precisely to get good common mode rejection ratio which we won't go into but suffice it to say if we've got one vote on this point here relative to ground we'll have naught point 9 o-- 9 o-- 9 o-- 9 repeat up at that point there and that becomes our virtual ground bingo I have that same voltage there and then we have a one point one volts here that has X and then you subtract that from that that and you get X amount of current flowing through here which then must flow through the 10k which has 1.0 not 909 voltage across it subtract the difference there it's exactly the same configuration as before with the biased voltage but then we'll left with an put voltage of minus 1 so I've amplified the difference in our input signal by the gain here 10 it's not a terrific differential amplifier but it works so we've timed our op-amp that is a differential amplifier anyway but pretty unusable we've actually made it into a pretty usable differential amplifier Beauty just combines both those techniques and there's lots of tricky stuff like this you can do with op amps and just briefly another one of these tricky configurations goes back to their name the operational amplifier and one of those mathematical operations the integrator won't go in integrals and all that sort of stuff but what we can do a basic inverting configuration here except instead of a feedback resistor we have a feedback capacitor what does that do well our standard input voltage here following the rule no current flows in but we have a virtual ground of course rule number 2 so if that's 1k and that's one volt there where we have one milliamp flowing through that resistor where does it flow kind of flow into the op-amp it's got to flow up here and through the capacitor so you've got effectively a constant current of 1 milliamp you've just made this is now a constant current flowing there through this resistor and when you have a constant current flowing through a capacitor you'll end up with a well in this case it's going to ramp negative down like that if our input go if our input is a step and it goes up like that the constant current because it takes time to charge your capacitor the voltage on the capacitor will increase like that I say increase because it's an inverting amplifier so it's going to go negative but that's what it does and that's an integrator and that is actually a mathematical integral of your input signal anyway that's way too much Theory more than I wanted to do and longer than I wanted to take actually but suffice it to remember that these two rules of op amps allow you to analyze practically any configuration and as a bit homework I go recommend you look at the summon op-amp configuration the summing amplifier and figure out how it works because you're going to be using those two rules to figure it out so I'll leave that one up to you but enough of that let's head on over to the bench here and see if we can measure some stuff make sure I wasn't bullshitting you about this virtual ground stuff let's check it out sounds a bit suss see if it really works all right we're at the breadboard let's take a look at an inverting amplifier here because I wanted to show you that virtual ground point there just to show you that there really is no signal there it actually vanishes in quote marks when you go from the input here to here and then it magically reappears at the output because that's how an op-amp works as I've explained anyway it got a jellybean lm35 eight here it's actually a dual op-amp so we've just start tied off the terminate at the top op-amp here could probably do a separate video on that on how to properly terminate op amps that might make an interesting video thumbs up if you want to see that one anyway here we go I've got it configured I've got a 10k input resistor here 100k feedback so we've got a gain of 10 the formula of course is the feedback resistor on that one bingo easy times 10 so I'm going to feed 2 volts peak-to-peak input here we should get 20 volts peak-to-peak on the output so we're using pretty much near the maximum supply rail of the ln 3 v 8 in this case on parent from plus minus 15 volts so we have a split supply so our ground reference our input signal is reference to ground I should actually draw that on there there we go that's clearer so our input is referenced a to ground and our non-inverting input here is reference to ground and they'll output is reference to ground also but for signals to go negative for output signals to go negative we need a negative rail on here so we're using minus 15 volts so plus 15 to power at minus 15 as well so 30 volt total supply on there allows us to go positive and negative signals input and output let's go over to our power supply here it is plus minus 15 also got dual tracking on there and you notice that I've joined the supplies here generating a split supply so this one actually becomes the negative so this is our positive 15 from here to here and this is our negative 15 relative to here because we've strapped the positive one over and tada there we go we're feeding in now one year we've just got a 1 kilohertz low frequency signal 2 volts peak-to-peak here on the input and you can see our input and output waveforms and these imports are of course all our AC coupled and their bandwidth limited as well to 20 megahertz to reduce the noise and we're using our high resolution mode as well so we get some boxcar averaging in there and that's why we've got a nice crisp waveform like that beautiful so what happens if we turn our bandwidth back to full in this case it's my 1 gigahertz at Tektronix 3000 series and we turn off high-res mode go back to sample mode there we go we get our nice fuzzy waveforms because we've got that massively high bandwidth that's the advantage you can go into averaging of course but high-res mode does boxcar averaging just cleans it up of course you can do envelope mode look at that pretty horrible waveform so when looking at this sort of stuff you definitely don't want to use your regular mode you want high-res mode if you've got it there you go we're getting exactly what we expect look at that 2 volts peak-to-peak in roughly 20 volts out there's probably going to be some error due to the resistors in here anyway we can it out times 10 and of course the blue the blue waveform there is the input that's 500 millivolts per division so we get in our 2 volts peak-to-peak and our output is 5 volts per division so which is the yellow waveform there and look at that and of course because it's an inverting amplifier the output is exactly 180 degrees out of phase it's inverted so at the moment I'm probing the input and the output now you want to see the virtual ground in you what happens if I move my input probe the blue waveform here from the input over to this you'd expect to see the signal but as I have told you and as you should trust me let's move the probe over that is our virtual ground point look flatters attack the signal has vanished magic but of course you know it's not magic it's just standard op-amp behavior with virtual ground on the input that's how an op-amp works and know the current hasn't magically vanished the current is going through the resistor Ohm's law still holds current is changing because we've got an AC resistor here there's AC current flowing through this resistor and it's all flowing up here but this point by nature of the op-amp action and the negative feedback that is a virtual ground our op-amp rule number two their inputs are the same the op-amp changes the output here in order to ensure that that point is equal to that input there easy and that's why we don't see any signal on there so trip for young players when you're probing around circles like this don't think is signals vanished virtual ground remember your op-amp rules always now actually chose the lm35 eight for a reason because it is not like a regular op-amp and not quite like a rail-to-rail op-amp it's sort of halfway in between check it out here we go it eliminates the need for dual suppliers okay you can use it as a single supply op amp but as I said you can use any op amp as a single supply op amp but this one is extra special in that allows direct sensing near ground so then V out also goes to ground so effectively it's it's not rail derail it won't go up to that all the way to the positive rail on the important output but it will go down to ground or the negative Orvil and because an op-amp doesn't have a ground pin it's the negative rail so even if we power it from split suppliers plus minus 15 like we are now it'll still go down to that minus 15 volt pin all that pin 4 it'll go down the input will this input here will allow to sense all the way down to the negative rail and also the output will go all the way down to the negative ray'll and I'll demonstrate but we've got to look at here as a couple of things on the datasheet now input common mode range and a voltage range here as we said it goes all the way down to that negative pin or zero volts as they're calling it here but on the positive side this op-amp will not go since all go to the output less than 1.5 volts below or above one point 5 volts below the positive rail v plus there so if we've got an output signal of 10 volts for example the voltage range says if we want to get an output voltage of 10 volts peak well we need a v+ rail of at least 1/2 volts above that so 11 point 5 volts so what we're going to do is lower the voltages here on these rails we're going to lower V Plus from 15 volts down to 11 point 5 and around about that 11 point 5 volts because we're getting 10 volts peak on the output 20 volts peak-to-peak 10 volts peak we should start seeing distortion or clipping of our waveform at around about 11 a half volts let's see if we do okay so here we go we're 15 volts I'm going to drop it down by 0.1 volts at a time notice that it's lit supply its dual tracking so our waveform is still looking good still looking good but we expected to start clipping around about 11 1/2 it may not be precise this is not an exact value on the datasheet but there we go live and a half it's still there still there there we go it's starting to clip it's starting to clip you can see it it's actually about eleven point two volts there but you can start to see that waveform flatten out now I wind down even more because this is a not a symmetrical supply op am it actually goes down to zero we don't start seeing clipping on the bottom here on the bottom rail until a significant time after that now we're getting both but I wind it back up there and that's about eleven point one volts but we're seeing that clipping on the top and we won't see it on the bottom the time after so there you go just be aware of that and if we had a a were even a worse op-amp in this respect like a LM 741 or something like that they can't even go down to the negative rail we would start to see these rails clip right roughly at the same time and you remember that open-loop gain I was telling you about how large is it well it tells you a couple of ways in the datasheet not all data sheets will have it but this one does large DC voltage gain so it doesn't say it's open-loop gain but that is effectively the DC voltage gain is the gain of the dip the inherent differential amplifier in there and they put it in dB so you use your 20 log formula you reverse that and you get about a hundred thousand and likewise here on the datasheet they've got another way to tell you it's called now it's called something different it's got the large signal voltage gain there it's specified for a certain rail but there we go typically a hundred and they specify it in volts per millivolts so if if you divide a hundred volts by one milli volt what do you get same figure 100,000 there's your open-loop gain so there's just a quick out practical demonstration showing the virtual ground effect there and also the voltage rail limitations positive and negative I should do another part of this video on op-amp limitations practical limitations things like that that would be interesting thumbs up if you want to see that one but I've got I'll leave you with one last thing and I won't explain it I'll leave it to you to try and figure out I chose these values higher than what I had on the white board there I chose them for a reason let's lower them down to 10 K and 1 K here and see what happens with this specific op-amp lm35 8 hmm let's drop these down still quite high values 1 K and 10 K they're not you know 10 ohms or something like that but let's give it a go and there it is a 1k input resistor 10k feedback resistor zakah the same gain exactly the same input signal but what's that little funny business going on in there and over there hmm and if we measure our virtual ground point Wow look at these little spikes there and they're corresponding to that little bump in that waveform interesting so as Professor Julius um the Miller said why is it so I'll leave that to you to figure out catch you next time you you

Channel: EEVblog

Views: 2,068,349

Rating: 4.9406843 out of 5

Keywords: Operational Amplifier, opamp, how it works, tutorial, how to, inverting amplifier, non-inverting amplifier, amplifier, virtual ground, opamp buffer, breadboard, circuit, oscilloscope, waveform, power supply, split supply, power rail, opamp comparator, differential amplifier, opamp rules, open loop gain, closed loop gain, gain, closed loop, negative feedback, tips, fundamental friday, theory, practical

Id: 7FYHt5XviKc

Length: 49min 31sec (2971 seconds)

Published: Sat Apr 05 2014

Reddit Comments

[u/Shadow703793]

If you're interested in electronics, I highly recommend watching this as OpAmps are very useful in a lot of projects.

I also recommend reading "Op Amps for Everyone Design Guide" by Ron Mancini if you are interested in a deeper understanding. You can find the PDF on Google. It's on Ti's website.

[u/vulcant]

Thank you for posting this! I'm just learning to analyze these in class and this clarifies a lot!

[u/[deleted]]

Speaking of op amps here is more than you ever wanted to know about them. Has a great treatment of basic circuit analysis too:

http://www.ti.com/lit/an/slod006b/slod006b.pdf

[u/[deleted]]

I've needed this for months, I know this isn't a constructive comment, but man, thank you.