2.2. Analogue electronics.Transistors (1)

 A transistor is an electronic componen that can be used as an amplifier, or as a switch. It is made of semiconductor material. Transistors are found in most electronic devices.

There are different types of transistors. A very common one is the “bipolar junction transistor” or “BJT”. And it usually looks like this:

It has three pins: Base (b), collector (c) and emitter (e). And it comes in two versions: NPN and PNP. The schematic symbol for the NPN looks like this:

The transistor works because of something called a semiconducting material. A current flowing from the base to the emitter “opens” the flow of current from the collector to the emitter.

Let's see it with a hydraulic simile.

Water flow (litre per minute) to represent current. Water flow at the base causes larger flow at the emitter-collector (or collector-emitter depending transistor type), which is 'amplifier mode'.

There will be a point where no matter how you increase flow at the 'base', the current at emitter-collector is just 'stuck' (limited by the maximum flow of emitter-collector). This is 'switch' mode (or saturation mode). 

 In a standard NPN transistor, you need to apply a voltage of about 0.7V between the base and the emitter to get the current flowing from base to emitter. When you apply 0.7V from base to emitter you will turn the transistor ON and allow a current to flow from collector to emitter.

Let’s look at an example:

In the example above you can see how transistors work. A 9V battery connects to an LEDand a resistor. But it connects through the transistor. This means that no current will flow in that part of the circuit until the transistor turns ON.

To turn the transistor ON you need to apply 0.7V from base to emitter of the transistor. Imagine you have a small 0.7V battery. (In a practical circuit you would use resistors to get the correct voltage from whatever voltage source you have)

When you apply the 0.7V battery from base to emitter, the transistor turns ON. This allows current to flow from the collector to the emitter. And thereby turning the LED ON.

The transistor is also what makes amplifiers work. Instead of having just two states (on or off) it can also be anywhere in between “fully on” and “fully off”.

A small “control current” can then control how big a portion of a bigger “main current” that is going to flow through it. Thereby, the transistor can amplify a signal.

We use transistors in almost all electronics and it’s probably the most important component in electronics.

If you want to know more about transisors  http://www.explainthatstuff.com/howtransistorswork.html