Transistor Basics
This page is going to focus primarily on Bipolar Junction transistors. I have spent a large amount of time studying transistors and I feel the current methods of teaching the operation of transistors are not adequete. Too many sources I have found gloss over the important parts and bore the reader with endless physics of their operation. So hang on to your chairs and lets get started.
The invention of the transistor brought out a new age in electronics. The vacuum tube made possible the home radio but the transistor brought world communication into your palm. Trasistors are active components. Their operation depends on a source of power. They have three pins, one connects to the base, one connects to the collector, and one to the emitter. Lets look at a visual aid:
Here we see the circuit symbol for the NPN transistor. I will focus on NPN transistors as their operation tends to be more intuitive. On the right I have drawn how current flows through the transistor when it is operating. PSC reminds us I am using Passive Sign Convention. In the begining it was thought that the charge which flowed through conductors was positive. Thus it was stated current flowed from the positive terminal to the negative terminal of a source. We still retain this ideology when analying a circuit. Using PSC, positive current flows into the positive side of the terminal of an element. Now back to transisors.
In an NPN BJT, a positive current at the collector will flow to the emitter and a positive current at the base will flow to the emitter. The emitter current is a combination of the base current + collector current. Here is the clinching point: The positive current at the collector can ONLY flow to the emitter IF a base current is applied. The collector will roughly look like a 15 Mega-ohm resistor until the right amount of base current is applied. Note that the collector-emitter portion does not behave as a variable resistor, it behaves like a current source. So the most important take away here is that the base current controls the collector current.
Is it just current at the base that matters? No, the base-emitter portion of the NPN is a PN junction, a diode. This means the base-emitter circuit will drop 0.7V (roughly) so you must apply a bare minimum of 0.7V to the base to get the transistor to turn on. Lets clear this up with another visual aid.
The area boxed in the light grey rectangle is the equivalent circuit of an NPN transistor. An equivalent circuit is an equal reprensentation. Engineers use this representation to greatly simplify calculations regarding transistors. The terminals labled B, C, and E correspond to the base, collector, and emittter respectively. Just to reiterate, the base-emitter circuit is a diode and the colletor-emitter circuit behaves as a current source. If you understand the somewhat abstract concept of a current source you may wish to skip the next paragraph.
I struggled with the concept of current sources for a while. I understood a voltage source which could create a potential difference between its terminals and given a conductor of certain resistance, a certain current would flow (V=I*R). But how could a current source in effect 'force' current to just flow. It turned out I missed an important part of their operation. A current source produces a voltage between its terminals but it adjusts this voltage to only allow a certain amount of current to pass. For example, a current source produces 10mA. Thus if the conductor between its terminals is 100ohms, it will adjust the voltage at its terminals to 1V. If the resistance changes to 1kohms then it will adjust the voltage at its terminals to 10V. This is known as a constant current source (or independant current source). It maintains its current and is represented by a circle with an arrow inside it designating the direction of current flow. The transistor has an arrow inside of a diamond. This symbol represents a dependant current source. Depending on some other circuit parameter, the depedant current source will adjust its current.
As I had just stated in the last paragraph, the collector-emitter current source is a dependant current soure. I did not indicate on the equivalent circuit what the dependant current source relies on for simplicity but I will tell you now. The depent current source is linearly related to the base current, its a current dependant current source. If there is X amount of current flowing through the base, then the collector-emitter current source will pass X times a factor amount of current. The factor is called the gain of the transistor. For example a transistor might have a gain of 100, then if 1mA flows into the base, 100mA will flow from the collector to the emitter. In many engineering textbooks the gain of the transistor is refered to as beta and is represented by the greek symbol. In industry it is just called gain and if you look at any transistors data sheet, you will see a parameter labled hfe, sometimes called forward gain, this is synonimus with the beta used in textbooks.
Thus we can generate a varying voltage at the collector terminal, a varying current at the collector terminal, or use the transistor as a switch! To do this we must use the modes of the transistor. It has four modes of operation: Cut-off, Foreward-Active, Saturation, and Inverse Active. In cut off there is no base current and the transistor does not conduct from the collector to the emitter, it is off. In forward active the transistor actively varies the current between its collector and emitter based on the amount of base current flowing. In saturation, the transistor passes as much current possible and looks nearly like there was a piece of copper wire replacing it. Finally inverse active is normally not used and occurs when the collector and emitter terminals are swapped.