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## Operational amplifier

Current time:0:00Total duration:11:34

# What is an operational amplifier?

## Video transcript

- [Voiceover] We're gonna 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 anything that you put an electronic signal in, and you get out a larger
version of the signal. So this will 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 as 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. 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 some sort of plus
voltage that 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 gain,
usually with a symbol A, is something like 10 to the
fifth to 10 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 that's
distinctive about op-amps is that they have 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 wanna make a plot of
this equation right here, just so we get a good idea
of what it looks like. And the axis here are v in and v out, where v in specifically
equals v plus minus v minus. v plus minus v minus,
that's the input signal to the op-amp, and we're gonna
apply the gain factor to it to get v out. So that'll look like
this, something like this, it's gonna 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 fifth or 10 to the sixth, 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 saturates. It 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, is quite linear. It goes through zero, and this is where we
use it most of the time. So now I wanna 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 in, where v in, of course,
equals v plus minus v minus. So these are the two electrical properties that are gonna allow us
to analyze these circuits. Analyzing these circuits is
actually gonna be pretty simple. So you're probably wondering,
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 minus supply, minus V, and there'll be a ground pin, there'll be a ground node on here, like that. And when this is used in a circuit, there'll be, over to the side, there'll be two power supplies, and this will be 12 volts, 12 volts is a real typical value, and there'll be another one, and this is a plus 12
volts supply as well, and they'll be connected together, and this node right between them will be the 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 gonna do
is we're gonna assume that all of this stuff
is always hooked up, and we're just gonna use
an even simpler symbol, just the three terminals like that, and you'll know that all of
the rest of the power supply's hooked up that way. And the thing to keep in mind is there's a large minus 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 to look at the way a
change in voltage on the input modifies the output. 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 v out goes down, so that's the inverting, it's
called the inverting input, and this is called the
non-inverting input. So on a 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, gonna run out of colors. Let's do the same thing
for the inverting input. If the inverting input goes
down, what does the output do? It goes up, it goes in
the opposite direction. That's the way to think about this 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 wanna share with you, something to be aware of. You're gonna 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 the 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
wanna do is just glance and see what order these symbols are in. 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. Okay, let's move on and build
something with our op-amp.