- [Instructor] When you need to amplify a signal,
or buffer a signal, filter a signal,
or simply add two signals together,
look no further than the operational amplifier,
or op-amp for short.
An operational amplifier is an electronic component
that acts as a voltage amplifier,
with an incredibly high amount of gain.
It's DC coupled,
which means it can be used to amplify
both AC and DC signal components,
it has two differential input terminals,
and has a single-ended output.
Op-amps have become popular in modern electronics
because they're fairly cheap, easy to use,
and they can implement a wide range of operations
for processing electrical signals.
If you look at the internal circuitry
of an operational amplifier,
you'll see a complex network of components.
For example, the op-amp schematic shown here
contains 20 individual transistors, that are interconnected
to form three separate amplifier stages.
The thing that makes op-amps so great,
is that to use them,
I don't need to understand how all of this complex,
under-the-hood circuitry actually works.
I can treat that complex op-amp circuit
as a single component,
and I just need to remember a few simple rules to use it.
The schematic for an operational amplifier
looks like a sideways pointing triangle,
with five terminals.
The two terminals on the left are differential inputs,
the terminal on the top with the plus sign
is called the non-inverting, or positive terminal,
and the terminal on the bottom with the minus sign
is the inverting, negative terminal.
As a differential amplifier, the input signal to the op-amp
is the difference in voltage between those two terminals,
subtracting the inverting input voltage
from the non-inverting input voltage.
The impedance of the input terminals on an op-amp
is designed to be so high that, for most practical purposes,
I can basically treat the op-amp
as if it has an infinitely high input impedance.
So, when I'm designing an op-amp circuit,
I can pretend like no current will flow
into either input terminal.
That approximation keeps things simple,
and is often referred to
as one of the golden rules of op-amps.
The output terminal on the right side is single-ended,
meaning the output voltage will be referenced
to the circuit's common ground.
The output terminal is designed to have a very low
output impedance, so it can provide lots of current
to whatever load is connected to it,
to produce the desired output voltage.
The last two terminals on the top and bottom
of the op-amp symbol,
are connected to the positive and negative voltage lines
that provide power to the op-amp,
which are called the power rails.
An op-amp cannot produce an output voltage
that is greater than the positive rail voltage
or less than the negative rail voltage,
so the range between those two supply voltages
needs to be big enough to encompass the full range
of the expected output signal.
Different types of op-amps
will be able to handle different supply voltage ranges
and will have their own requirements for the relationship
between the output signals they can produce,
and the supply voltage levels.
So, be sure to check the data sheet for that,
when you're choosing which type of op-amp to use
for a certain circuit.
In schematics, it's common to hide the power rails
to reduce clutter in the drawing,
but, even if they're not shown,
you always need to connect the power supply rails.
Op-amps are an active component
that require an external source of power to function.
For prototyping circuits on a breadboard,
I usually use op-amps that come in an 8-pin,
plastic dual in-line package, or PDIP, form factor,
which is a rectangular package with two parallel rows
of four pins.
Those pins are spaced apart just right,
so that the component can straddle the drop
that runs down the center of the breadboard
with one row of pins on each side.
I typically use a black wire
to connect the negative supply rail to the op-amp,
and a red wire to connect the positive supply rail.
The pins on the PDIP are numbered
counterclockwise around the package,
and the surface of the package
will always have some sort of dot or orientation marking
to indicate which side is which, so you can find pin #1.
Some op-amp models, like the 741 op-amp,
only contain a simple amplifier
within an 8-pin PDIP package.
But other models, like the 358 op-amp,
can have two separate amplifiers
packed into a single package.
The location of the input, output, and power supply pins
will vary for different types of op-amps,
so always check the data sheet
to make sure you're connecting the part correctly.
It's easy to destroy an op-amp
by accidentally connecting the wrong pin to power,
and unfortunately, an op-amp usually doesn't show
any visible signs that it's been destroyed.
To check whether an op-amp has been broken,
you'll have to measure the output signal
with an oscilloscope, and decide if it looks
like it's supposed to.
Since it functions as a differential amplifier,
the output voltage from the op-amp
will be equal to the difference
of the non-inverting input voltage,
minus the inverting input voltage,
times a gain factor known as the open-loop gain,
abbreviated here as AOL.
Op-amps are designed to have
an incredibly large open-loop gain,
usually over 100,000, which makes them incredibly sensitive
to small differences between their input terminals.
For example, if the inverting input terminal was at 1 volt,
and the non-inverting input terminal was at 1.001 volts,
the difference between those two terminals
is just 1 millivolt, but, when the op-amp scales that
with an open-loop gain of 100,000,
it turns that tiny 1 millivolt differential input
into 100 volt output signal.
Now, as I mentioned earlier, the op-amp can't generate
output voltages that are greater than, or less than
its power supply rail voltages, so,
if I had my op-amp powered by plus and minus
12 volt power sources,
that 100 volt output signal would get clipped off.
At the very most, it can only be 12 volts,
and depending on the actual capabilities
of the op-amp I was using,
it would probably be even slightly less than 12 volts.
If I swap those two input signals,
so that the inverting input voltage
was slightly higher than the non-inverting input,
then the op-amp would see that difference as -1 millivolt,
and its output would saturate in the other direction,
at -12 volts.
When the op-amp is used in this way,
it's called the open-loop configuration.
The op-amp simply scales the input
by its enormous gain factor, and as you can see,
it doesn't take much input to saturate the op-amp's output.
That open-loop behavior can be useful for some applications,
like using an op-amp as a comparator,
which I'll cover later in this course,
however, to keep those wild voltage swings under control,
it's much more common to use a circuit configuration
that provides an external path from the op-amp's output,
back to its input terminals,
which creates a closed-loop configuration
that provides feedback.