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## Electrical engineering

### Course: Electrical engineering > Unit 3

Lesson 1: Operational amplifier# What is an operational amplifier?

The "operational amplifier" has two differential inputs and very high gain. Willy describes the symbol and properties of an op-amp. Op-amps are the backbone of analog circuit design. Created by Willy McAllister.

## Want to join the conversation?

- At7:17, are BOTH of the power supplies, POSITIVE power supplies?(5 votes)
- Hello Wagner,

Yes, the power supplies are identical. For example, you could use two 9 VDC batteries. Notice how they are connected at7:24. The ground makes one power supply appear positive and the other appear negative. The op amp takes advantage of this by alternatively "pulling" the load to the positive supply and then to the negative supply.

Regards,

APD(12 votes)

- What are the internal workings of the operational amplifier? Is it a transistor or something?(3 votes)
- Hi. How do i differentiate an inverting op amp from a non inverting op amp?(3 votes)
- Opamps themselves are not inverting or non-inverting. It's how you hook them up that creates either of these properties. The opamp videos that come next show the difference.(8 votes)

- At11:05, why might the the orientation of the symbols be changed?(3 votes)
- The opamp triangle symbol is symmetric, and the "pointy" output end aways (usually) points to the right. But there is nothing preventing the person creating the schematic from putting the + input on top or the - input on top. So you always take a quick glance at the signs to avoid making a bad assumption. My habit when I draw opamp schematics is to make the top opamp terminal the one that takes in the main input signal coming from the left. This can be either the + or - input depending on the opamp configuration I'm using. Each designer tends to have some sort of habit, but we are not all the same.(5 votes)

- what will happen to the op amp if the differential inputs are given with interchanged polarities(3 votes)
- Hello Venkata,

The short answer is that the direction of the output will change.

In practice nearly all op-amps operate using a negative feedback loop. If you connect the terminals incorrectly the feedback loop will be broken. Instead of having helpful negative feedback you end up with positive feedback. The circuit will either oscillator or more than likely, lock up on one of the power rails.

Sorry, if you are new to op-amps I have included many new terms. May I suggest that you print this note and set it aside. Continue your study of op-amps and take a look back from time to time to see if you understand the concept.

Please leave a comment if you would like to continue the conversation.

Regards,

APD(3 votes)

- What is the relation between the open loop gain and closed loop gain of an op-amp?(2 votes)
- Open Loop gain refers to the gain of the opamp itself (just the triangle symbol). When you include the opamp in a circuit with resistors, there is always a resistor that connects from the opamp output back to one of the inputs. That forms the 'closed loop'.

If you connect everything up right the Closed Loop gain of the circuit will be determined only by ratios of resistors. The open-loop gain of the opamp drops out of the computation of closed-loop gain.(4 votes)

- At6:06he mentions about 20 to 50 transistors and resistors being used so if vacuum tubes were used instead of transistors what would be the number of vacuum tubes?

Asking just outta curiosity...(2 votes)- Hello Amresh,

This is a complex question. In modern op-amps you will find both PNP and NPN transistors. Know that there is only one type of vacuum tube - think of it as an NPN transistor. Consequently, the circuits must be designed differently.

You may be interested in the Philbrick K2-W - link below. This is one of the first recognizable op amps on the market. It uses two each dual triodes (12AX7 vacuum tubes) as the active elements. By modern standards the circuit is very inconvenient with its + and - 300 VDC power rails and the 6.3 V (likely AC) for the tube heaters.

Fun fact - the vacuum tube identifier tells you something at a glance. The first number (12) indicates the heater voltage. For this particular tube the heaters may be run in parallel or series. Philbrik chose to use a parallel 6.3 V connection.

The last number in the tube identifier (7) tells you the number of active components. Here we have a dual triode (2 x 3) plus the heater.

I'm curious what has you thinking about vacuum tubes? Please leave a comment below.

Enjoy,

APD

Ref:

http://www.philbrickarchive.org/k2-w_refurbished.pdf

http://www.philbrickarchive.org/(2 votes)

- Is it possible to operate a UA741 op amp with a single power supply?. And if so, Should the voltage be twice of the dual supply voltage?(1 vote)
- Yes, you can use a uA741 with a single supply. See an example here from the Texas Instruments datasheet: http://www.ti.com/lit/ds/symlink/ua741.pdf in Section 9.2 and figure 12.(3 votes)

- At10:05, what does he mean by "v+ goes this way"?? does that mean raising the value of +v? same question with -v also..(1 vote)
- Hello Karthikeyan,

V+ physically describes the non-inverting input terminal to the op-amp. It is also a description of the input voltage. We need to look at the context to determine which thing we are talking about.

Similar statement for V-.

Vo physically describes the output terminal. The same term is also used to describe the output voltage as measured at the output terminal.

The op amp will respond to the difference between V+ and V-. If V+ is greater than V- then Vo will swing positive. Likewise if V- is greater than V+ the output will swing negative. The amount of swing is (V+ - V-) * Gain. Where gain is a very large number.

BTW I read your profile where you described your search. May I recommend: taking a look at

https://www.youtube.com/adafruit

Please let me know if you have any other question.

Regards,

APD(3 votes)

- what does it mean to have a negative voltage?

I mean, if i use a battery of 1.5v, then the positive terminal is at +1.5v and the -ve at 0v right?

Or is the other terminal at -1.5v?

But that would mean a 3V potential**difference**right?(1 vote)- Voltage is a relative measurement. When you say a voltage value, it is always with respect to some other point. Even if you don't mention the other point explicitly, there is always another point. If it is not mentioned, that means it has to be assumed, and you have to know that everyone talking about the voltage shares the same assumption about the other point.

If you have a 1.5v battery, the positive terminal is at +1.5v**relative to**the negative terminal. It is also equally correct to say the negative terminal of the battery is at -1.5v**relative to**the positive terminal. If you say "the voltage of the negative terminal is 0v", that means is you are assuming the "other point" needed to specify voltage is also the negative terminal.

Voltage is kind of like height. The top of my head is 5' 9" above my feet (+5'9"), and I can also say my feet are 5' 9" below the top of my head (-5'9"). Usually when we talk about the height of a person the assumed reference for 0 height is the bottom of your feet.(3 votes)

## Video transcript

- [Voiceover] We're gonna talk about the operational amplifier,
or op-amp for short, and this is the workhorse
of all analog electronics. The operational amplifier,
it's a type of amplifier. An amplifier is anything that you put an electronic signal in, and you get out a larger
version of the signal. So this will be an amplifier
with some sort of gain, and if I put a signal x in here, usually a voltage or a current, then the signal that comes
out here is A times x, and that's what we mean by amplification. And a signal, that I've shown here as x, is anything that we're interested in, it could be a voltage or a current, and when we put it through an amplifier, we get a larger version of it. So this is a really common
activity in electronic design. Now, when we talk specifically about an operational amplifier,
the symbol for that, we use for an operational
amplifier, is a triangle. It has two inputs, one is the plus input,
one is the minus input, and it has an output, and it also has two power supplies to it. There's some sort of plus
voltage that goes into it, and some sort of minus voltage. So this is the abstract
symbol for an op-amp. So when we say the word op-amp, we have some specific properties in mind. One is op-amps have high gain. So in this case, the gain,
usually with a symbol A, is something like 10 to the
fifth to 10 to the sixth. Really, really high. Another thing we think about
when we talk about op-amps is that they're used
for feedback circuits. And we'll talk about feedback
in the next couple of videos and what that means, but that's the application
that we use op-amps for. And the third that's
distinctive about op-amps is that they have this kind of input. This kind of input here is referred to as a differential input. So an op-amp usually
has differential inputs, and that's as opposed to something we call a single-ended input, which
will be just one wire. And what a differential input means, it says, we have, we can
label the voltages here. We'll call this v out,
we'll call this v plus, and we'll call this input v minus. And differential input means that v out equals the gain times v plus minus v minus. So the output signal here is
proportional to the difference in the voltage between
these two signals here. So I wanna make a plot of
this equation right here, just so we get a good idea
of what it looks like. And the axis here are v in and v out, where v in specifically
equals v plus minus v minus. v plus minus v minus,
that's the input signal to the op-amp, and we're gonna
apply the gain factor to it to get v out. So that'll look like
this, something like this, it's gonna be a very steep line, and the slope of that line, the slope of this line is A. So the slope is going
to be 10 to the fifth or 10 to the sixth, something like that, very, very vertical. Now one of the properties
of this is that v out cannot go above or below
its power supply voltages. So on this plot here,
that's called saturation. If v out gets up to V
plus, we say it saturates. It looks like this, it
goes flat basically here and here, where this voltage value here is minus the power supply, and this voltage right here
is the positive power supply. But over this range here,
over this range in here between those two points, is quite linear. It goes through zero, and this is where we
use it most of the time. So now I wanna talk a
little bit more detail about what this symbol means
here and what's inside it, and how it's actually
connected up in a circuit. So we talked about the
voltage behavior of an op-amp. This is v plus, v minus, and v out. There's one thing more
that we need to know, and that is the current, this current right here and this current right here, for an op-amp, an ideal op-amp, is zero, no current flows in here. So this op-amp is just sensing
the voltages at these points but no current flows in. So this is the second key
property of an op-amp. The first one is the voltage behavior, v out equals the gain times v plus minus v minus. Another way we can write this
is v out equals A times v in, where v in, of course,
equals v plus minus v minus. So these are the two electrical properties that are gonna allow us
to analyze these circuits. Analyzing these circuits is
actually gonna be pretty simple. So you're probably wondering,
what is inside here, what's going on inside here? So what's inside here is, somewhere between 20 and 50 or
so transistors and resistors, sometimes capacitors. These are really complex
designs, and for right now, if we just concentrate
on the two properties that we have here, we'll be
able to use these circuits even without understanding
exactly what's inside. Suffice to say it's a
differential amplifier with really high gain, and with just that knowledge, we can work out how these circuits work. So let me do a couple of more details on how this thing is actually hooked up. So we have a plus terminal
and a minus terminal. There's more terminals on this, there's a power supply like this, that's plus big V, and there has to be a minus supply, typically a minus supply, minus V, and there'll be a ground pin, there'll be a ground node on here, like that. And when this is used in a circuit, there'll be, over to the side, there'll be two power supplies, and this will be 12 volts, 12 volts is a real typical value, and there'll be another one, and this is a plus 12
volts supply as well, and they'll be connected together, and this node right between them will be the ground node, that's the voltage reference, and these two guys will
be hooked up like that, the two power supplies
will be hooked up this way. So, with respect to ground,
this node is at minus 12 volts and this node from ground
is at plus 12 volts. And ground is right in the middle. And when we measure v
out, we'll measure it with respect to this ground node. So this is the voltage where we measure plus or minus v out right there. And what we're gonna do
is we're gonna assume that all of this stuff
is always hooked up, and we're just gonna use
an even simpler symbol, just the three terminals like that, and you'll know that all of
the rest of the power supply's hooked up that way. And the thing to keep in mind is there's a large minus voltage, there's a large plus voltage, and the ground level, the ground node, is right in between, so positive voltage is high on the page, negative voltage is low on the page, and v out can go both positive
and negative around ground. So that's your voltage
framework to keep in your head. And the op-amp that we've been looking at has a symbol like this, and we know that v out
equals some huge gain times v plus minus v minus, so this is a differential input, here's v plus, here's v minus. One way I think about this is to look at the way a
change in voltage on the input modifies the output. Let me label v out here. So, if there's a change on the input, say v plus goes this way, because it's a plus sign, that means that v out goes this way. Now, if I change it over to v minus, v minus is on the negative input. If v minus goes up, then v out goes down, so that's the inverting, it's
called the inverting input, and this is called the
non-inverting input. So on a non-inverting input, up goes up, and on the inverting input, if you go up on the inverting input, you go in the opposite
direction on the output. Let's say that the positive
input, the non-inverting input, went down this time, and that means what? That means that v out will go down, and just do the same thing over here, gonna run out of colors. Let's do the same thing
for the inverting input. If the inverting input goes
down, what does the output do? It goes up, it goes in
the opposite direction. That's the way to think about this symbol when you see it on a schematic page, is how do these signals
translate through the device? Positive, non-inverting signals
go in the same direction, inverting signals go in
the opposite directions. Okay, and here's one final
trick I wanna share with you, something to be aware of. You're gonna see a symbol
like this on a page, the same op-amp. It's the same op-amp, but
it's written on the page with the negative, with the
inverting symbol on the top and the non-inverting
symbol on the bottom. So, as you look at a schematic
that has an op-amp in it, one of the first things you
wanna do is just glance and see what order these symbols are in. Does it look like that or does it look like that? And keep that in mind as
you're reading the circuit and trying to understand what it does. Okay, let's move on and build
something with our op-amp.