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So the last video we were looking at this five volts signal coming in here that, uh, you know, it's switching between zero and five volts, but it doesn't.
It's not able to drive a whole lot of current.
In fact, it was able to drive less than a 1,000,000 AMP.
And we're trying to turn an led on that requires 20 millions.
And I mentioned that we could use these little guys here.
These transistors and the transistors would work, as you know, basically, ah, you know, a current amplifier or or a switch.
And in this video, I want to talk about how that how that's actually gonna work.
So if we go and look at this is basically a picture here that I've drawn of a transistor and hopefully it looks like the ones we were just looking at, and, um, there are basically a couple different types of transistors and and just just to be clear about what we're talking about here, the types of transistors we're gonna be talking about is kind of the most generic sword.
And that's a bipolar bipolar junction transistor.
So a bipolar junction transistor and should be another s in there, and this is kind of the most common sort of transistor.
Or usually, if someone you know you might even see this referred to is just a bipolar transistor or even just a transistor.
Someone just says transistor, and they don't specify.
They're probably talking about a bipolar junction transistor, Um, and it's an and they often look like this.
They could come in different sorts of packages, but this is this is the sort that I have.
Um, and there are two types, I guess, of transistor.
There's that you might come across.
There's N p N and there's PNP.
And in this video, I'm actually not going to really talk about PNP at all.
Um, other than just to mention that it is.
It's another type of transistor that exists.
I find that in the work that I do, um, I'm using NPM transistors.
I don't almost 99% of the time.
It's not that often that I find myself using a PNP transistor, particularly um, in digital electronics.
You know, we do use them from time to time, and perhaps I'll make another video that talks about the differences here.
But rather than confuse it with all the differences.
Um, I'm just gonna focus on one type of transistor, which is the most common that you'll come across, which is the bipolar NPM transistor.
And this is what it might look like.
And it's got three leads here to connect to the circuit, which is a little bit unusual.
Um, if you're used to looking at things like resistors or capacitors or even in doctors certainly batteries, all of these things have to Leeds, which kind of makes sense when you put him into a circuit you can put him in in Siri's.
You could put him in in parallel.
Three leads is a little bit different, but hopefully, hopefully it'll it'll start to make sense.
And just so ah, when we're talking about this, the leads have each have different names, and so just so we know what they are, there's an emitter of base and a collector.
And this is this is how they're laid out in this particular transistor that I have that the transistors that I'm using Scrabble in here and take a look.
It's ah to end 39 04 So to end 39 04 and That's just one type of bipolar and PM transistor there are.
There are lots of other types that are very, very similar.
To what?
I'm what I'm describing this as particular attributes that you can you can look up in the data sheet, but but basically, this is the number of different transistors that are very similar to the one I'm talking about.
Um and so Ah, the three leads are the emitter.
Let me make that a little clear.
My handwriting is not that great here.
Let's see.
Emitter base and collector.
So, um, inner Basin collector, those air the three leads.
And if we look at the skirt circuit schematic for this transistor, what we're going to see is the way that it's drawn is it's usually drawn like this.
And there's a little arrow on this guy and huh These are the three leads coming off, and sometimes you'll see a drawn with a circle around it.
Sometimes that circle won't be there.
That's just a matter of preference, really.
And this lead over here is the base.
This guy up here is the collector, and and this guy down here is the emitter.
And so this is the circuit diagram for for an N P.
And the way it works is basically, um, the transistor allows a current to flow, um, in basically in this direction, and I'm gonna use sort of conventional current flows.
So this we're going to say this is our positive, uh, voltage up here.
This is our negative voltage here.
And so current is flowing in this direction.
Of course, we know that electrons actually for flow from negative to positive, but in conventional current flow, we talk about it going this in this direction, and the current will flow in this direction from the collector to the emitter and the The way we talk about that is we actually call this the collector current collector current, and sometimes you'll see that abbreviated his eye subsea.
It's a current of the collector.
And so this collector current will flow.
Um, based on whether there is another current flowing from the base to the emitter.
So there's another current flowing over here from the base to the emitter.
So the base is slightly more positive than than the emitter in this case.
And so this we call the base current based current and that will sometimes be abbreviated his eye sub b current of the base.
And so the way that the transistor works is that it basically will automatically adjust the collector current based on the base current.
And so what you'll see is you'll see that the collector current.
What the transistors trying to do is is trying to make sure that the collector current is equal to the base current times, times some value, and it's usually a pretty large value.
Um, usually like 100 to 200 100 is pretty common.
So times 100.
So this transistor is going to say, if there's some sort of base current flowing here, then I'm gonna take that.
I'm gonna multiply it by 100 and I'm gonna try to make the collector current equal to that.
So another way to look at this, um, kind of a neat little, ah sort of analogy here is you can almost think of the transistor.
If I if I kind of zoom in on what's going on inside that transistor is.
I have the base here and the bass comes in and there's actually you have to you have to make sure that the base the current flowing from the base to the emitter is is flowing in that direction because there's a, um effectively a diet inside the transistor.
So this is the base up here, and this down here is gonna be our emitter, and so you're gonna have a current flowing in this direction, and you could actually put a little m meter in here.
So this is this is gonna be, like a little meter that's gonna register How much current is flowing?
This is gonna look at our base current.
I be over here and based on what that little meter reads, we're gonna have effectively.
What is a variable resistor that allows the transistor the transistor?
You'd almost think of the transistor as a variable resistor because it has the ability to adjust how much current flows through it by sort of adjusting this variable resistor.
And then, of course, there's another little measure over here of amperage.
And this guy is the, uh is gonna be our collector, and so you can almost think of they're being a little guy inside the transistor who has got his hand on this on this variable resistor and he's looking at.
He's looking at the base current and he's thinking to himself, I want to look at this base card and then I want to adjust this variable resistor so that when I look at the collector current, then the collector current is equal to the base current times 100.
And this, of course over here is measuring the collector current.
I said, See, So this little guy is inside the transistor.
He's got his hand on this variable resistor and he could he could move this up and down to two.
Ah, adjust the resistance between the collector and the emitter you control almost Think of it is adjusting the resistance between the collector in the mirror to allow ah current to flow from the collector to the emitter.
But he's looking at both of these and meters he's looking to see Okay, I want to see how much current is flowing from the base to the emitter and whatever that is, I'm gonna adjust this thing to see if I can get the current flowing over here from the collector to the emitter to be 100 times what I see over here.
So this guy is kind of looking at both of these meters and he wants He wants to make sure.
So he's sort of like thinking to himself This is a little thought bubble here.
He's thinking to himself.
Okay, I want to make sure that this is 100 times what this says, and he's gonna just this up and down to do that.
And and that is basically what a transistor does you can almost think of it of as a variable resistor that is controlled by this other current over here.


How a transistor amplifies current | Digital electronics (6 of 10)

林宜悉 2020 年 3 月 28 日 に公開
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