# Transistors

#### Jweise-cc

Joined Sep 30, 2005
1
Hello Eveyone,

I know this is NEWBY question but you gotta start somewhere. So for you advanced digital devices people. if you can take a couple minutes out of your time it would be appriciated.

Why Does amplification occur in transistors? I'm looking for the actual theory

And if I hook up a AC signal generator to a circuit with resistors. Why would i use a resistor after the emmiter(After the resistor it grounds)

Schematic included

Come on somebody?!?!?!!?

#### Brandon

Joined Dec 14, 2004
306
Originally posted by Jweise-cc@Sep 29 2005, 11:41 PM
Hello Eveyone,

I know this is NEWBY question but you gotta start somewhere. So for you advanced digital devices people. if you can take a couple minutes out of your time it would be appriciated.

Why Does amplification occur in transistors? I'm looking for the actual theory

And if I hook up a AC signal generator to a circuit with resistors. Why would i use a resistor after the emmiter(After the resistor it grounds)

Schematic included
[post=10717]Quoted post[/post]​
The amplification occurs from the ratio between the base current and the collector/emitter current within the transistor which controls current flow through it.

You will see this term refered to as β , beta.

Typically for the 390x models β will be about between 80 upwards of 200. So for every 1ma into the base, you have the potential to get 80x to 200x of that running through the source and emitter. The reason the base current controls the current flow in the device is that the base current 'charges' a silicon channel between the collected and emitter. The more current flow charging the channel the more current that can flow through it.

The resistor at the emitter can serve a few purposes. Sometimes you don't want the emitter to get pulled to ground. By doing that you would loose much of your input signal since the largest voltage drop you could have if the emitter was tied to ground would be 0.7 volts on the base. If you put more, it would just bias the transitor more on and dump much wasted current. By adding the emitter resistor you permit the emitter of the transistor now to flucuate between a range of voltages, nearly ground upwards to nearly the source voltage minus a few tenths of a volt for the transistor being in saturation. This also permits the base voltage to move over a wide range without since it will be about 0.7v above the emitter at all times when it is conducting.

Hope this has helped you.