# bipolar transistors

Discussion in 'Homework Help' started by smarch, Mar 14, 2009.

1. ### smarch Thread Starter Active Member

Mar 14, 2009
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transistors and electronics are quite new to me, so I don't know a whole lot and I am struggling quite a bit with analogue electronics,
atm i'm doing transistors, and i done an experiment where i kept Vce constant at 2v and took down readings for Ic when increasing Ib by 0.005mA each time.
I am asked what is it about this characteristic that makes the transistor so useful. I am having trouble wording it, is there any explanation or way of putting it that i can understand transistors a bit better.

2. ### hgmjr Moderator

Jan 28, 2005
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Take a moment and read the AAC ebook section on BJT transistors. Hopefully you will find it helpful.

hgmjr

3. ### mik3 Senior Member

Feb 4, 2008
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I think that the useful thing you have to say in your answer is the gain of the transistor (Ic/Ib). It is the gain that makes a transistor so useful.

4. ### smarch Thread Starter Active Member

Mar 14, 2009
52
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cheers for the replies.

How/why is this gain so useful though, I am finding it hard to put into words, what is its funtion in a circuit.

5. ### ismellsmoke Member

Feb 10, 2009
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with it you can amplify a small input signal to a larger output signal. Depending on the setup of your cirucuit (common collector/common emitter/common base) you can invert the phase, amplify the current or amplify the voltage.

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
A very simple and common usage/example of usefulness is when driving an LED, Relay, or other high current device from a Logic chip that can only source 2mA.

The 2mA is enough to allow the transistor to "switch" a 50mA or more load, which can then switch a MOSFET to control a 400V, 20A motor. They are amplifying the digital logic.

With audio, the output of a microphone is very tiny, so the small change in base current from microphone input is set up (biased) to make a larger change in collector current, after a couple stages, a loudspeaker can be driven with clarity from a microphone.

7. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Hi Smarch:

Good advice from everyone so far. I might add that you don't really need to know the physics of how a transistor works...there are LOTS of different models for this, and some of them seem to be conflicting. I'm not even sure the physicists really know. But, again, it's not too important.

The overall BEHAVIOR of a transistor is what's important, and much easier to understand than the internal physics! The important thing is this...a small amount of current in the base terminal controls a much larger current in the collector terminal. This fact alone opens up a world of possibilities.

Hope this helps a bit.

Eric