Silly transistor question

Mr. Richard

I am looking at a simple NPN transistor (2N3904). I understand the concept of what happens and how to use it. I think I fell asleep during my college electronics class, and I have been wondering “where does it go?”
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If I·send a voltage to·the base, voltage is permitted to flow from the collector to emitter. What happens to the signal voltage? Is it added to the voltage coming from the collector and travels out the emitter?·If so, do I need to combine the two (signal and collector) to have a true value of the power potential at the emitter?
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Please correct any misunderstanding or poor terminology I have used!!
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Thanks for the help
Josh


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Magic Smoke Theory of Electronics –
Inside every electronic part there is magic smoke.
The magic smoke is what makes everything work.
If you release the magic smoke, the part stops working!

David B

You really need to use the expression of current flowing through the transistor junctions rather than voltage.

If you ground the emitter and allow current to flow through the base-emitter junction then the base resistance drops and allows a larger current to flow across the collector-base-emitter path. Yes, the emitter current will be the sum of the base and collector currents, but usually the base current is so much less than the collector current that it is ignored.

Since this is still just the case of current flowing through resistances, the traditional power formulas still apply. You can measure the current through each junction with an ammeter, measure the voltages at each pin and calculate the power dissapated by the transistor.

If you mean what power of an external device the transistor is able to switch, that depends on how much current the transistor can pass, which depends on how low the transistor resistance is able to go and to how much heat it is able to pass to its case and to a heat sink, and what voltage the collector can stand in its off condition.

David

Mr. Richard

David B,

Thanks, I will use current and sleep better tonight.

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Magic Smoke Theory of Electronics –
Inside every electronic part there is magic smoke.
The magic smoke is what makes everything work.
If you release the magic smoke, the part stops working!

LoopyByteloose

I started electronics with tubes and transistors have always been a bit daunting: maybe because of that.
Tubes amplify voltage whilst transistors generally amplify current.

The Art of Electronics spends a great deal of effort trying to make clear that transistor amplification is a bit complex, but can be handled by following their concepts. And it also points out that 'gain' is not really an exact figure, just a ballpark estimate. When you buy a transitor, it may actually range from something like 50-200x gain. This seems to be the reality of semiconductor production. If you need exact gain, you need to calibrate. But this is usually not the issue with digital because you want it full on or full off

Rated Voltages are generally guidelines of peak operating values. I use many transistors that have a 60V max at 5V operation. It just assures they won't burn up if an excessive voltage spike comes from somewhere [noparse][[/noparse]like the wrong battery].

Power rating is really the key to actual durability because it is the heat that creates a destructive thermal runaway. Again, I use devices that are often rated 10 to 100 times the actual need.

Also, unlike tubes that require exactly the same tube for replacement; you can often find another transistor of more robust character that will fit. For novices, this often causes great confusion. For example, the 2n3904 is a handy transistor, but the 2n2222 will easily replace it and handle more power. So why even buy a 2n3904 to keep on hand? You will find people telling you to use one or the other in many cases as the modern microprocessor is using so little of the transistor's true capacity.

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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan