transistor question

[MAXIMUS]

how can there be a difference, besides the amount of voltage or current or whatever it can handle, between two transistors? isnt a transistor effectivly just a switch? do different transistors have to get a different amount of voltage applied to the base before the "switch" will flip or something?, or is it just any current applied to the base that will activate (deactivate?) the transistor?  and how is electricity applied to the base of a transistor to flip the switch if there is no circuit between the base and anything else?

Tell me if i am way off... Im desperately trying to understand...
:?  

P.S. I know that a pot controlls the amount of resistance between two terminals, but what is the middle terminal on a pot for?

Im confused and a newbie.
Thanks Guys!

[niftydog]

simple stuff first.  A pot has three terminals so you can set up a variable voltage divider.  A voltage divider divides a voltage by the ratio of two resistors in series.  (Google "voltage divider" for more info).

Code Select Expand

  VCC
   +
   |
  .-.
  | |<-- variable voltage
  | |
  '-'
   |
  ===
  GND


ie; a variable version of this voltage divider network;

  VCC
   +
   |
  .-.
  | |
  | |
  '-'
   |
   o---o fixed voltage
   |
  .-.
  | |
  | |
  '-'
   |
  ===
  GND


A lot of the time we use pots as "rheostats" which are literally just variable resistors.  Like this;

Code Select Expand

  VCC
   +
   |
  .-.
  | |<--o
  | |   |
  '-'   |
   |    |
   o----o
   |
  ===
  GND


Now, about that big-ass can of worms you opened!

A transistor CAN be a switch, but it doesn't have to be.  There are different characteristics for different transistors that govern the useful parameters of a transistor.

Between the OFF state and the ON state there is a "gap."  That is to say, it doesn't switch instantly from on to off or vice versa.  Whithin this gap is  a "linear region" and we can use this to amplify signals.

To "bias" a transistor is to HALF switch it on, or switch it on just a little bit, enough to have it sitting in the middle of the linear region.  By adding an AC signal to the biasing voltage, we can make tiny fluctuations in the base current result in large fluctuations in the collector current.

This is where the GAIN thing comes in.  For a given base current two transistors with different gains create two different collector currents.

I don't intend to go right into this, as I said, you've just opened up yourself a nice big can of worms and it's not my job to clean it all up for you!

But there are other factors, such as the frequency response of the transistor (think of that as how fast the transistor can switch on and off) and the noise figure (how much noise the transistor makes when switching) the temperature characteristics, the dynamic resistance, the input and output capacitance, the reverse bias voltage, the saturation current etc etc etc.

Then there's, as you say, the current, voltage and dissiapation limits.  There's a lot of factors to consider.

If we could make the ideal transistor, then we'd only need one type and it would do all of the jobs we need it to.  Unfortunately for us, making the ideal transistor would seem to be impossible.  So, we sacrifice one parameter to get another, or make circuits that have small transistors feeding medium transistors feeding big transistors so that we don't expect one transistor to do all the work!

I don't know if I explained this very well, but between me and all the other folks here, you'll get an idea.  There's a number of websites offering tutorials and such, here is a starting point: http://www.tpub.com/content/neets/14179/css/14179_62.htm

[brett]

Hi.

Don't worry about asking these kinda questions.  They indicate a healthy interest.

First, the usual not very helpful advice - see the many good articles on the web. e.g. http://www.101science.com/transistor.htm

To quote the website;

A transistor is a small electronic device that can cause changes in a large electrical output signal by small changes in a small input signal.  That is, a weak input signal can be amplified (made stronger) by a transistor.  

That's a better description of a transistor than a switch.  It CAN be a switch, if the changes in the control system (the base to emitter current) are large enough to stop and saturate the flow of current from the collector to the emitter.   Think of it like a fawcet, it CAN be fully off or on, but it can also be used to regulate a more moderate amount of water flow - and that's how we use most transistors in audio circuits (neither fully "off" nor fully "on")

I'll let the web link and other, more expert people describe the details.

have fun