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

Hello, welcome to the basics. Friday, today we are going to take a look at the

operational

amplifier

or better known as the really important opamp building block. It is absolutely essential that you understand how they work. There are two ways to learn about

opamps

. 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 do it the easy way. So

what

is an opamp or

operational

amplifier

? Well, the name op amp comes from the fact that when they were first used. developed, they were developed to do mathematical operations, hence the name operational amplifier and back then we didn't have digital computers, they had them, they used them for analog computers, so analog mathematical operations addition subtraction integration differentiation things like that, even those things of really difficult calculations,

opamps

could In fact, we now do these operations in Hardware of this digital software garbage, that's where they came from, although today we don't have analog computers, we still use them for those mathematical operations.

You can turn an opamp into an integrator, for example. You can make it into a summer that's just an adder and stuff like that, so they're really useful circuit building blocks, but the main thing we're going to look at is the op amp as an actual amplifier because that's

what

they're most commonly used for. and probably also what you mainly use them for, so an op amp is essentially just an amplifier, yes it can be used for those math operations, but essentially what it boils down to is that it is a differential amplifier and what that means is that It has two entrances.

Here what we will talk about and an output and it has some gain because amplifiers have a gain and what they do is take the difference between these two input signals and amplify them by their internal gain or what is called open loop gain. and it gives you an output voltage, but op amps can't really be used as differential amplifiers on their own, although that's what they are quite confusing, but it's an important aspect to understand, so why can't you use this only as a differential amplifier input signal? here is an output signal with some gain, well the answer is that they are not designed to be used as differential amplifiers, oddly enough, because they are essentially differential amplifiers, that was the hard circuit you saw here before, it was actually the internal circuit of an opamp that shows it as a differential amplifier, but hey, let's forget differential amplifiers.

I shouldn't even mention it, but it's important to understand the workings of how an opamp actually works. Now the reason they don't work is the differential amplifiers. because the opamp the gain of the natural gain the internal natural gain of the op amp is huge and that's the first thing you need to know about opamps is that it's 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 tried to use an op amp like this with no external circuitry and just fed it, you know, like a mute into the input here, the gain is so big that the voltage output will be so large that it is simply not a practical device at all, which is why you never see an op amp without any external circuitry or what is called negative feedback, which brings us to our first practical application for the op amp, which It's a comparator, but before we look at that, we'll look at the symbol here now, an opamp is usually drawn as a triangle like this, it has two inputs here and one input here, sometimes it can be reversed depending on the ease of drawing your circuit and the way the signal flows. but it's exactly the same now these two inputs here one is the positive input it's called non-inverting input easy to remember because it's positive the inverting input is also easy to remember because it's negative negative inverts something so that's the terminology that you should I'll use when referring to opam it is very important to get the terminology correct otherwise you sound a bit silly now there is an output pin here easy and there are two power supply pins one positive and one negative which we will talk about also. so I mentioned that the gain of an opamp naturally inside is designed to be huge, almost infinite, so what happens if you just feed voltage into the input here?

Well, let's say we have one volt across our non-inverting input here and we have 1.01 volts. or slightly above 10 m volts or even 1 mol above this here, well, the amplifier will actually amplify the difference or try to amplify the difference between these two inputs, so the output put here will be this huge gain, like a million times that of a mole. It will try to generate hundreds and hundreds or thousands of volts and well, it can't do that because, well, your circuit only knows 5 10 15 volts, something like that, so your output will saturate, so if you have 1 volt. here and let's say 1.1 volts here, then its output will go up to v+, it will just saturate on the positive voltage, so we have a comparator and in the same way, if we change those voltages so that the non-inverting input is larger than the inverting input, even in a small amount.

Bingo, your output will go from positive and hit the negative rail down here so you can see it's only used as a comparator. It's going to be a very crude comparator and you can use an opamp as a comparator in a pinch, but they're not as good as a proper comparator that you can actually buy. They are designed to be comparators, but hey, you actually can. use opamps as comparators, but that's what happens if you plug in an opamp without any feedback and what's called is open loop setup because there's no loop, there's no loop, the loop is open and we'll close the loop in a minute. but with an open loop setup like that, an opam is just a comparator, so now that we got that little nonsense out of the way of the Oddball comparator setup for the opam, let's take a look at where opamps are really useful and that's as proper amplifiers now to do that like I said we need to go from the no feedback open loop setup to add what's called negative feedback and therefore the t-shirt negative feedback and once you do that the amplifiers operational devices will become incredibly useful and powerful devices.

There are two rules with opamps, that's all you have to remember, it's fantastic, that's how easy opamps are if you know these two rules, if you remember these two rules you can analyze practically any opamp circuit, you can't go into the details essential. from the performance of it perhaps, but you can look at a schematic and understand how it works and the two rules are very simple, rule number one, no current flows in or out of these inputs, so nothing flows in or out of these two tickets. pins, that's it, nothing flows in or out, regardless of how you connect this circuit, whether it's the open loop comparator setup we saw before or whether or not it's a Clos Loop setup and an inverting or non-inverting amplifier.

I'm going to see that nothing flows in or out of 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 we just looked at with the comparator, which is why I made the comparator first. Although it might have been a little confusing to start that way, most people start opam explanations with these two rules, but I wanted to show you that comparison first because to highlight that rule number two does not apply or only applies to configurations of closed loop with negative feedback now rule number two is that the opamp does everything it can internally correct internal circuitry which we won't get into but it does everything it can to keep these two input voltages equal now the opamp actually can't change your input voltage.

If these are inputs you have no way to remove a voltage and keep them the same but you can do it with feedback and that is why this rule only applies to Clos Loop configurations so that the op amp only has control over its output but if you If you have feedback, you will change this output voltage to make sure that this input is equal to this input here and that is a very powerful rule of opamps and if you see a Clos Loop configuration like this, you can be pretty sure that that rule will apply so use these two rules let's look at the simplest possible opamp conf setup and it's not this it actually has no external components so what you have is the output connected to the inverting input like this and you Feed your signal or your voltage to the non-inverting positive input. so and this is called opamp buffer, so using our two rules it is very easy to analyze this opamp buffer circuit, let's say let's just do DC because opamps, the other thing is that opamps are DC coupled amplifiers, they can amplify signals DC and AC. very important property then, but let's do the DC case, we are feeding a volt to our non-inverting input here, what do we get at the output of our opamp?

Well, see, rule number two always applies when you're, when you have feedback in a circuit in an opamp circuit, the opamp tries to keep these two input voltages identical, so because of the rule, this inverting input here will be equal to this pin up here, the op an will make sure that by driving this output to get this input to match this one, so we have a volt here, then we have a volt here and since it's only connected by a little bit of wire, we will get 1 vol here, that's why it is called buffer, it is not. an amplifier because there is no gain 1 volt in 1 volt out minus one volt in minus one volt out whatever the voltage is within uh within the limits of the power supply voltages here, what's the point?

That rule number one? out of the inputs so nothing flows, so if you have a load here, I don't know, it could be some kind of sensor or whatever, it could be a low pass filter for example, like if you were feeding a pulse with a modulated signal from your microcontroller or something and then you want to buffer that voltage there because no current flows into the input, this opamp doesn't disturb your sensor or your circuitry which you're actually trying to do, it's what's called a very high input. High impedance is essentially an open circuit, so it doesn't disturb anything you plug into it, but the op amp has what's called a low impedance output, so we can drive a reasonable amount of current, you know, milliamps, Something like this can go up to a couple hundred milliamps for your power op amps, but it can generate a reasonable amount of current, so it buffers the signal, a high-impedance signal, and gives it an output.

Low impedance that just lets you drive. things with a sensitive input like that pretty easy and very useful setup the opamp buffer now the next setup we're going to look at is what's called a non-inverting amplifier and this is where we tame our opamp Beast that huge, unwieldy gain that changes everywhere with temperature and it's horrible anyway it has this huge gain unusable as a differential amplifier but as a single ended amplifier that's what single ended means its power input here and it's always referenced to ground , we can use this as a single termination amplifier and we can control that gain by adding negative feedback and I won't explain the negative and positive feedback in the mechanisms and how it works because well, that's for a more advanced topic, but anyway here we feed a feedback resistance. like we did before it shorted out, but we put a resistor in there and put a resistor back to ground, so what it's doing now is this input, the inverting input is taking a small portion.

R this feedback resistor which we'll call RF is always bigger than R1 here, so here we have a voltage divider that feeds back a smaller part of the input and that's essentially what negative feedback is: you take a part of the output and you put it back to the input and there is You have to remember a very simple formula for this non-inverting amplifier setup and I won't try to derive it, but the gain of this amplifier or what is called AV, that is the actual terminology used, AV is just gain, you can use gain equals RF. the feedback resistor divided by R1 going down to ground here plus one, you need to add that plus one there, very easy if we have a 9k feedback resistor and a 1K resistor to ground here, our gain is 9k on 1K. or 9 + 1 our gain is equal to 10, so if we feed 1 volt into the input here we will get 10 volts inthe output easily and since we have positive and negative rails that will go in, we can power AC or DC. signals here over ground so we can feed uh 1 VT negative here and we'll get -10 volts so there you have it that's the basic setup of a non inverting amplifier and you might see weird setups there might be a capacitor at through here or something like that we won't get into this one, but you know the setup is the same if you see your input going to the non-inverting input and the feedback goes back to the inverting input, you know it's a non-inverting input. amplifier and this formula here applies and from this formula you can also see why our buffer amplifier had a gain of one before because our feedback resistance is zero, it was zero ohms, so zero at R1 here, which was infinite , then zero at more than infinity or very large.

The value is 0 + 1 so our gain is one, that's why our buffer had an easy gain, the math doesn't lie, so now we move on to the second of our two main setups that we've already looked at. In 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, as before, we could say it. it was a non-inverting one because of the signal coming into the positive input here, the non-inverting input, hence the name non-inverting amplifier, our signal now goes to our uh inverting amplifier pin, that's why it's called an inverting amplifier and you'll notice that I changed the two symbols here, the positive one is now at the bottom, our op amp hasn't changed.

I just did it visually to, you know, make it a little bit easier here and that's what you'll commonly find in schematics and CAD packages and all kinds of things, you might find them flipped upside down, back to front, screaming or scrolling the place, some pointing down for various feedback passes and all sorts of things, it's exactly the same opamp, it's just visually different, you can draw it however you want now our inverting amplifier, this is the one we have the same as before, we have our resistor feedback, our negative feedback goes to, in this case, our inverting amplifier pin instead of our non-inverting one. one, so now we're feeding our input through the resistor here, so it's a different setup, our signal isn't going directly to the non-inverting pin and this brings up our next really important concept with opamps that you really need to understand. and this is where rule number one really comes into play when trying to analyze this.

It's called virtual earth. Stay with me, so once again, how do we break this down? Always go back to your two rules. What is our second rule here? The opamp tries to hold the input. In fact, the voltages will be the same if you have this non-inverting configuration and you haven't hit the rails yet, so if the amplifiers are operating normally within the normal limits of your power supply rail, these two inputs will always be the ones. themselves, so, eh. actually we are grounded our non inverting input, here it is grounded, we are forced to ground it, it is never going to change, so what is the inverting input here that will work well?

Of course, rule number two will be identical, it will be the same, so this point will also be ground or Zer volts, so this seems almost a meaningless circuit because look at rule number one, there is no current flowing in or out. outside, so there's no current going in or out of that. pin and is connected to ground, we have both pins connected to ground and no current flows in or out. What is the point of having an opam? It's a very confusing concept, but once you understand it, you're like, oh, that's easy and it's pretty brilliant, so remember opam. does what you need on the output drives it to any positive or negative voltage to make sure this inverting pin here is the same as the non-inverting pin down here makes them the same we force this pin so you can't change this pin all you can do is change the voltage through the nature of the feedback resistor here to make this zero and believe me, we will do a practical measurement of this in a minute and this node here will actually be zero volts and this Confuses the hell out of a lot of beginners, they build their upam circuit, they start polling and they have their input signal here.

You know it's a 1khz 1vt sine wave for example, and so they measure this side of the resistor and the signals there, they measure this side of the resistor and it's ground, the signal is gone, where did it go? H strange but true, so let's follow this and use our rules and see if we can analyze this circuit one more time, the DC case to do it. Easy, we have one volt on the input here, 1 volt positive with respect to ground, of course, now we've said it before, believe me, we'll measure it later, but this pin will be connected to ground, it will be zero volts.

It's always there, so all we have is one volt across our R1 here, which is 1 K, so we're going to have 1 milliamp flowing through there, where does it flow right? It doesn't flow here to land, how can it? why not. current rule number one no current flows in 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 a volt across that 1K resistor law of ohms should always be B, so that current is flowing, believe me, it can't flow into the input pin when we know it is high impedance, so it should flow here like this through this 10K resistor and it comes from the output .

Remember this op amp has internal circuitry, it has an output buffer so it can drive currents in and out of the various supplies back there and that's where the current sinks and that's the tricky part about this, our current now has been forced up to this node here and is flowing through In this case, our RF feedback resistor, which is 10K, I've made it 10 times larger. You'll see why in a minute it should be flowing through there, so we should have a voltage drop across that resistor once again. Ohms law must always be obeyed, so if we have that 1 milliamp flowing through our 10K, we will have a 10 volt drop across this resistor with positive here and negative here, aha, these voltages are with respect to ground here now, this is where you get a A little bit complicated, this positive voltage here is we're going to get plus 10 volts across that resistor there, but because this pin is positive, but we're forced, we know that this pin is zero .

Well, we know it's zero because we forced it. of the amp action and rule number two here in what's called virtual ground which I'll talk about in a minute, so we have that means if this is ground this is positive so we have minus 10 Vols coming out of here Bingo, there's our inverting amplifier 1 volt in minus 10 volts out, so our gain, our formula, AV gain is equal to r f into R1, there is no + one with the inverting amplifier, the plus one only applies to the other non-inverting configuration , so as an opamp action, we'll call it. and the negative feedback here at this point, this node here on the inverter pin is what is called virtual ground because normally in this configuration it is actually grounded because we have grounded this pin, we don't need to be able to power other voltages . this pin and it scrolls and does all kinds of other things, but it's still called, even if you put another pin in here, it's still called virtual ground because it's virtual, it's not real, it's not tightly bound if it would be tightly bound to the ground if it really we tied him up. that PIN to connect to ground this wouldn't work because all of our current would flow through here like through this resistor to ground and around like this and then this output here well, I wouldn't know what to do, the output would be zero because there would be a difference of zero volts here, remember it's still a differential amplifier as such, so we have a difference of zero volts here, we're going to take out zero, we would have no current flowing through here and we would have zero volts, so you can see it doesn't work unless you tie that hard terrain, but when it becomes virtual terrain by the nature of the opamp action, everything works magically.

I hope that makes sense because once you get it, it's really easy, so functionality-wise. It's pretty much exactly the same as the non-inverting amp except it's inverted and that's it, and the gain formula is slightly different, but other than that it works exactly the same, but that magic virtual terrain is at play in this setup and, of course, as with op amps. They are DC coupled, so they operate on DC signals. You can just feed a fixed DC voltage like I said 1 VT DC would give minus 10 volts output in this case with these value resistors or we can feed a 1 V peak uh for - Peak or RMS uh sine wave for example , above the ground, so it's centered on the ground, so this is the blue waveform here, let's say it's 1 volt, it's not the scale, but you'll get the idea and then our output will be the inverse of that , when the input increases, the output becomes negative because it is now an inverting amplifier.

Of course, one of the disadvantages of the inverting amplifier compared to the non-inverting one we looked at 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 you can change the gain equation and max everything out, that's where you want a non-inverting amplifier or at least a buffer that some people will actually follow. Put a buffer on the input here and then drive the inverting amplifier, but normally in those types of cases you would probably use a non-inverting amplifier.

Now we need to dig into this and talk about power supplies, split rails and everything. This kind of stuff and, uh, single supply op amps. I'll try to be as brief as possible, but you saw in this configuration that the op amp only has two power pins. Well, it's usually called v+ and V minus, now V minus, you can plug that in. to ground there is nothing, regardless of what the datasheets tell you, there is nothing inherent in opamps that makes them truly a single supply opamp, so you can take an opamp that has v+ and V minus and connect it grounded that way, there's nothing to stop. you, as long as it meets the minimum voltage specification and does not exceed the maximum, etc., so what if we did that?

In this case, our input or our non-inverting input is also connected to ground here, well, now it becomes a problem that you get into. Practical limitations of op ANS We have been talking about what is called an ideal opamp up to this point these rules here are not strictly true. I lied, but there is still a fantastic way that even professionals used to analyze these circuits as first order. first pass, no current flows in or out, if you've been watching my videos you'll know I did a previous video on this talking about input bias current, so tiny little currents can flow in and out of these pins . depending on what type of operating application you have and that's a really practical limitation of these things and the other one is the one I talked about in the previous video, which I'll link below if you haven't.

Seen, the inputs can't necessarily go directly to the rails, whether it's a positive negative reference to ground or whatever, so you can get what are called rail-to-rail op amps or rail-to-rail input op amps in In this case, if you had a rail-to-rail input op amp, then Yes, you might be able to get away with having the inverted input and the non-inverting input tied to the ground like this, but wait, what's the point of that? If you only have ground, this is an inverting amplifier. it inverts its signal so if you supply one volt it will try to make the opam try to give it one or less 10 volts but how do you do it when the supply is negative like this it doesn't work? you have to um it doesn't have space to do it so your opamp should always be on in the configuration that you expect your input signals to be referenced so if we were to use the inverter opamp configuration like this with a single supply rail like this and we wanted to amplify the AC signals.

Signals can't go negative this way, they can go negative at the input, but you'll never get that negative voltage at the output, but you still want to amplify your signal cleanly this way. Well, what we need to do is this zero point needs to go all the way down like this, so we need to offset, so if that's Zer volts, we need to offset our input waveform, our input and output reference, in a certain amount of voltage,how much? Well, typically half your supply rail for Max maximizes your clearance, how do we do that? I hinted before feeding it, if this is v+, you would go to v+ in two, you would feed that voltage to the middle of the rail there.

Usually you do that by just putting a resistor like that going to v+ and a resistor down there going to ground and to the voltage divider bingo, there's the half rail, so we're compensating our voltage here, our virtual ground. Remember this is still called virtual land even if you don't go. If so, the voltage here will be equal to the voltage here because of our second opamp rule, so if our power supply is 20 volts, for example, this point here would be half that if we do them, you'll know exactly the same value, of course. make them the same value on half rail so we will have an offset voltage here at this point and that will shift our waveform up and we will see that in the practical experiments below as I said some time ago you may see some other components around here, like some capacitors and things like that around the circuit, that effectively change the bandwidth of the circuit, because we're not going to get into that.

I'll have to do a second part of this video that covers opamp bandwidth and stuff like that. I've done one on C cascade opamp bandwidths which I'll link below, but suffice it to say that an ideal opamp we've been looking at has infinite bandwidth, its frequencies and signals infinite, but in practice, no, Of course, your practical op amp could have a bandwidth of one MHz or a bandwidth of 100 khz or something like that. You know it might be a nice, fast 100 mahz bandwidth, but you will always have bandwidth that changes with your bandwidth gain or gain product. and I've made a separate video, I'll link to it, but sometimes you might see a little derivation limit there, it could be, you know, 10 inhalations or 100 inhalations or something like that and that's just derived from the frequency response to that and in the same way. you might see a little limit on something like this, for example, if you have um, if you're compensating for this by using a single supply like this, you know, I won't go into details, but basically any noise at this point here will be amplified. and captured in that virtual terrain, so you're going to get noise in your output signal, so you're going to be able to put a big butt, you know, a one or 10 microfarad cap.

Here, for example, and actually making that virtual land really noise-free, but well, that goes beyond the basics. A small mistake I noticed. Wow, my formula here for the inverting amplifier needs a negative in front because the gain is actually negative, so that's how it is. the gain is not in this case it is not 10K it is not 10 it is minus 10 ups, so let's briefly return to this voltage rail topic, uh, briefly because it is something that is quite confusing because there is no round pin on an opamp, There is only the positive and the negative, so Well, where does your reference go?

Well the reference is part of the external circuit, in this case let's go back to our non-inverting amplifier setup, here is our reference to ground and then our positive and negative supply is here like this, plus 15 vols and minus 15 vols if we want to power a signal that is both positive and negative, if we are only feeding a signal that is positive above ground then this here could be tied to here like this and then it has to be above so the output can't magically go negative it can only become negative to your ground reference if you have that minus 15v rail there, clear as mud and just like the inverter setup, if we wanted to power this from a split supply we could ground it like this and then we can add a voltage of bias here like this to compensate for the voltage and then you'll be able to get into all sorts of weird and wonderful things with AC coupling, these amplifiers, all the operating configurations that we looked at have been DC coupled, but you can actually couple AC . them, that's why you start with SE capacitors at the inputs and outputs of the opamps.

Now here's a complicated setup that I'll touch on briefly that combines the two different setups that we've seen before and a couple of things that we've seen before. the differential amplifier you know how I said opamps are essentially a differential amplifier that's how they work but you have to do it but they do it in open loop configuration so they're hopeless they're useless for that but yeah you combine the N, the investment. amplifier setup that we just looked at, so we have the feedback here, our signal comes in, it's a standard inverting setup and we have exactly those two resistors that we saw before to bias that voltage, but instead of going to the supply rail, we do that . our other differential input and bingo becomes a differential amplifier.

I'll let you do the actual calculation yourself to find out, but basically the difference we're powering if we're powering 1 volt here and 1.1 volts here. We have a difference of 0.1 volts and the gain of this amplifier is exactly the same as the R2 inverting configuration on R1. Before we used RF. I'll call it R2 here, so R2 in R1 10K in 1K we have it G and you have to add negative. So it's a gain of minus 10, but because our bias voltage is not fixed, it's actually the differential input signal. Look, look what happens. We have one volt here.

We have our divider here. R1. These two values ​​are the same. R1 is equal to R1. here R2 is equal to R2 here they have to be precisely matched to get a good common mode rejection ratio which we won't get into, but suffice it to say that if we have one volt at this point here relative to ground, we will have a repeater of 0, 9999 at that time. point there and that becomes our virtual ground Bingo, we have that same voltage there, then we will have our 1.1 volts here which have across the 10K which has 1.0 99 volts across it, subtract the difference there, it's exactly the same setup as before with the bias voltage, but then we'll be left with an output voltage of minus one, so we've amplified the difference in our input signal by the gain here 10 is not a great differential amplifier, but it works, so we have tamed our op amp, which is a differential amplifier anyway, but pretty much unusable.

In fact, we've turned it into a pretty usable differential amplifier. The beauty is simply combining both techniques and there are many complicated things like this that can be done with opamps and briefly another of these complicated configurations goes back to its name, the op amp and one of those mathematical operations, the integrator. I won't get into integrals and all that kind of stuff, but what we can do is a basic inverting setup here, except instead of a feedback resistor we have a feedback capacitor, what does that do? Our standard input voltage here follows the rule of no current flowing, but we have a virtual ground, of course, rule number two, so if that's 1K. and that's a volt where we have 1 milliamp flowing through that resistor, where it flows, it can't flow into the opamp, it has to flow up here and through the capacitor, so effectively you have a constant current of 1 milliamp.

I just did this now it's a constant current flowing through this resistor and when you have a constant current flowing through a capacitor you end up with a well, in this case it's going to decrease negatively so if our input goes if our input is one step and it goes up like this the constant current because it takes time to charge a capacitor the voltage on the capacitor will increase so I say increase because it's an inverting amplifier so it will go negative but that's what it does and that's an integrator and that's actually a mathematical integral of your input signal anyway, that's a lot more theory than I wanted to do and more time than I wanted to actually take, but just remember that these two rules of opamps allow you to analyze practically any configuration and As homework , I recommend that you look at the sum op amp setup and figure out how it works because you're going to use those two rules to figure it out, so I'll leave it up to you, but Enough of that, let's go to the bench here and see if we can measure some things.

Make sure you're not lying to yourself about this virtual terrain stuff. Let's check it out. It sounds a bit suspicious, let's see if it really works. on 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's really no signal there, it actually disappears in quotes 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 like I explained it anyway I have a Jelly Bean lm358 here it's actually a dual op amp so we just tied the top op amp here I could probably do a separate video on how to properly terminate uh opamps that could be an interesting video um thumbs up if you want to watch it anyway here we go.

I have it configured. I have a 10K input resistor here. 100K feedback, so I have a gain of 10, the formula of course is the feedback resistor on that Easy Bingo multiplied by 10, so I'm going to feed a 2 volt input peak to peak here, we should get 20 peak-to-peak volts at the output, so we're using pretty close to the max supply rail of the lm358. In this case, I'm feeding it more than 15 volts, so we have a split supply, so our ground reference, our input signal is a ground reference. Actually, I should draw that there. There we go, that's clearer, so our input is referenced to ground and our non-inverting input here is referenced to ground and our output is also referenced to ground, but so that the signals become negative, so that the signals from output become negative, we need a negative rail here. so we're using minus 15 vol so plus 15 to power minus 5 as well so the total 30 volt supply there allows us to go in and out of positive and negative signals so let's go to our power supply here it's more - 15 vols.

I have dual tracking there and you'll notice that I joined the supplies um here generating a split supply so this one actually becomes negative so this is our positive 15 from here to here and this is our -5 relative with here because We've tied the positive and Tada, here we go, we're feeding our own. We just received a low frequency signal of one khz, 2 volts peak to peak. Here at the entrance and you can see our input and output waveforms. and these inputs of course are all AC coupled and their bandwidth is also limited to 20 MHz to reduce noise and we are also using our high resolution mode to get a Box Car average there and that is why we have a Nice sharp waveform like that, beautiful, so what happens if we put our bandwidths back to maximum?

In this case, it's my 1 GHz tectronics 3000 series and we turn off high resolution mode, we go back to sample mode, there we go, we get our nice diffuse waveforms because we have that enormously high bandwidth, that's the advantage that you can use to average of course, but the high resolution mode makes Box Car averaging just clean it up, of course you can do the surround mode, look at that pretty horrible waveform, so when you look at this kind of thing, You definitely don't want to use your normal mode, you want the high resolution mode, if you have it there, come on, we're getting exactly what we expect.

Look at that 2 volts peak to peak at about 20 volts, probably going. There may be some error due to resistors here anyway we are getting our 10 time and of course the blue waveform is the input which is 500 MTS per division so we are getting our 2 volts peak to peak and our departure. it's 5 vs per division so what's the yellow waveform there and look at that and of course because it's an inverting amplifier the output is exactly 180° out of phase it's inverted so by the time I'm testing the input and output, now you want to see the virtual terrain, don't you know what happens if I move my input probe, the Blue Waveform here from the input to here?

You would expect to see the signal, but as I told you and since you should trust me, let's move the probe over that is our virtual ground point. Look at the point. Flat attack, signal is gone, magic, but of course, no, it's not magic, it's just a standard op amp. Behavior with virtual ground at the input, this is how an op amp works and no, current does not. magically disappeared, the current passes through the resistor, the ohms law still holds, the current ischanging because we have an AC resistor, uh, here there is AC current flowing through this resistor and everything flows up here, but this point by the nature of the opamp action and the negative feedback which is a virtual basis our rule number two of the op amp, the inputs are the same, the op amp changes the output here to ensure that point is equal to that input there easily and that's why we don't see any signal there So catch the young players when you are exploring circuits like this.

Don't think the signs disappeared. Virtual Earth. Always remember your exit rules. I actually chose the LM 358 for a reason because it is not like a normal op amp. it's not like a rail to rail opamp it's halfway between see it come on it eliminates the need for dual supplies ok you can use it as a single supply opamp but like I said you can use any opamp as a single supply . opamp, but this one is very special and that allows direct sensing close to ground, so the V output also goes to ground so effectively that it's not rail to rail, it won't go all the way to the positive rail on the input and output , but it will pull down everything negative to ground because an op amp doesn't have a ground pin, it's the negative rail, so even if we power it from plus -15 split supplies like now, it will still pull down to that. -5 pin Vol or that pin 4 will go down the input, this input here will allow it to detect up to the negative rail and also the output will go up to the negative rail and I will demonstrate, but what we have to see here is a couple of things on the sheet data: our input common mode range and our voltage range here, as we said, goes all the way to that negative pin or zero volts, as it is. calling it here, but on the positive side, this opamp will not make sense or go to the output less than 1.5 vols uh below or above 1.5 vols below the positive v+ rail there, so if we have a 10 volts output signal, for example the voltage range says that if we want to get an output voltage of 10 Vols Peak, we need a v+ rail of at least 1 and 2 volts above that, i.e. 11.5 volts , so what we're going to do is reduce the voltages here on these rails that We're going to lower v+ from 15 volts to 11.5 and around that 11.5 volts because we're getting a 10 volt peak at the output 20 volts of peak to peak 10 volts peak we should start to see distortion or clipping of our waveform at about 11 1/2 volts let's see if we do well so here we go we have 15 volts.

I'm going to lower it 0.1 volts at a time. You will notice that it is divided. Supply dual tracking, so our waveform still looks good. looks good, but we expect it to start clipping around 11 1/2, it may not be accurate, this is not an exact value in the datasheet, but there we go, 112, it's still there, there we go, it's starting to clip it's starting to clip you can see it it's actually about 11.2 volts there but you can start to see the waveform flatten out now I reduce it even more because this is not a symmetrical supply it actually goes low to zero, we are not beginning to see cuts. at the bottom here, the bottom rail until a significant amount of time after that, now we're getting both, but I bring it back up there and that's about 11.1 volts, but we're seeing that clipping at the top and we won't see it on the bottom for a while then there you go, just keep that in mind and if we had an even worse op amp, in this respect, like an lm741 or something that can't even go down to the negative rail, we would start seeing these clips at about At the same time, you remember the open loop gain.

I was telling you how big it is. It tells you a couple ways on the datasheet, not all datasheets will have it, but this one has a large DC voltage gain, so no. I'm not saying it's open loop gain, but that's effectively the DC voltage gain is the inherent differential amplifier gain there and they put it in DBS, so you use your 20 log uh formula, invert it and you get about 100,000 and the same thing here in the data sheet they have another way of telling you it's called now it's called something else it has a huge signal voltage gain there it's specified for a certain rail but there we go normally 100 and they specify it in volts per molt, so if you divide 100 volts by 1 molt, what do you get?

The same figure is 100,000. There's your open circuit game, so just a quick hands-on demo showing the virtual ground effect there and also the positive and negative limitations of the voltage rail. I should do another part of this video on opamp limitations, practical limitations, things like that, that would be interesting. Approved if you want to see it, but I got it. I leave you one last thing and I won't explain it. I leave it to you. Try to find out that I chose these values ​​higher than the ones I had on the board. I chose them for a reason, let's lower them to 10K and 1K K here and see what happens with this specific lm358 H opamp, let's lower them still quite high. 1K and 10K values, they're not, you know, 10 ohms or something, but let's try it and there's a 1K input resistor, a 10K feedback resistor, exactly the same gain, exactly the same input signal, but What's this funny little thing going on? there and there H and if we measure our virtual ground point, look at these little peaks there and there corresponding to that little bump on that interesting waveform, as Professor Julius S Miller said, why?

So I leave it to you. to find out I'll catch you next time