Monday, August 31, 2009

Basic electronics - transistors and semiconductors

Countdown Timer: 5 weeks 1 day to go

Right, we covered a fair bit under the banner of basic electronics, including resistors, capacitors, inductors etc. Now we need to do a wee bit on transistors. The course syllabus, as outlined in the Exam Notes and Sample Paper document from IRTS, says the following needs to be known:

- PNP- and NPN-transistor
- Amplification factor
- Field effect vs. bipolar transistor (voltage vs. current driven)
- The transistor in the:
- common emitter [source] circuit
- common base [gate] circuit
- common collector [drain] circuit
- input and output impedances of the above circuits

There was a question in Sample Paper 3 which asked the candidate to identify a diagram. I have included the question here on the right. It would be a good exercise to learn the diagrams of the four main types of transistor in case this comes up, and also to have some sort of rudimentary knowledge of what a transistor does.

Here is a brief introduction to the transistor from Wikipedia:

In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal.

The transistor is the fundamental building block of modern electronic devices, and is used in radio, telephone, computer and other electronic systems. The transistor is often cited as being one of the greatest achievements in the 20th century, and some consider it one of the most important technological breakthroughs in human history. Some transistors are packaged individually but most are found in integrated circuits.

And a nice simple explanation from the Williamsons Labs site:

A Transistor can be thought of as a device that is active in only One Direction: It can draw more or less current through its load resistor (sometimes referred to as a pull-up resistor).

It can either Source Current or it can Sink Current, it Cannot do Both.

Types of Transistor:

Types of transistor (from this website):

There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.

The leads are labelled base (B), collector (C) and emitter (E).

In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs.


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