One_Lung

iTrader: (1)

just wondering. i never really got my head round it at college Are there any electronics boffs who know?!?
cheers onelung

S55 HOT

To what level of detail would you like an answer ?

At a high level it's like a variable switch - 3 connections - electric current flows between 2 of the connections based on the level supplied to the third.

That enough or are you after something more molecular ?

One_Lung

iTrader: (1)

I done a BTEC HNC in insturmentation so have a relative understanding of electronics-i sought of know about collecter,base,emitter and about hfe and NPN or PNP but its operation still confuses me.

TopBanana

Are you asking what it does or how it does it?

AndyC_772

iTrader: (2)

I presume you're after a description of what it does, rather than how it actually works. How it works is to do with electric fields in doped semiconductors, and I've managed the last five years as an electronic engineer without having to revise this stuff

What it does, however, is pretty straightforward. You have three terminals: Base, Collector and Emitter. In an NPN transistor, conventional current flows through the device coming IN through the collector and OUT through the emitter.

The amount of current that flows is controlled by the base. With no current flowing in to the base, the transistor is switched off and no current can flow between collector and emitter either. If you do allow a small current into the base, a corresponding and larger current can flow between collector and emitter.

The ratio of the collector current to the base current is called the gain, or hfe. This is usually around 50-200 or so, but isn't terribly consistent across the transistor's operating range or between different transistors. Most circuits are designed to work well regardless of the exact value of the gain.

The base-emitter junction can be modelled for circuit design purposes as a diode. So, if the base voltage is LOWER than the emitter, no current will flow. For any base current to flow at all, the base voltage must be approx. 0.6V HIGHER than the emitter. This voltage then stays pretty constant regardless of the amount of base-emitter current flowing. If it increases by much, you're cooking the device.

When the base current exceeds a certain value, the transistor is as 'on' as it can get. The transistor is then deemed 'saturated', the collector-emitter voltage is very low and the collector-emitter resistance is also low. This is good for low-loss switching, but bad in high speed designs - transistors take time to get out of this condition.

With a PNP transistor, operation is fundamentally the same but with all polarities reversed, so current flows IN through the emitter and OUT through the collector (and base).

Clear as mud?

Andy.


[Edited by AndyC_772 - 4/16/2003 12:53:52 PM]

scunnered

iTrader: (1)

So what's the forward current transfer ratio of a common emmiter amplipier?

boomer

There is quite a lot of useful info about semiconductor physics here

Also, try http://www.howstuffworks.com/ and do a search, although the electronics part of the site was down just a minute ago.

mb

hades

I take it you don't want to go into electrons and holes, band-gaps and all that sort of thing at a molecular level?

Certainly hope not, cos I'm not sure I remember it well enough to explain!

One_Lung

iTrader: (1)

ok i get it now sought of. i understand it at a molecular level aswell sought of. electrons and holes and depletion layers and stuff cheers