Peter: Hello.
My name is Peter Wheeler, and today I'd like to talk to you about bipolar junction transistors.
Before we jump into the transistor, I'd like to create an analogy which you maybe comfortable
with.
Water, water in the house: let's look at how that comes to us.
The city has large pumps which bring the water from the lake or a well, something to that
effect.
Lots of pressure coming out of these pumps.
As you know what a fire hose looks like, and a hydrant, obviously lots of pressure.
We don't have that same type of pressure in your house.
Even though it's available, we don't use that same type of pressure.
We can't.
We'd blow out the pipes, so we do something like this.
Now that we're in your house, we need to control that water flow , the various sinks that you
have.
We use a device we call a faucet, like this.
To control the faucet, we use a tap.
Now we don't create all the water in the world and it goes nowhere.
It has to return at some point in time through the ground back to where we pull it from.
Why am I talking about this?
What we have here is a pressure that is developed.
What we have here is a flow of water, the water coming out like this.
What we have here is a tap to control that flow.
You can have it off.
You can have it hard on, show you get the full flow of the water.
Or you can do something in between.
It doesn't take a lot of effort to turn that tap to control that water compared to the
amount of pressures that comes out of the pumps from the city.
This is a water analogy.
You have push, you have flow, and oh yes, we have here a little bit of, let's say, resistance
to that flow so we can control the flow and the pressure that comes to us in some degree.
We have a similar analogy to this in electronics.
Let me draw something over here.
Now, wanted a source for this pressure.
This is a symbol for a battery.
A battery is a device that stores a lot of charge on one side and no charge on the other.
The essence of this device is what's on one side wants to run to the other side so that
it can spread itself out.
Because charge doesn't like to be all crowded together, just like people.
They like to spread out.
We have a source.
What we're going to do is we're going to take this.
This is our piping.
Some people talk about plumbing, but that's plumbing.
This is wiring.
We're going to put it into a device that looks like this.
This will be our faucet equivalent.
What I'm going to do is, like I said, you need a full circuit.
You don't have an infinite source of water.
We have to replenish.
We don't have an infinite source of charge.
We have to complete the circuit.
We'll do that.
Now, as this is sitting here, it's something floating in space.
What we want to do is reference it to mother earth so that we have a reference of what's
above, what's below, the situation that we have on mother earth here.
Here's Canada, if you're familiar with the geography.
This is mother earth.
This is our reference point.
That is a symbol for ground, which earth is.
That is our return.
Now, in order to look at this and see the analogy, this is a resistance.
The symbol for resistance is ohms.
A gentleman named Ohm was an engineer who discovered this.
We name it in his name.
This is the symbol for a transistor.
The components of this transistor are the base, the collector, and the emitter.
Makes sense.
Collect, send it back out, emit.
Now, this is our source measured in volts.
What we will see is a current flow.
I'm going to label it with I measured in amps, amperes, amps for short.
From this we're going to move to the next video after a short break, but before I go
there, what you need to know then is, as we discussed, this is the transistor.
These transistors, the way we're looking at this particular one right now, we can actually
redraw it like this.
These are diodes.
This is N material, the negative side of the diode.
This is P material, this is P material, and this is N material.
This will be labeled as an NPN transistor.
You'll need to know that in a diode, if it's silicon, there is a pressure drop of approximately
.7 volts across the anode to cathode junction.
We'll have that to take into consideration when we start talking about transistors.
What else do we need to know?
We need to know also that in order to understand just how much flow we have, you got push,
you got flow, but you got pushback.
Given the push and the pushback will determine what the flow is.
The law that we use there is one called Ohm's law, which you should be familiar with, but
let's just go over it again.
What that says is, the push equals the flow ... let's do it this way ... times the pushback.
I can change that around a little bit.
We can say that the flow equals the push over the pushback.
One more time, we can say the pushback is equal to the push over the flow.
Simple math.
Now, that, the diode dropped ... Ohm's law, sorry ... the diode dropped, and one last
thing.
You turn this ever so slightly, yet you control a lot of flow.
The ability to give a little and get a lot back is known as gain.
If we look at the gain, what it really is is ... I'm just going to show it as a big
capital A for now ... is out over in.
If you can think about this for a few minutes, we'll come back and we'll use these concepts
of gain, Ohm's law, push equals flow times pushback, and a voltage drop across the semiconductor
junction of .7 volts for silicon, for silicon.
The other concept is there is a ground reference here to mother earth, one more time.
We'll pick this up in the next video.