This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.

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From the course by Georgia Institute of Technology

Introduction to Electronics

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This course introduces students to the basic components of electronics: diodes, transistors, and op amps. It covers the basic operation and some common applications.

From the lesson

Op Amps Part 1

Learning Objectives: 1. Develop an understanding of the operational amplifier and its applications. 2. Develop an ability to analyze op amp circuits.

- Dr. Bonnie H. FerriProfessor

Electrical and Computer Engineering - Dr. Robert Allen Robinson, Jr.Academic Professional

School of Electrical and Computer Engineering

Welcome back to Electronics.

Â We're starting module two, which is on Op Amps.

Â Lor lesson objectives here are first to introduce operational amplifiers,

Â to describe their ideal behavior, and then to introduce two particular circuits.

Â An, a comparator and a buffer circuit, which use Op Amps.

Â So operational amplifiers, or what we call Op Amps, are specialized circuits made

Â up of transistors, resistors, capacitors, and are fabricated on an integrated chip.

Â They're actually fairly complicated, internally.

Â But the nice thing about Op Amps is that they've got a fairly easy input to

Â output sort of behavior.

Â So, if I look at this as being my input to my circuit.

Â V sub n. And then this being the output, V out.

Â The, I can come up with a fairly simple expression relationship for

Â the input to output behavior.

Â So the uses of.

Â Op Amps is ann amplification.

Â So I can amplify a voltage signal, make it larger, or

Â I might be able to boost the power from the input to the output.

Â We typically use Op Amps in active filters.

Â What do I mean by active?

Â Active, an active device is something that has its own power supply.

Â So if I'm looking at this right here, V sub s and, you know,

Â I've got V sub s and minus V sub s gives me a power supply.

Â So any active element is one that has its own power supply to it.

Â Now in another use of Op Ampsers in Analog computers, and this is the old style

Â computers that we used years and years ago before they had digital computers.

Â They had Op Amps in them.

Â And that was one of the basic components of them.

Â Lets look at Op Amps in circuits.

Â And as I mentioned this is the power supply.

Â These don't actually have to equal each other.

Â I might have a, a different plus value and a diff with a different minus value.

Â And then I've got my output and my two input terminals.

Â So actually I've got five inputs to this circuit.

Â Now this circuit is, in, is fabricated on this integrated chip and I have pins

Â to this integrated chip that allow me to connect to the internal circuitry of it.

Â Now common values of power supply the V sub s is 10 volts to 15 volts.

Â Now a symbol, a circuit symbol for an Op Amp looks like this.

Â Now, notice that I dropped the power supply.

Â The reason I dropped the power supply in this symbol is because the power supply is

Â what makes it work, but it does not affect the circuit equations that

Â we use in analyzing Op Amps in circuits.

Â So again, the power supply doesn't affect our circuit equations.

Â In most circuits.

Â As I mentioned, it's an active element, because it has its own power supply.

Â And the other thing is that we're ignoring that,

Â V sub s in the symbol of it [SOUND] Lets first examine open loop behavior of this.

Â So I've got my op app here and

Â I'm going to have a value a which is a scaler of v plus minus v minus.

Â The difference in other words between these two input terminals.

Â So, this A is actually the slope.

Â And typically, A is really large, say on the order of like, you know, 10,000.

Â And we often tall, call that the open loop gain [SOUND] And

Â with that being very large and

Â it doesn't take very much for this to do what we call saturate.

Â When a difference, this difference,

Â doesn't get very large before it reaches this value here where it's constant.

Â We call that saturation.

Â [SOUND] And often times, we look at the input to this circuit as Vin,

Â and it's a difference between these two values, so this would be called Vin.

Â So with a very small value of Vin, it saturates.

Â Now let's do a quiz on that value.

Â Now in the quiz we had to 10,000 and

Â we had v of s is equal to ten, and so what value of this

Â of the voltage input before it saturated it was one milliamp minus one milliamp.

Â So in other words, very small value of V in made this saturate.

Â So we'd have what we'd say, a very small range of operation for the linear range.

Â This is the linear range right here,

Â very small range of operation, when we've got what we call, the Open Loop Behavior.

Â A comparator circuit is one that utilizes an Op Amp in its open loop configuration.

Â So we've got the open loop configuration here,

Â and we've just repeated what this value looks like.

Â In this particular circuit, we're assuming that for the most,

Â most of the time we are operating not in a linear region, but in a saturation region.

Â So in other words, if vin is greater than 0.

Â In other words, v plus is greater than v minus, we have a value of vs.

Â If v plus is less than v minus, we have a value of minus vs.

Â So, that gives us an indication whether these root,

Â we are looking at a difference really between these two voltages.

Â If one, if V plus is greater than V minus,

Â we have got a positive value out of our comparator.

Â And if V minus is, if V plus is great less than V minus,

Â we have got a negative value.

Â So, it's an indicator, which of these is larger than the other.

Â As an example of a comparative circuit, let's assume that we've got

Â a sine wave voltage going into V plus, whereas C is just some constant.

Â And we connected V minus to ground.

Â So our input voltage is actually some constant times the sine of omega t.

Â Now this is our comparator circuit right here.

Â And we're assuming that C is a fairly large value.

Â So that we're into the saturation region almost immediately, as we go through 0.

Â So if this is our sine wave, of our input.

Â Voltage.

Â That we're in the linear region very, very small amount of time.

Â So that means we're almost always in saturation.

Â So my output will actually have a value of vis of s

Â anytime I've got a positive value of vn.

Â And then when I go through ze, the 0 crossing, it becomes negative.

Â It goes to minus Vs.

Â And it stays that way as long as we're on the negative cycle of this sin wave.

Â And then when we go positive again, it goes positive.

Â So, in other words, we've converted our sin wave into a square wave.

Â By sending it through a Comparator.

Â Let's examine a model, or different models for Op Amp behavior.

Â Now as I mentioned before, what's internal to an Op Amp is a lot of transistors,

Â capacitors, it's rather complicated, but

Â we can come up with a fairly simplistic model of it.

Â That looks like this over here.

Â And in this model we've got an, what we call an input impudence, R sub i.

Â And R sub i is actually very large.

Â And because it's large, the current running through it is very small.

Â So this current i, is very small.

Â And we have this dependent source right there, and that dependent source

Â gives us the output, A times Vm, which we've already seen before.

Â For ASM gain.

Â Now this is a model for an We wanted to come up with this simpler model.

Â And this is what we're going to call the ideal model here.

Â Now, in the ideal model.

Â Instead of saying i is very small, we're going to say it's zero.

Â The input current at both of these terminals,

Â we're going to set equal to zero.

Â And since that current is zero, another words this current going through this

Â resistor zero then this voltage is going to be equal to zero,

Â the voltage across these terminals is equal to zero.

Â So this right here is equal to zero.

Â And we've also got this equal to zero and that equal to zero.

Â So that's my ideal model.

Â We're going to use that in analyzing op amp circuits.

Â The simplest circuit we're going to examine is what's called a buffer circuit.

Â A buffer circuit is made by just connecting the output back to

Â the negative terminal.

Â And this is what we call a feedback loop.

Â And because it's

Â a feedback loop we often call that closed loop and that's the difference.

Â Before I used the term open loop.

Â Now this is called a closed loop or a closed loop around here.

Â A lot of the op ant circuits we're going to be looking at.

Â Actually have a resistor in this.

Â But all of the other Op Amps circuits re, we'll examine aside from the Comparator

Â all have this feedback loop, with our without a resistor in there.

Â Now the buffer circuit has this relationship between

Â the output to the input.

Â V in is equal to V out.

Â We want to examine how you would come up with that, equation.

Â And is, it, to examine that, it's actually easier to re-draw this circuit,

Â showing some sort of reference ground.

Â because this V in is a node voltage, and it's with respect to some ground.

Â So, let me, go ahead and re-draw the input, and

Â the output voltage is this way, because I want to do a K-V all around this.

Â I'm going to do a K-V-L from the ground, up across the input,

Â across the input of the up amp and then back to the output.

Â So this is back to ground, so I've done a complete loop there, and I,

Â I want to show these equations.

Â The way I do a KBL is, I will go to here.

Â When I see a ne, negative sign on an element, I negate, I add in it, minus

Â voltage and go in Now actually, when I go from here to here, I'm gaining potential.

Â And I come around here, I'm losing potential, but just for my sake and

Â a lot of student sake, it's just easier whenever to use the convention.

Â Whenever you come a minus sign, subtract it.

Â Whenever you come to a plus sign first, you add it.

Â So coming across here,

Â I have the minus V n and I've got the voltage drop across this input.

Â Well with the ideal.

Â Die ideal op amp behavior that would be zero volts.

Â And then coming around here, I get to the plus V out is equal to 0.

Â And that gives me the equation back for buffer circuit.

Â So in summary, we've shown that op amps are active devices.

Â Active again, meaning they've got their own power supply.

Â And they can be used to filter or amplify signals, linearly.

Â Just to comment about linear, what I mean by linearly.

Â We've got an input and output characteristic.

Â And in the open loop,

Â we show that we've got this range right here, before we saturate.

Â This range right here, is what we call, the Linear Range.

Â So if we stay in that range, we stay in what we call linear range.

Â Now we looked at ideal op amp models,

Â in particular, the model required this assumption.

Â So any of the following lessons, we will be developing a lot of op amp circuits,

Â and we will always go back to this model right here.

Â Where these assumptions are, are true.

Â We looked at two particular types of circuits.

Â A comparative circuit, and a buffer circuit.

Â So the remainder of module 2, we will go in more depth in the buffer circuit,

Â we will cover a number of different amplifier configurations with op amps,

Â and we'll look at differentiators and integrator circuits.

Â And then we'll go on to look at active filters.

Â Now 'd like to encourage you again to go to the forums and ask and

Â answer questions.

Â We really appreciate your participation,

Â especially those of you who are on there answering questions.

Â You do a, a great favor to us and to the rest of the class.

Â Thank you.

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