Welcome back to Electronics.
This is Dr. Ferri.
In this lesson, we will look at buffer circuits.
In a previous lesson, we introduced the ideal op amps and we actually did
introduce the buffer circuit, but only to show how to analyze ideal circuit.
In this particular lesson, I'm going to go a little bit deeper into buffer circuits,
because we want to introduce physical op amps in circuits and
examine the buffer circuit in a physical circuit.
First of all, we need to understand why we use buffer circuits.
Well, we use them to boost power without changing the voltage wave form.
Now, a buffer circuit seems like a pretty obvious input-output relationship.
V in is equal to V out.
In that case, why would you use it if V in is equal to V out?
But, it turns out that if I've got an input voltage,
a lot of times those input voltages are power limited.
Remember that power is equal to i times V.
So, with the same V,
I would need to have maybe a large current to get a certain power out of it.
So, if my output voltage where to need to draw a lot of current.
I might need to have this amplifier in there because it has its own power supply.
It's got Vs and -Vs.
So it has its own power supply so
it can boost the power without changing the voltage.
We're going to demonstrate that in a circuit in a couple of minutes.
But first lets talk a little bit about the limitations here.
In a physical opp amp, we can only have a linear
range here between the negative power supply
and the positive power supply.
Again, we've got a VS and a minus VS right there.
In all of our cases, and normally an op amp is
operated in the linear range, where you had this V in is equal to V out.
This sort of Nonlinearity is called the Saturation.
So we say it saturates at the plus and minus,
sometimes you call it the rails, and those are the power supplies here.
This power supply, this value, might not equal this, it might be different.
In which case, this would saturate at whatever value it was.
Now lets look at an example of a circuit without the buffer.
We want to demonstrate why we need the buffer circuit.
So looking at this particular example, we've got a voltage source and
it is applied directly to a resistor, so that means that
V out is going to be equal to i times R and that is equal to VM in.
Ok, now suppose V in is equal
to a sine wave, A sine mega t.
That means I would be equal to A over R sine omega t.
Now if R is fairly small, that means the current would be very large, and maybe
our voltage source cannot supply that kind of current, it's current limited.
In that case, we would have a problem with this equation
if I cannot supply the current needed to give me this voltage.
Now let's look at a physical op amp circuit.
So here I've got a resistor.
Actually it's two resistors in series with one another.
The total resistance here is 100 ohms and I've got it connected to a ground
along this rail so everything along this upper rail is a ground.
This is the power supply.
It's also the oscilloscope.
So we're going to be generating a sine wave across
this resistor and we're also measuring the result in sine wave with the oscilloscope.
Now let's take a look at our screen.
I'm showing an oscilloscope read-out of a sine wave.
This is the voltage across the resistor.
If I increase the source amplitude, the function generator amplitude,
right now it's reading about 300 millivolts.
If I increase the amplitude, I should still see a sine wave, but
here's the problem.
It starts clipping and it clips because I can't generate that much current
in order to keep producing that sine wave over that particular resistor.
Now, we see that we had problems with that circuit so
let's replace the circuit with one with a buffer in it.
So what I wanna do is basically break this circuit right here and
add this op amp into it, the buffer circuit.
So we've got the same resister here, and we've got the same source right here.
We're just adding the buffer circuit in the middle.
In order to do that, we need to understand how an op-amp circuit works,
what a physical op-amp looks like.
And it is made out of an IC, integrated chip.
And this is an example of one.
Now notice that, if you can see it, there are these notches up at the top.
And the notches in integrated chips are used to tell you where the 1 PIN is.
We will call this the 1 PIN, 2, 3, 4 and then
we're going to count up the other side, 5, 6 7 and 8.
And any specifications on chips like this, they always tell you
what internally these PINS are connected to.
So this op amp by itself, just the op amp, is internal to this chip.
We connect it to these terminals through these PINS so
the PIN 2 right here, is connected to the V minus.
And PIN 3 is connected to V plus.
The minus V is connected to PIN 4.
V out is connected to PIN 6 plus Vs is connected to PIN 7.
So if I wanna connect up this circuit, I would connect the resistor to PIN 6 and
the voltage source to PIN 3.
So now let's take a look at the physical behavior of this circuit.
Let's rebuild this circuit from before but now with an op-amp in there.
This op amp, I put it on this board straddling the center line so
that I separate all of these pins there.
This is PIN one, PIN two, will need to go to PIN six.
So that's PIN two, PIN six is there.
And in order to connect it to my resistor, I need to connect
from PIN two or PIN six over to the resistors.
So, now I've got the resistors in line.
This is leading back, this is ground right here and
these two are my input voltage and my osciliscope reading.
Now let's look at our oscilloscope.
I'm measuring the output across the resistor, and
I see that it is a clean sine wave, with a volt peak to peak, of two volts.
Remember with just the resistor there, and
no op amp, the best I could do was 400 millivolts before it started clipping.
Well let me go ahead and increase the amplitude here.
And as I'm increasing it, I still get a clean sine wave across my resistor and
I can go up to ten volts peak to peak which is the most that my
function generator gives.
And I still have a voltage across the resistor of ten volts peak to peak.
Again, without the op amp, the best I could do is 400 millivolts.
So in summary, we've shown that buffer circuits
boost the power without changing the voltage waveform.
And that's really helpful in cases where our voltage source is current limited or
power limited.
Because our op amp has its own power supply, it's able to boost the power.
And we demonstrated physical op amp circuits and showed how to wire them.
In our next lesson we will do some basic amplifier configurations.
Now please go to the forum and ask any questions that you have one the homeworks.
And please try to answer questions that other people ask.
Thank you.