- [Instructor] There's a wide variety
of operational amplifiers that have
different characteristics making them more suitable
for some applications than others.
And as with most things in life,
you can expect to pay more for the op-amp models
that have better performance characteristics.
For the op-amp circuits I'll be building in this course,
I'll use the LM358 operational amplifier.
It's a relatively low cost op-amp that works well enough
for most low-power hobbyist applications.
And as an added bonus, the LM358 actually contains
two separate op-amps within a single package,
which can come in handy when building complex designs
with multiple op-amps.
Although there are other op-amps on the market
that have better performance characteristics,
I've found that the 358 is a good go-to part
for my own projects.
And it's a good one to start with for beginners.
As I describe some of the key characteristics
to consider when choosing an operational amplifier,
I'll be looking at the datasheet for the LM358.
To find that datasheet,
I'll go to my search engine
and type in the part number, LM358,
and the word datasheet.
I see that there are several datasheets available
for the LM358 from different manufacturers.
And for this video, I'll look at this datasheet
from Texas Instruments.
This particular datasheet covers four related
operational amplifier models.
The LM358, 258,
158, and 2904.
Most of the characteristics will be in common
amongst those four models,
but where they differ I'll be focused on the LM358.
The features list on the first page
highlights several of the key characteristics
for this op-amp such as the range of supply voltages
that it can handle.
The LM358 is designed to be flexible
so I can use it with both single and dual voltage supplies.
When used with a single supply of voltage,
the negative supply rail should be connected to ground,
and the positive rail can be given a voltage
anywhere between three to 32 volts.
In dual supply mode,
the positive and negative voltage rails
can handle anywhere from 1.5 to 16 volts
in the positive and negative directions.
Located a bit farther down on the features list
is the open loop differential voltage gain,
which for the LM358
is typically around 100 DB.
100 decibels corresponds
to a voltage gain ratio of 100,000.
That means when the op-amp is used
in the open loop configuration without any feedback,
it only takes a tiny 100 microvolt difference
between the input terminals to produce
a whopping 10 volt output voltage.
However, an op-amp output a voltage
that's greater than it's power supply rails.
So, if this op-amp was being powered
by plus and minus five volt supply rails
even though the open-loop gain
should produce a ten volt output,
the op-amp can only really produce five volts.
Now, while and open loop gain of 100 DB is pretty huge,
the op-amp's ability to provide gain
will vary depending on the frequency of the input signal.
This graph compares the op-amp's open loop voltage gain
to the frequency of the input signal.
At very low frequencies, around DC,
the op-amp is able to provide lots of gain
up to that 100 DB.
But as the frequency of the input signal increases,
the op-amp's ability to provide voltage gain decreases.
When that input frequency gets up to around .7 megahertz,
the LM358 op-amp will, at best,
be able to provide a gain of one.
The unity gain bandwidth parameter
tells me the highest frequency
at which I can use the op-amp, with a gain of one,
before it will begin distorting my signal.
When I start working with signals over .7 megahertz,
I'll start to run into problems.
It's possible to find other operational amplifiers
that have a much higher bandwidth,
but since most of my hobbyist projects involve signals
that are down in the kilohertz range,
the LM358 works fine for me.
Most of the other parameters in this feature section
are covered later in the datasheet,
so I'll continue on down.
The third page of the datasheet gives me information
about the device package and pin configuration.
The LM358 contains two individual operational amplifiers
within a single eight pin PDIP package.
However, both op-amps share a single power supply.
The positive supply voltage will be connected
to pin eight, and the negative supply voltage,
or ground if you're using a single-sided voltage supply
should be connected to pin four.
Pins one, two, and three on the left side of the package
correspond to the input and output terminals
of one of the op-amps.
And pins five, six, and seven
on the other side of the package correspond
to the input and output terminals
for the other op-amp.
Continuing down to page four of the datasheet,
the absolute maximum rating's table
describes the maximum supply and input voltages
that the op-amp can safely handle before breaking
as well as its range of usable operating temperatures.
Towards the bottom of that page is another related table
with several recommended values
for those operating conditions if you want to play it safe.
On the fifth page is a sizable table to describe
the components electrical characteristics.
And there's quite a bit listed here
so I'm just going to focus on some of the key parameters.
This op-amp has a typical input offset voltage
of three millivolts,
which describes the amount of voltage difference
that will appear between the two input terminals
even when they're shorted together.
Ideally, since op-amps are differential amplifiers,
when there is zero difference between their input terminals
the output voltage should be zero.
However, real world op-amps are not perfect.
Even if their terminals are shorted together
so that the inputs are exactly the same,
the internal workings of the op-amp
will see those inputs as being slightly different
from each other, around three millivolts different.
That input offset of three millvolts
may not seem like much, but if the op-amp is being used
in a circuit with a gain of 100,
then the output voltage will be off by 300 millivolts.
Some models of operational amplifiers
provide offset null terminals that can be used to tune
the op-amp with external components to compensate
for that offset voltage.
Unfortunately, the LM358 does not have
offset null terminals, so I'm stuck with what I get.
Although it's common to pretend like operational amplifiers
have an infinitely high input impedance
that prevents any current from following
into their two input terminals,
in reality the op-amp does require a tiny amount
of input current to function.
The LM358 will allow an input bias current
of around 20 nanoamps to flow into the input terminals.
That current is necessary to bias the internal transistors.
And since op-amps are not perfect components,
the amount of current flowing into each of the terminals
will differ slightly from each other.
As described by the input offset current,
these bias an offset current parameters
can come into play for certain op-amp circuits
that are beyond the scope of this course.
For simplicity, as I look at op-amp circuits
later in this course, I'll continue to use the assumption
that the op-amp has an infinitely high input impedance,
and that no current flows into its input terminals.
The output current and short circuit
output current parameters describe the amount of current
the op-amp can handle at the output terminal.
Which is designed to have a very low output impedance
so the op-amp can provide the necessary current
for a variety of attached loads.
The output current is listed for two different conditions.
When the differential output voltage is positive,
the op-amp will act as a current source,
which means it pushes current out of it's output
terminal to raise the output voltage.
And when the differential output voltage is negative,
the op-amp will act as a sync,
which means current will flow in the other direction
drawing into the output terminal to lower
the output voltage.