- [Instructor] One of the simplest op-amp circuits
that utilizes feedback is called a voltage follower.
It's created by simply connecting
the output of the op-amp directly back into
the negative inverting input terminal.
The input signal for the voltage follower goes into
the non-inverting or positive terminal,
and the output signal is simply the output from the op-amp.
The reason this circuit is called a voltage follower
is because the output voltage follows,
or matches, the input voltage.
Since the output voltage is the same as the input voltage,
the voltage follower circuit has a constant gain of one.
You'll often hear that referred to as unity gain,
which means that the signal passing through the circuit
is neither amplified nor attenuated.
At first glance, a circuit that outputs
the same voltage signal that you put into it,
that may not seem very useful,
but even though the input and output voltage is the same,
this circuit is useful because the impedance
at the input and output terminals is different.
The voltage follower takes advantage
of the op-amp's incredibly high input impedance
and low output impedance to serve as a buffer
between two circuits.
To maximize the voltage that's transmitted
from a source to a load,
the impedance should be bridged
so that the impedance at the load
is significantly larger than the source impedance.
If the source and load do not already have
that impedance relationship,
maybe the source impedance is
slightly greater than the load,
then you'll need to use a buffer circuit to bridge them.
The op-amp's impedance is so large,
it'll be significantly greater than the source,
so that connection will be bridged,
and the op-amp's output impedance will be
very small compared to most loads,
so that connection will also be bridged.
Since this circuit acts as a buffer with a gain of one,
it's often called a unity gain buffer,
and I'll show you seven common uses
for it later in this course.
But, how exactly does an op-amp act as a unity gain buffer?
By shorting the output from the op-amp directly back
to the inverting input terminal,
the voltage at those two terminals will always be the same.
The inverting input voltage will be
equal to the output voltage.
Now the op-amp itself doesn't know
that those two terminals are connected to each other,
but it doesn't care.
The op-amp simply looks at the difference in voltage
between its two input terminals,
which happens to be equal to the input voltage
minus the output voltage,
and then the op-amp adjusts its output
based on what it sees.
When the voltage at the input and output
of the voltage follower are the same,
there will be zero difference
between the two input terminals
because of the direct feedback.
The op-amp sees that the circuit is in equilibrium,
so it'll keep on outputting the same voltage.
Now, if the input signal that's connected
to the non-inverting terminal goes up,
that will create a positive difference
between the two input terminals.
The op-amp will see that positive difference
and respond by increasing the output voltage
until the circuit returns to equilibrium.
If the voltage at the non-inverting input drops,
that'll create a negative difference
between the two input terminals,
and the op-amp will respond to that
by decreasing it's output voltage
until that difference returns to zero again.
To demonstrate that behavior,
I've built a voltage follower circuit on my breadboard
using a variable voltage source as the input signal
and a one kilo-Ohm resistor connected to
the output terminal as the load.
I've connected a five volt source
to the op-amp's positive supply terminal
and the negative supply terminal is connected to ground.
My oscilloscope is showing the input voltage
on channel one in yellow
and the output voltage on channel two in blue.
Since the output matches the input,
those two traces are directly on top of each other.
So I'll adjust the position of channel two down
a little bit so I can see both signals.
The input voltage is currently set at 2 1/2 volts
and if I wiggle it around a little bit,
I can watch as the output voltage follows it.
However, if I raise my input voltage up
several volts higher, the output follows up
until it reaches five volts and then it stays there.
The op-amp cannot output a voltage that's higher
than the positive supply voltage I gave it,
which is five volts.
Similarly, if I lower the input voltage until it's negative,
the output follows it down until it reaches zero volts,
and then it stops there.
Since I connected the op-amp's negative supply terminal
to ground, it'll never be able to output
a negative voltage below zero volts.
As long as the input signal to the voltage follower
is within the usable range of the op-amp's
positive and negative supply voltages,
the output signal will look the same,
but if the input signal exceeds
that threshold in either direction,
then the output signal will get cut off at those limits.
This is a form of distortion called clipping,
and it's not just limited to the voltage follower.
Clipping can occur in all op-amp circuits
if the output signal tries to exceed the voltages
that the op-amp can actually produce.