- [Voiceover] So now I wanna talk a little bit about
- [Voiceover] So now I wanna talk a little bit about

the concept of Feedback.
the concept of Feedback.

This is a, this is a really important concept.
This is a, this is a really important concept.

It was developed in the 1920s, the idea of using Feedback,
It was developed in the 1920s, the idea of using Feedback,

and it was done at, at Bell Labs,
and it was done at, at Bell Labs,

Bell Telephone Laboratories.
Bell Telephone Laboratories.

Remember we talked about this on the op amp,
Remember we talked about this on the op amp,

this being the, the non-inverting input,
this being the, the non-inverting input,

so if this voltage went up, this voltage went up,
so if this voltage went up, this voltage went up,

and the negative sign is the inverting input,
and the negative sign is the inverting input,

so these, these voltage here and here,
so these, these voltage here and here,

move in opposite directions.
move in opposite directions.

And the key to understanding what Feedback is,
And the key to understanding what Feedback is,

is to pay attention to this inversion path,
is to pay attention to this inversion path,

and these non-inverting paths.
and these non-inverting paths.

We're gonna start over here with just isolating
We're gonna start over here with just isolating

the voltage divider part of our circuit,
the voltage divider part of our circuit,

so that's copied over here.
so that's copied over here.

And when you think about, let's look at what happens
And when you think about, let's look at what happens

if V out goes up.
if V out goes up.

And what happens to V minus in this case?
And what happens to V minus in this case?

Well V minus goes in what direction in a voltage divider?
Well V minus goes in what direction in a voltage divider?

It goes up.
It goes up.

So this is a, this is a non-inverting structure.
So this is a, this is a non-inverting structure.

If V out goes up, then the thing we care about V minus,
If V out goes up, then the thing we care about V minus,

goes up as well.
goes up as well.

And likewise if V out goes down, then V minus goes down.
And likewise if V out goes down, then V minus goes down.

So that's isolating just on this part of it,
So that's isolating just on this part of it,

we have no inversion happening around here.
we have no inversion happening around here.

So we go around here now, and let's look
So we go around here now, and let's look

at what happens on this path here,
at what happens on this path here,

if we go up in V in, then we know V out goes up.
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 we then change colors to, let's try this.

If V minus goes up,
If V minus goes up,

then V out goes down.
then V out goes down.

That's the inverting path.
That's the inverting path.

So there's, so there's one inversion in this circuit,
So there's, so there's one inversion in this circuit,

and it happens right here where V minus goes to V out,
and it happens right here where V minus goes to V out,

so that's where the inversion is.
so that's where the inversion is.

So now let me set up just an example circuit.
So now let me set up just an example circuit.

We'll set R1 equal to R2.
We'll set R1 equal to R2.

And from the last video, we developed a gain expression,
And from the last video, we developed a gain expression,

and we said that V out equals R1 plus R2, over R2,
and we said that V out equals R1 plus R2, over R2,

times V in, and with these, with these resistor values,
times V in, and with these, with these resistor values,

V out equals two times V in.
V out equals two times V in.

Alright, so this is equal to, two times V in.
Alright, so this is equal to, two times V in.

And what does that make this point here?
And what does that make this point here?

V minus, this is V minus,
V minus, this is V minus,

and from our voltage divider, we know a voltage divider
and from our voltage divider, we know a voltage divider

says that V minus equals V out times R2,
says that V minus equals V out times R2,

which is just R, over R plus R,
which is just R, over R plus R,

or V minus equals one-half, V out.
or V minus equals one-half, V out.

So, we have let's put a,
So, we have let's put a,

let's put a voltage on here.
let's put a voltage on here.

Let's put a real voltage on here.
Let's put a real voltage on here.

Let's say this is at one volt, alright?
Let's say this is at one volt, alright?

And going through our amplifier,
And going through our amplifier,

we know that V out equals two volts,
we know that V out equals two volts,

and that means that V minus equals one-half of V out,
and that means that V minus equals one-half of V out,

so V minus is one volt.
so V minus is one volt.

So this is one volt here.
So this is one volt here.

So let's say for the moment,
So let's say for the moment,

that something happens to the circuit,
that something happens to the circuit,

like we heated up or something like that,
like we heated up or something like that,

and let's say the gain goes up a little bit.
and let's say the gain goes up a little bit.

Now what that means is,
Now what that means is,

that this amplifier, which is amplifying
that this amplifier, which is amplifying

this voltage difference right here,
this voltage difference right here,

is gonna be a little higher,
is gonna be a little higher,

so the voltage here is gonna go up a little bit.
so the voltage here is gonna go up a little bit.

Let's use this color.
Let's use this color.

It goes up a little bit,
It goes up a little bit,

and that means that this output voltage
and that means that this output voltage

is gonna go up a little bit.
is gonna go up a little bit.

And we already decided from looking at this voltage divider,
And we already decided from looking at this voltage divider,

that if this point goes up, that this point will go up.
that if this point goes up, that this point will go up.

It goes up half as much, but it,
It goes up half as much, but it,

it goes in the up direction.
it goes in the up direction.

When this voltage goes up, that means
When this voltage goes up, that means

this voltage goes up, and now we find ourselves,
this voltage goes up, and now we find ourselves,

we're at the inverting input.
we're at the inverting input.

We're at the inverting input to the amplifier,
We're at the inverting input to the amplifier,

and that means what?
and that means what?

When a change at the inverting input goes up,
When a change at the inverting input goes up,

that means the output goes down.
that means the output goes down.

And that's in the opposite direction
And that's in the opposite direction

of the original change.
of the original change.

So this is the mechanism of Feedback.
So this is the mechanism of Feedback.

A went up a little bit.
A went up a little bit.

We thought that V out would go up a bit,
We thought that V out would go up a bit,

which meant this point goes up,
which meant this point goes up,

which meant it gets fed back to the input,
which meant it gets fed back to the input,

to the inverting input,
to the inverting input,

and then it goes back down,
and then it goes back down,

and this balancing act that's going on right here,
and this balancing act that's going on right here,

that is the mechanism, that is what we call Feedback.
that is the mechanism, that is what we call Feedback.

You get this Feedback effect,
You get this Feedback effect,

when this connection is made right here,
when this connection is made right here,

back to the inverting input, to the op amp.
back to the inverting input, to the op amp.

And in particular, because it's the inverting input,
And in particular, because it's the inverting input,

this is called Negative Feedback.
this is called Negative Feedback.

So this is the mechanism of Feedback,
So this is the mechanism of Feedback,

in particular, Negative Feedback and,
in particular, Negative Feedback and,

what it does for us is,
what it does for us is,

it provides us a way to exploit and to use,
it provides us a way to exploit and to use,

this enormous gain that these amplifiers have,
this enormous gain that these amplifiers have,

to create really stable, really nicely controlled circuits,
to create really stable, really nicely controlled circuits,

that are controlled by the values of the components
that are controlled by the values of the components

we attached to the, to the amplifier.
we attached to the, to the amplifier.

So that's the idea of Feedback,
So that's the idea of Feedback,

a really powerful idea,
a really powerful idea,

and really at the heart of analog electronics.
and really at the heart of analog electronics.