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What is an operational amplifier?

Video transcript

we're going to talk about the operational amplifier or op amp for short and this is the workhorse of all analog electronics the operational amplifier it's a type of amplifier an amplifier is is anything that you put an electronic signal end and you get out a larger version of the signal so this would be an amplifier with some sort of gain and if I put a signal X in here usually a voltage or a current then the signal that comes out here is a times X and that's what we mean by amplification and a signal that I've shown here is X is anything that we're interested in it could be a voltage or a current and when we put it through an amplifier we get a larger version of it so this is a really common activity in electronic design now when we talk specifically about an operational amplifier the symbol for that we use for an operational amplifier is a triangle and it has two inputs one is the plus input one is the minus input and it has an output and it also has two power supplies to it there's a some sort of plus voltage it goes into it and some sort of minus voltage so this is the abstract symbol for an op-amp so when we say the word op-amp we have some specific properties in mind one is op amps have high gain so in this case the the gain usually with the symbol a is something like ten to the fifth to ten to the sixth really really high another thing we think about when we talk about op amps is that they're used for feedback circuits and we'll talk about feedback in the next couple of videos and what that means but that's the application that we use op amps for and the third thing that's distinctive about op amps is that they have this this kind of input this kind of input here is referred to as a differential input so an op-amp usually has differential inputs and that's as opposed to something we call a single ended input which will be just one wire and what a differential input means it says we have we can label the voltages here we'll call this V out we'll call this V plus and we'll call this input V minus and differential input means that V out equals the gain times V plus minus V minus so the output signal here is proportional to the difference in the voltage between these two signals here so I want to make a plot of this equation right here just so we get a good idea of what it looks like and the axes here are V in and V out where VN specifically equals V plus minus V minus V plus minus V minus that's the input signal to the op-amp and we're going to apply the gain factor to it to get V out so that'll look like like this something like this is going to be a very steep line and the slope of that line the slope of this line is a so the slope is going to be 10 to the 5th or 10 to the 6 something like that very very vertical now one of the properties of this is that V out cannot go above or below its power supply voltages so on this plot here that's called saturation if V out gets up to V Plus we say it's saturates and looks like this it goes flat basically here and here where this voltage value here is minus the power supply and this voltage right here is the positive power supply but over this range here over this range in here between those two points it's quite linear it goes through zero and we this is where we use it most of the time so now I want to talk a little bit more detail about what this symbol means here and what's inside it and how it's actually connected up in a circuit so we talked about the voltage behavior of an op-amp this is V Plus V minus and V out there's one thing more that we need to know and that is the current this current right here and this current right here for an op-amp an ideal op-amp is zero no current flows in here so this op-amp is just sensing the voltages at these points but no current flows in so this is the second key property of an op-amp the first one is the voltage behavior V out equals the gain times V plus minus V minus another way we can write this is V out equals a times V n where V end-of-course equals V plus minus V minus so these are the these are the two electrical properties that are going to allow us to analyze these circuits and the analyzing the circuits is actually going to be pretty simple so you're probably wondering like what is inside here what's going on inside here so what's inside here is somewhere between 20 and 50 or so transistors and resistors sometimes capacitors these are really complex designs and for right now if we just concentrate on the two properties that we have here we'll be able to use these circuits even without understanding exactly what's inside suffice to say it's a differential amplifier with really high gain and with just that knowledge we can work out how these circuits work so let me do a couple of more details on how this thing is actually hooked up so we have a plus terminal and a minus terminal there's more terminals on this there's a power supply like this that's plus big V and there has to be a minus supply typically a - supply - V and let me a ground pin to be a ground node on here like that and when this is in a used in a circuit they'll be over to the side there'll be two power supplies and this will be 12 volts a 12 volts volts is a real typical value and there'll be another one and this is a plus 12 volt supply as well and they'll be connected together and this node right between them will be their ground node that's the voltage reference and these two guys will be hooked up like that the two power supplies will be hooked up this way so with respect to ground this node is at minus 12 volts and this node from ground is at plus 12 volts and ground is right in the middle and when we measure V out we'll measure it with respect to this ground node so this is the voltage where we measure plus or minus V out right there and what we're going to do is we're going to assume that all of this stuff is always hooked up and we're just going to use an even simpler symbol just the three terminals like that and you'll know that all the rest of the power supply is hooked up that way and the thing to keep in mind is there's a large - voltage there's a large plus voltage and the ground level the ground node is right in between so positive voltage is high on the page negative voltage is low on the page and V out can go both positive and negative around ground so that's your voltage framework to keep in your head and the op-amp that we've been looking at has a symbol like this and we know that V out equals some huge gain times V plus minus V minus so this is a differential input here's V plus here's V minus one way I think about this is is to look at the way a change in voltage on the input modifies the output I mean let me label V out here so if there's a change on the input say V plus goes this way because it's a plus sign that means that V out goes this way now if I change it over to V minus V minus is on the negative input if V minus goes up then the out goes down so that's the inverting it's called the inverting input and this is called the non-inverting input so on the non-inverting input up goes up and on the inverting input if you go up on the inverting input you go in the opposite direction on the output let's say that the positive input the non-inverting input went down this time and that means what that means that V out will go down and just do the same thing over here I'm going to run out of colors let's do the same thing for the inverting input if the inverting input goes down what does the what does the output do it goes up it goes in the opposite direction that's a way to think about the symbol when you see it on a schematic page is how do these signals translate through the device positive non-inverting signals go in the same direction inverting signals go in the opposite directions okay and here's one final trick I want to share with you some to be aware of you're going to see a symbol like this on a page the same op-amp it's the same op-amp but it's written on the page with a negative with the inverting symbol on the top and the non-inverting symbol on the bottom so as you look at a schematic that has an op-amp in it one of the first things you want to do is just glance and see what order these symbols are in or is it does it look like that or does it look like that and keep that in mind as you're reading the circuit and trying to understand what it does ok let's move on and build something with our op-amp