Electronic – Why does a BJT enter saturation

resistorstransistors

enter image description here

I'm a beginner in EE, learning about the forward active mode of transistors. The equation, Ic = Ib * BETA, and Ic=Ie * ALHPA, are referred to as the ways to figure out what base resistor you should have when running forward active mode. What is Ic and Ie? Do these mean the amount it lets in, or some ratio?

Along with this, how do I use these numbers? On this site, https://learn.sparkfun.com/tutorials/transistors they give an example where they say Ic is 100mA. From my understanding, that means that if it hooked straight up to the positive and negative terminals it passes 100mA. But what if there are resistors in series? What effect does this have?

Thank you very much. I'm sorry if there is a term for this I don't know, by all means give me something to google.

-edits-

Thank you for the name suggestion.

Best Answer

The equation, Ic = Ib * BETA, and Ic=Ie * ALHPA, are referred to as the ways to figure out what base resistor you should have when running forward active mode. What is Ic and Ie?

\$I_c\$ is the collector current. That is, the amount of charge flowing in to the collector per unit of time.

\$I_e\$ is the emitter current. In a circuit theory class, this will be the amount of current flowing in to the emitter terminal. But in every-day usage, we might define it to be the current flowing out of the emitter terminal as that is more likely to be a positive number. A good source will provide a schematic with arrows indicated the direction to be considered positive for every current they want to talk about.

Do these mean the amount it lets in, or some ratio?

\$I_c\$ and \$I_e\$ are currents. \$\beta\$ and \$\alpha\$ are ratios.

Usually in EE a variable named \$I\$ will be a current.

When talking about BJT's, the important ratio is \$I_c/I_b\$. This is often designated by \$\beta\$ or \$h_{fe}\$. (\$\alpha\$ might sometimes also be important but it's much more rare for it to come up in a practical circuit solution)

On this site, https://learn.sparkfun.com/tutorials/transistors they give an example where they say Ic is 100mA. From my understanding, that means that if it hooked straight up to the positive and negative terminals it passes 100mA.

If they say the current is 100 mA, they mean the current is 100 mA in whatever circuit they're talking about. If that circuit has the collector connected directly to the power supply, that's what they mean. If they're talking about a circuit with resistors, then they mean the current flowing in to the collector in that circuit.

But what if there are resistors in series? What effect does this have?

If there's a resistor (and nothing else) between the power supply and the collector, then any current into the collector will have to flow through that resistor to get to the transistor. Ohm's law will tell you what the voltage drop must be across the resistor for that current to flow.