 This video we're going to be walking through a transistor circuit that has been saturated and trying to calculate what your base resistor would be, which is this fellow right here, to determine what size this is to get this transistor at saturation. Now when something is saturated, when you do the calculations, if you calculate that this voltage, your voltage at your collector, is greater than your source voltage, you have not figured out a way to create energy out of nothing, so don't get excited about that. What you have is a saturated circuit, which means that your voltage across your CE, your collector to emitter, is going to be 0.1 volts. So I'm going to walk through this and show you how you can determine the size of your base resistor to get to saturation. Now let's get a few terms out of the way. Q, as in my last transistor video, is just a symbol for transistor. We have our collector resistor and our base resistor and what we're going to do in our last video, what we did was work our way from here all the way up to here and that. We're going to do the opposite now. We're going to start with this and work our self backwards. So let's start sending some values to these. We've got a battery here, we've got a VCE that we know is going to be 0.1 volts. Let's start punching the numbers. 24 volts source, it's at saturation as I told you, so this means that this is 0.1 volts. Now as I've said before, this is one circuit here, so I have to go 24 volts minus 0.1 volts to get the voltage at the collector resistor, which doing the math in my head becomes 23.9 volts. So now I have 23.9 volts here. We need to work our way backwards to the base. I need to give myself a collector resistance here, so let's just pick a number. I'm going to go with 10 ohms, make it easy on us. So as I've got a 10 ohm resistor here, I know I have 23.9 volts across here because this is in saturation. Using Ohm's law, 23.9 divided by 10 gives me my collector current, which is 2.39 amps. Now I didn't tell you what the beta was of this transistor, so you can assume the beta is 100. If I have 2.39 amps on the collector and a beta of 100 here, I'm going to go 2.39 divided by 100 to get my base current, which ends up being 23.9 milliamps. We're almost there. Now I need to figure out the base resistance, well in order to do that I need to have a voltage here. Now I have this is a complete circuit. I also have this as a complete circuit. So if I have 24 volts here, remember from the last video we have a 0.7 volt drop from here to here if it's a silicon diode, which means that 24 minus 0.7 is going to give me my voltage at the base resistor. Doing the math in your head should be 23.3 volts. And the other thing that's left now to do is kick in Ohm's law. 23.3 volts divided by 23.9 milliamps will give me my base resistor to achieve saturation. I've got 974.9. I don't know why I didn't just round that up to 975. 975 Ohms. That is the size that this resistor needs to be at least to put this in saturation. If this is any smaller, now let's walk through this. If this is smaller then the current's going to go up on this side. If the current goes up on this side it's going to appear to go up on this side, which means that voltage will be larger. But it can't. You've hit saturation. You've got a sponge which is full. You're not going to have this go above 23.9. And that's saturation folks.