Main content

## Operational amplifier

Current time:0:00Total duration:5:56

# Feedback

## Video transcript

- [Voiceover] So now I wanna
talk a little bit about the concept of Feedback. This is a, this is a
really important concept. It was developed in the 1920s,
the idea of using Feedback, and it was done at, at Bell Labs, Bell Telephone Laboratories. Remember we talked about
this on the op amp, this being the, the non-inverting input, so if this voltage went
up, this voltage went up, and the negative sign
is the inverting input, so these, these voltage here and here, move in opposite directions. And the key to understanding
what Feedback is, is to pay attention to
this inversion path, and these non-inverting paths. We're gonna start over
here with just isolating the voltage divider part of our circuit, so that's copied over here. And when you think about,
let's look at what happens if V out goes up. And what happens to V minus in this case? Well V minus goes in what
direction in a voltage divider? It goes up. So this is a, this is a
non-inverting structure. If V out goes up, then the
thing we care about V minus, goes up as well. And likewise if V out goes
down, then V minus goes down. So that's isolating
just on this part of it, we have no inversion
happening around here. So we go around here now, and let's look at what happens on this path here, if we go up in V in, then
we know V out goes up. If we then change colors
to, let's try this. If V minus goes up, then V out goes down. That's the inverting path. So there's, so there's one
inversion in this circuit, and it happens right here
where V minus goes to V out, so that's where the inversion is. So now let me set up
just an example circuit. We'll set R1 equal to R2. And from the last video, we
developed a gain expression, and we said that V out
equals R1 plus R2, over R2, times V in, and with these,
with these resistor values, V out equals two times V in. Alright, so this is
equal to, two times V in. And what does that make this point here? V minus, this is V minus, and from our voltage divider,
we know a voltage divider says that V minus equals V out times R2, which is just R, over R plus R, or V minus equals one-half, V out. So, we have let's put a, let's put a voltage on here. Let's put a real voltage on here. Let's say this is at one volt, alright? And going through our amplifier, we know that V out equals two volts, and that means that V minus
equals one-half of V out, so V minus is one volt. So this is one volt here. So let's say for the moment, that something happens to the circuit, like we heated up or something like that, and let's say the gain
goes up a little bit. Now what that means is, that this amplifier, which is amplifying this voltage difference right here, is gonna be a little higher, so the voltage here is
gonna go up a little bit. Let's use this color. It goes up a little bit, and that means that this output voltage is gonna go up a little bit. And we already decided from
looking at this voltage divider, that if this point goes up,
that this point will go up. It goes up half as much, but it, it goes in the up direction. When this voltage goes up, that means this voltage goes up, and
now we find ourselves, we're at the inverting input. We're at the inverting
input to the amplifier, and that means what? When a change at the
inverting input goes up, that means the output goes down. And that's in the opposite direction of the original change. So this is the mechanism of Feedback. A went up a little bit. We thought that V out would go up a bit, which meant this point goes up, which meant it gets fed back to the input, to the inverting input, and then it goes back down, and this balancing act
that's going on right here, that is the mechanism, that
is what we call Feedback. You get this Feedback effect, when this connection is made right here, back to the inverting
input, to the op amp. And in particular, because
it's the inverting input, this is called Negative Feedback. So this is the mechanism of Feedback, in particular, Negative Feedback and, what it does for us is, it provides us a way
to exploit and to use, this enormous gain that
these amplifiers have, to create really stable, really
nicely controlled circuits, that are controlled by the
values of the components we attached to the, to the amplifier. So that's the idea of Feedback, a really powerful idea, and really at the heart
of analog electronics.