These are These are similar to what we've kind of already been exploring, but I want to think of him in a little bit different way in that.

So each of these has kind of an input and output, and I want to think of these inputs and outputs in terms of voltage.

Um, and of course, transistors are current switches.

But, um, if this input, for example, is is a voltage than that'll, if it's a positive Leslie, let's say it's plus five volts then that's going to create a current and of course, the switch the transistor on it.

If it zero volts, then there will be no current and the transistor will will be off.

So we can think of this in terms of these imports, either being on or off five volts or zero volts, which which will help us out when we get into digital logic, where we where we deal with ones and zeros and so and in fact we can actually start talking about ones and zeros.

Now, as long as we keep in mind that a zero means zero volts and a one means plus five volts.

Um and so I'll actually just talk in terms of zeros and ones being our inputs and outputs because our input can either be a 00 volts or a one plus five volts.

And and actually, what we'll see is that the output is also gonna be either one of these.

So let's just take a look at this circuit on the left to start with and see what happens when its input is zero volts.

So if I have zero volts here and actually make a little table down here just to keep track of what's going on, So I have an input and my input can be either a zero or a one and Ivan Output, which we will see can also be either a zero or a one.

So if my input is 00 votes coming in, then the transistor is gonna be off because there's no current.

Remember, we have this this base current that has to flow in order for the ah, In order for the collector current to flow, that should be Hi, collector current to flow.

So with no based current flowing there's no collector current flowing.

And so there are output is not is not connected in any way to five volts cause the transistor is cut off.

And so the output is connected through this resistor to ground.

So if we measure the voltage between ground and our output, we get zero volts.

And so our output in this case is zero input a zero on our output zero.

Now, let's look at the case where our input is one.

So for input is one which remember means plus five volts.

So we're using.

We're using five We're using one to symbolize plus five volts.

Or I guess another way to think of it is we're using plus vie faults as as kind of our our symbol for for the notion of one.

But anyway, um, if our input is one, then we're gonna have that five volts, and so we're gonna have a current flowing across from our base to our emitter.

And so when that current is flowing, assuming there's there's enough current, we're assuming that there's enough current to get the transistor into saturation.

Which means that current will flow unrestricted from plus five volts to ground So when that's happening, if current is flowing unrestricted from plus five volts to ground if we were to measure our voltage from our output between output and ground output is basically directly connected to plus five volts.

Because we have, we have current flowing unrestricted from that plus five volts.

So in this case, our output is going to be one.

So this at first glance doesn't seem to be terribly interesting.

We put five volts in over here.

We get five bolts out over there.

We put zero vaulting over here.

We got zero votes out over here, but of course we saw before that this output contrive a whole lot more current than then the input might be able to so we might have our input have a relatively low amount of current, But then our output is able to drive more current, and they're a couple different names for for this circuit that I want you to be familiar with.

Um, one of them is is is this is called an emitter follower.

And let me let me rewrite this.

I apologised.

My handwriting is terrible.

Um, emitter follower, emitter follower.

And the reason this is called in a mid or follower is basically because the emitter, this is the emitter of our courses of the collectors is the base of our transistor.

The emitter follows the input.

It follows the base.

So the emitter follows the base.

So this is called an emitter follower.

Another name for this, um, might be a buffer.

And, um, basically a buffer just is a is a logic device that takes whatever its input is and gives it to you as the output.

And there's a symbol for the buffer that will use when we get into digital logic.

And it's basically just a triangle like this, and this is our input over here, and this is our output over here.

And so whenever you see this symbol, when we get into digital logic, it basically means a circuit that does this and it may not be implemented exactly like this.

With a transistor and a resistor, it might have other components in it.

Um, it might be a little bit more complicated, but effectively it will do this.

It will take whatever voltages on the input, and it'll give, and it'll give that same voltage level in the output.

And actually, it may take a range of voltages that signify one on the input and give out maybe a little bit more voltage on the output.

It may boost the voltage, but but effectively, whatever your definition of zero is on the input.

If if that's what you have there, then you'll get whatever your definition of zero is on the output.

And whatever your definition of one is, if it's plus five volts, if you see that on the input, then that's what you'll see on the output.

That's what a buffer does.

Um, this this circuit is also called in the middle Followers.

You might see both terms now moving on to this circuit.

We actually just built one of these.

You may recall, um, but we can We can go through the same process here and look it our input and our output.

And if our input can be either zero or one, if the input is zero, then the transistor is There's no current flowing across the base are our base.

Current is zero because our voltage is zero, so no current flows and so the transistor will be cut off and our output will be tied through this resistor, um, two plus five volts.

And so our output will be one and another name for this resistor that you might might might come across is called a pullup resistor pullup resistor.

And that's because it basically pulls up this output.

So this output, um, is being pulled up to five volts unless the transistor turns on, um and so Well, actually, let me just back up a second.

So when this is zero, the output is one.

When the input is one, you just erase thes when our input is one, then the transistor is We now have current flowing.

So this is five volts coming in here.

And so we have current flowing to ground.

And so the current flowing across the base is going to create a current flowing across the collector to the emitter.

And with this current flowing here, um, our output is now gonna be at a lower voltage because our output is effectively going to be connected to ground.

And so we're gonna have zero coming out here.

And this is this is what we saw in the last video way built one of these ah, and so winner and put his one.

Our output is zero.

And so, like I said, this is kind of couple pulled up resister because if if the transistor is is conducting, um, or if it's if it's in saturation, our output is kind of connected to ground.

But if the transistor goes away, then the resistor pulls it up to plus five volts.

And that's why it's kind of call a pullup resistors so you'll see that you might also see pulled down resistor.

You could think of this as a pull down resistor.

So when the transistor is off, our output is pulled down to ground.

But then, when the transistor comes on the transit, the transistor is going to give less resistance than this resistor.

And so our output voltage will will go up to plus five volts or very nearly plus five volts.

So with that in mind, there are also a couple names for this circuit over here.

Um, and actually, the most common name that you'll see for this is called an inverter inverter.

And there's also a logic symbol for that, and it looks very similar to the buffer, except that it has a little Ah, a little bubble on its output.

So this is our input over here.

And this is our output over here.

And likewise, whenever you see this symbol, um, in a logic diagram, it basically means that we're talking about a circuit like this.

And again, it doesn't have to be exactly the circuit with one transistor one resister.

It just has to be a circuit that obeys this particular this particular truth table.

So whenever whatever the input is a zero, the output will be a one.

Whatever the input is the one, the output will be zero.