Lecture 4 1 Intro to Op Amps part 1 of 2

ECE1250 from the University of Utah. This video gives an example of an inverting amplifier op amp circuit calculated with ideal and non-ideal assumptions.

>> Hello, this is Dr. Cynthia Furse at the University of Utah.

Today, we're going to talk about Operational Amplifiers or Op Amps.

Remember that electrical engineering is about what you can do to a voltage.

For example, a resistor converts voltage to current, and current to voltage.

A voltage divider divides the voltage, and a current divider divides the current.

We can think of this as the operation of a voltage divider or a current divider.

Here's the circuit that goes with that operation.

Why do we need an Op Amp?

Because there are many more operations that we would

like to do than the devices that we have today.

For example, an Op Amp circuit can amplify or multiply voltages.

It can de-amplify or divide them.

It can add, subtract, compare.

It could switch voltages,

and many more operations can be done with Op Amp circuits.

So, what is an Op Amp?

The Op Amps we use are going to come into the little chip like this.

There are eight pins on this chip.

This will fit into your protoboard and you can see chips one, two, three, four,

and here, the pin numbers start with this little dot here at the top.

On pin two, we have an input Vn,

that's our negative input,

and there's a Vp input on pin three,

that's the positive input.

Pin six has the V output.

So, there are two voltage inputs and there's one voltage output.

This triangle right here is the symbol that we use to represent the Op-Amp.

The Op Amp is different than other devices that we have used in the past.

A resistor, for example,

always acts like a resistor even if we don't have it connected to power.

But an Op Amp only acts like

an operational amplifier if we connect it to its outside power supply,

Vcc and minus Vcc.

This is very important.

Keep your eye on the Vcc as we go through this lecture.

This is what's inside the amplifier.

This is what's inside that triangle.

Here is the negative input,

the positive input, and the output.

You can see there are a series of transistors that are hooked up in

this fashion that make the operational amplifier do what we want it to do.

However, these transistors only work if they are connected to Vcc and minus Vcc.

This is why the amplifier is an active device.

Without this power, it does not act like an operational amplifier.

Okay. So, now that we know that we have

this triangle that represents an operational amplifier,

if we want to hook it up,

we're going to hook it up this way with plus and minus Vcc.

But that's complicated to draw,

and we really just remember that we have Vcc and we normally draw it this way.

Another thing to remember is all of those transistors in there,

they divide the frontend and the backend of the amplifier in effect.

So, even though we might have a positive current here and a negative current there,

because there are all of those transistors and the power supply,

sometimes current gets added and sometimes current gets subtracted.

We don't really know in advance.

So, all we can say is that i zero, the output current,

is not equal to the sum of the two input currents.

Now, let's talk about Op Amp Gain.

Op Amp Gain is intrinsic to the amplifier.

It's controlled by how the transistors are put together.

It's sometimes also called Open Loop Gain.

Op Amps have a very high voltage gain

typically on the order of 10 to the fourth to 10 to the eighth.

They also have a linear response.

So, the gain tells us that the output is equal to the gain times the input.

In this case, the input is considered to be

the difference between the positive and negative inputs of the Op Amp.

So, if we have a voltage gain of 10 to the fourth,

it would be linear of something like this,

a voltage gain of 10 to the eighth would be steeper.

Because we have Vcc,

remember I said follow Vcc,

it limits the actual output of our operational amplifier.

We cannot put out more power or

more voltage than we were able to put in to our amplifier.

So, our amplifier circuit always saturates at plus Vcc

and minus Vcc because of the power supply limitations.

Let's see how that saturation can help the operational amplifier acts like a switch.

Here is a circuit where we hooked up our amplifier and we want to

be able to control if a red or a green LED turns on.

So, we've connected our negative input right here to ground along with our two LEDs.

Remember that the LED only turns on if current

goes through it in this direction because of its diode nature.

So, let's see what happens if we put a voltage,

let's say, two volts, on our positive input.

In that case, remember that Vp minus Vn is the thing that we're interested in,

and that's two minus zero in this case.

So, it's two volts, two volts minus zero volts.

Multiply that by a very large value, let's say, 10,000.

So, our output voltage tries to go up to be 20,000.

Well, the limitation of the power supply limits that,

let's say, to 12 volts.

So, the output voltage right here is 12.

Aha! But that's very good because that gives us a twelve volt difference across

our LED that drives the current in this direction and turns on our red LED.

The green LED is trying to drive current in this direction,

but the LED acts like an open circuit because of a diode and it doesn't turn on.

Now, let's see what happens if we put a negative voltage on Vp instead.

Then, Vp minus Vn is going to be minus two volts.

Multiply that by a very large number and it tries to go to, say, minus 20,000.

But it's limited by the power supply voltage,

so it goes to minus 12.

Well, if it's trying to drive current through here,

the red LED acts like

a diode and doesn't allow current to go through. It doesn't turn on.

Oops! Sorry, I'm not trying to drive current in that direction.

I'm actually trying to pull current in that direction.

The green LED, however,

does turn on because here goes the direction of the current for the green LED.

So, if we make Vp positive,

we can turn on the red LED,

and if we make it negative,

we can turn on the green LED.

This allows us to have a circuit that acts like a switch for our two LEDs.

Now, let's talk about Op Amp Gain versus Circuit Gain.

The op-amp gain is always high and it's intrinsic to the inside of the op-amp.

It's also called the open-loop gain or the voltage gain.

Gain is typically defined as the output divided by the input, and in this case,

the input is considered to be the difference of the two inputs.

Circuit gain, on the other hand,

we may have an op-amp in here and connect it up in this fashion.

Connect the output right there.

The circuit gain tells us about the op-amp that has

its gain A for all of the other resistors around it also.

Depending on how we connect up our op-amp circuit,

gain can be large or small.

If it's large, larger than one, that's amplification.

Less than one is deamplification.

If the gain is negative,

we call it inverting,

and if the gain is positive,

we call it non-inverting.

Here's a non-ideal op-amp equivalent circuit.

Notice that right here is a dependent voltage source that depends on

the gain and the difference between the two input voltages.

So, for a non-ideal op-amp,

we recognize that Ri is very large.

It's typically on the order of 10 to six to the 10 to the 13th.

A is very large, 10 to the fourth to 10 to the eighth.

The output resistance is very small,

one to a 100, and the supply voltage right here,

Vcc and minus Vcc,

and sometimes they draw it that way too,

is five to 24 volts.

Now, for an ideal op-amp, it's going to be different.

For an ideal op-amp,

we're going to assume that the input resistance is so large that it can be eliminated.

In that case, it's like having an open circuit right there at the input.

We can consider that the gain is so large that we can say Vp is equal to Vn,

and we can say that the output resistance is so

small that we can neglect it and consider it just as a wire.

The implications of this are that for an ideal op-amp,

we can say that Vp equal Vn,

and we can say that no current goes into the either input of this op-amp.

Sometimes it's also useful to remember,

right here, that R out is equal to zero.

You'll see why we need that in a minute.

Okay. So, the circuit gain depends on the circuit.

In this red box,

I have an example of a non-inverting amplifier.

Trust me for now that the output is going to be equal to

R1 plus R2 over R2 times Vs for this particular op-amp circuit.

In that case, we call this the gain,

G, and we can see that it's always positive.

That's because resistors are always positive.

So, it's non-inverting, it's always positive.

Now, let's see if this non-inverting amplifier can go to any value we want.

No. Because we have this Vcc and minus Vcc,

so we know that the gain has to be limited by Vcc.

So, the op-amp gain is intrinsic to the amplifier,

but the circuit gain depends on all of the resistors that we put around it.

So, in summary, here are the important things to remember about an op-amp.

Here is a model of a non-ideal op-amp.

We're actually not going to use that very much in

this class but I want you to remember these parameters.

We are going to use the ideal op-amp model extensively.

Remember that the important features of

the non-ideal op-amp are that the negative and positive input are equal,

that no current goes into either input,

and that the output is not the sum of the two input currents.

Remember also that we have Vcc and minus

Vcc that limit the output voltage of our operational amplifier.

Now, I hope you're wondering where the picture was on the front.

That is the Wild Horse Corral on Antelope Island.