Real Analog - Circuits1 Labs: Ch5 Vid1: Operation Amplifiers

Real Analog - Circuits1 Labs: Ch5 Vid1: Operation Amplifiers

Constructing operational amplifier based circuits.

>> In this video, we'll discuss the basics of

implementing operational amplifier based circuits.

Main points to look for are the fact that we have to

apply power to our Op-Amps to make them work,

and that the output voltage from the Op-Amp is limited by the power supply voltages.

These are probably two of the points that get

overlooked most often when we analyze Op-Amps circuits,

but they have a huge effect when we build and test a circuit.

First, let's just briefly go over the rules that we

use to analyze operational amplifier based circuits.

The Op-Amp circuit schematic symbol is shown here.

Op-Amps typically have five or more terminals.

The operational amplifier has two inputs and one output terminal.

The ideal operational amplifier rules are that there are no voltage difference

between the input terminals and no current flowing into the input terminals.

Two of the terminals V plus and V minus are used to provide power to the operational

amplifier.

The Op-Amp will not function if you don't apply power to these terminals.

To complicate things, these terminals are sometimes not even evident on circuit diagrams.

We just assume that they're there and let it go with that.

When we analyze ideal Op-Amps,

we generally don't assume anything about the voltage and current at the output terminal.

However, there are some practical limitations associated with these parameters.

The output voltage is restricted to being between the power supply voltage is

V+ and V- and the output current is limited by the design of the Op-Amp itself.

The Op-Amps in Digilent unlike, parts kit are provide

relatively little current but higher current devices are available.

Finally, one very important point relative to the voltages on this diagram,

they should all be relative to the same reference voltage.

This isn't a big deal if your using only the analog discovery,

since all of the voltages provided by it are automatically relative to the same voltage.

But if you're using other sources of power in your circuit,

you may need to physically interconnect them to ensure a common reference.

All the labs in this chapter specify use of

the OP27 Op-Amp from the digital and analog parts kit.

So, I want to spend a little time talking about

the physical pin descriptions for that Operational Amplifier.

The general comments I'll make for this Op-Amp will

be typical for most operational amplifiers,

but the specific pin descriptions may vary from Op-Amp to Op-Amp.

In order to find the pin descriptions for any given Op-Amp,

simply look up a datasheet online.

One of the first pieces of information on the sheet will generally be

a picture similar to this one with a description of the pin functions.

First, a couple of comments about Operational Amplifiers in general.

Op-Amps which can be used in a solderless breadboard are generally in what are

called dual in-line packages abbreviated as DIP.

This simply means that the pins are oriented in two rows of pairs.

The spacing of the pins allows the chip to be inserted in

a solderless breadboard with the central channel of the breadboard separating the rows.

This ensures that any pin is electrically isolated from any other pin.

Second, the orientation of the pins is specified by a notch at one end of the chip,

or a dot in one corner of the chip.

Many chips have both of these indicators.

If you're looking down at the chips with the notches at the top,

pin one will be in the upper left-hand corner of the chip.

Other pins are numbered successively in a counterclockwise direction.

Alternately, if you use the dot to orient the pins,

the dot is closest to pin one.

Now, let's take a look at some physical Op-Amps themselves.

This is the OP27 Op-Amp.

The Op-Amp type is printed directly on the top of the chip.

It has eight pins in two rows of four each.

It's been inserted into the breadboard correctly with

the central channel of the breadboard between the two rows of pins.

This particular Op-Amp has both a notch and a dot to indicate pin one.

The other pins are numbered counterclockwise,

so this is pin two,

three, four, five and so on.

>> The individual pin descriptions for the OP27 Op-Amp are shown here.

Pins two and three are the inverting and non-inverting inputs.

Pin six is the output terminal.

The positive voltage supply is connected to pin

seven and the negative voltage supply is connected to pin four.

Pins one and eight are offset voltage trim terminals.

These can be used to fine tune the Op-Amp behavior,

but we won't use them in this course.

Pin five doesn't do anything.

It's said to be not connected or NC.

We will always need to provide power to our circuit.

So, I'll go ahead and connect the voltage supplies

now before we wire up our example circuit.

This pin is the positive voltage supply,

this pin is the negative voltage supply.

I'll use V plus on my analog discovery for

the positive voltage supply and V minus for the negative voltage supply.

I generally connect my power supplies first. Since my experience has been that

forgetting to connect these as my number one silly mistake when wiring circuits.

For this video, we'll create

this inverting voltage amplifier circuit and investigate it's response.

If we analyze this circuit,

we'll find that the output voltage,

V-out, should be negative two times the input voltage, V-in.

We've already connected V plus and V minus to the positive and negative voltage supplies.

So, all that remains is to add the resistors in the circuit and apply an input voltage.

We'll use channel one of

our waveform generator to apply the input voltage to this circuit.

According to our circuit schematic,

we have a 20 kilo ohm resistor connecting the output terminal to the inverting input,

a 10 kilo ohm resistor connecting

our inverting input to the arbitrary waveform generators channel one,

which is this yellow wire,

and we need to ground the non-inverting input terminal.

We'll use channel one of our voltmeter to measure our output voltage.

We'll measure that between this terminal and ground.

To demonstrate our circuit,

first we'll have to apply power to the power supplies.

I'm going to turn on the voltage instrument.

Now, let's start by applying a relatively small voltage to the input,

say half a volt.

Turning on the waveform generator,

we see that our output is about negative one volts which is exactly what we would expect.

Increasing this to one volt,

the output is about negative two volts.

It's negative two times the input.

To emphasize the inversion property,

if I input negative one volt,

I should get positive two volts out.

Now, let's take a look at the saturation of the amplifier.

If we go up to two volts,

we get about negative four volts out which is what we would expect.

Increasing this to three volts, however,

just brings the output up to a little bit over four volts. We've saturated.

We've gotten as close to the voltage supplies as we can.

Increasing the input even more will not change our output.

That's really all there is to Op-Amps for now.

All of the lab assignments in this chapter you use

these basic tools, but apply them to different Op-Amp circuits.

Makes sure that you apply voltage to your Op-Amps for

all your circuits and the lab should be pretty straightforward.

The only thing to look for is that the ideal Op-Amp behavior breaks

down if the output gets too close to the supply voltage rails.