4.2 Models of Diode Behavior

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Introduction to Electronics

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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.

À partir de la leçon

Diodes Part 1

Learning Objectives: 1. Develop an understanding of the PN junction diode and its behavior. 2. Develop an ability to analyze diode circuits.

4.1 Introduction to PN Junctions14:15

4.2 Models of Diode Behavior8:45

4.3 Ideal Diodes6:01

4.4 Assumed States Method14:19

4.5 Ideal Diode + Voltage Source Model10:45

Solved Problem: Diodes 16:42

Solved Problem: Diodes 28:39

Rencontrer les enseignants

Dr. Bonnie H. Ferri
Professor
Electrical and Computer Engineering
Dr. Robert Allen Robinson, Jr.
Academic Professional
School of Electrical and Computer Engineering

Welcome back to Electronics.

This is Dr. Ferri.

In this lesson, we will look at diode behavior and their models.

So in the previous less, we looked at the physics of PN junctions, and

we tried to explain how they behave.

In this lesson, we will analyze the diode behavior and introduce diode applications.

We will describe different operating regions.

And then we will go through some simple diode models that approximate the actual

device, and make it simpler for us to design and

work with diode models or diodes.

Let's go back to some background.

We first introduced diodes as, to find them to be

a combination of a p type of substance joined to an n type of substance.

So PN junction.

And just to remind you, the p substance is really a p.semiconductor,

which means it has an excess of holes.

And the n substance here it

has an excessive of electrons because it's n.semiconductor.

And this combination is such that, current can flow fairly easy in one direction but

it does not want to flow in the other direction.

So think of it in an, analogous term, as a one way valve.

So if you think of pipes and fluid flow through a pipe you

can have one way valves for the fluid flows in one direction but

it doesn't want to flow in the other direction.

That's the same thing here with electricity.

So, there's a circuit symbol for the diode and it's shown here.

And it's symbolic for a good reason.

You could see this triangle looks like an arrow, and

the triangle's in the direction that the current is allowed to flow.

And then we also define the polarity of the voltage drop

across the diode using this form.

So, why do we use diodes?

Well, they have tremendous amount of uses.

A very simple device, and they come in a lot of different forms and

a lot of different type of diodes.

And they, they're just very useful.

Any time you build a circuit where you want to restrict current flow within

certain direction, use a diode.

They're also used very commonly in rectifiers.

Rectifiers are circuits that convert alternating current to direct current.

And they're used in all kind of electronic chargers, all electronic chargers in fact,

laptop chargers cellphone chargers, they all rectify the signal.

They convert AC from the wall to DC, which is what your electronics want.

The basic component in those is, is the diode.

Voltage regulators and limiters.

Voltage regulators are used to regulate the voltage to a certain level.

Limiters are for protection, so that you don't have too much voltage or

current into a circuit.

Light emitting diodes very, very common in the last few years.

They replaced light bulbs can,

incandescent light bulbs in a lot of applications such as traffic lights, and

brake lights, and flashlights are all LEDs.

In addition, you've got LED displays, which give you a lot, a lot of

choice in programming, displays to be be a lot more vibrant and inter, interactive.

AM detectors are detectors that convert a signal

that is AM modulated into an information signal, these are used in AM radios.

AM radios they, they take a signal which has a modulation on there,

that was used for, for communications.

And they demodulate it into sort of a crude ways using diodes.

So very common in AM radios.

Then, we also have electronic tuners that use diodes, and

photodiodes, which are, are photo detectors.

So in this module, we will cover a number of these applications and

show you how to analyze those circuits.

Let's look at the iV characteristics for a diode.

Now this is the current versus the voltage.

And, we're defining the current again as a current in this direction through

the diode and the voltage drop this way, where the polarity is shown here.

So I'm, I'm plotting the, the current versus the voltage, and

a typical diode has this characteristic curve.

And we define the different regions, operating regions.

The forward bias region is shown here, and

that's the region which the current will flow in the forward direction.

You can see that the current is positive here.

Now, reverse bias is virtually 0 current, and

that's because the current does not want to flow in this direction.

We get, maybe a little bit of leakage current.

Now the third region is the breakdown region, and that's when I,

I have a voltage so large but negative, that the diode breaks down and

it starts to conduct anyway.

Now, most diodes are operated in the reverse bias and the forward bias region.

So these two regions right here.

There are some diodes which are purposely designed to work in the breakdown region.

Now, I want to compare the iV characteristics of a diode to a resistor.

A resistor is a linear element, and you can see it's got a straight line here.

And the slope is 1 over R because V over R is equal i from Ohm's law.

Now, a resistor is very easy to use and,

and analyze in a circuit, but a diode by itself is not.

If I look at this curve, it's really hard to put that in a circuit and analyze it.

So we come up with some very simple models that we can analyze in circuit.

And the first of them is an ideal diode model.

Remember this is the actually curve right here.

And then the red line right here is the ideal diode model.

And you get the, the general shape.

It's, it's meant to work in the reverse bias region,

as well as the forward bias region.

So this becomes a forward bias region right here.

Now if I want to make my curve a little bit more realistic,

a little bit more accurate.

Then I look at this curve right here, where you can actually see the actual iV

characteristics having, has a knee in the curve right here.

So when it really starts to conduct a lot, we call that the the voltage drop.

It's like a threshold under which the below which the,

you don't get very much current and beyond which you get a lot of current.

So that's like a threshold right here.

The third curve, the third approximate model

is the ideal diode plus voltage source plus resistor model.

In this case, the voltage source is equivalent to giving us a threshold, and

then the slope here is equivalent to having the resistor in there.

That so instead of having the straight line vertical line here,

we get a little bit of a slope to follow the curve a little bit better.

Now they don't like they're very good curves, and

that's only because we've zoomed in on the model.

This is, if we look at the axis right here, we've kind of zoomed in on this, so

that we can show this, this characteristic.

This, near the curve right here, when it starts to go up, is in the range from say

about 0.2 volts to maybe about 4 volts, depending on the type of diode you have.

So it's fairly small.

And then down here in the breakdown region, in most diodes,

this is very large.

It could be 50 volts, 100 volts or something like that, it's very large.

So, if we're going to be operating in this range right here, but

may be with a larger voltage, it actually looks like a better curve.

So let's, let's look at that.

This is where I've zoomed out.

So, if I've zoomed down on my axis, and my breakdown curve is way over,

my breakdown reaches way over here, for example, and this is say,

1 volt right there, and this is maybe 30 volts.

And I'm operating at a much larger range, then you can see that

if I change my scale, then this approximation looks a lot better.

So in blue here, is the actual and in red is the ideal model.

So it really is not a bad model once you're,

if you're working in larger voltages.

So in summary, diodes have three operating regions.

The forward bias, where the current is allowed to flow in the forward direction.

Reverse bias, where the current does not flow.

And that's when the voltage drop is negative.

And then you've got the breakdown region.

And the basic concept that I want you to

understand is that ideal diodes only allow current to flow in one direction.

In our next lesson we will look at ideal diode behavior.

Thank you.

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