When does a transistor reach a virtual "zero ohms" state?

Question

When does a transistor reach a state where across the emitter and collector there is virtually zero ohms of resistance? I was thinking it would be when it allows all the current that can be drawn through that section of the circuit, but it seems like there would be more to it.

Answer 1

when you fry it and cause a short circuit.  otherwise, there will always be some resistance.

Answer 2

Depends on what "virtual zero ohms" means.  When it's saturated it has low collector to emitter resistance, certainly not zero, but low.  You could say it's "virtually zero" if the rest of your circuit is high impedance.

Answer 3

Every transistor has a parameter known as HFE, current gain between the base and collector. Say it is 100, typical.

If you have a collector circuit that limits the current to 10 ma, perhaps by a 5 volt supply and a 500 ohm resistor, and apply 1 ma to the base, then the collector would not be able to sink 100 ma, only 10ma, so it is "saturated", and the collector voltage falls to the region of 0.05 volts to 0.2 volts, depending on the transistor. That equates to a resistance of 5 ohms to 20 ohms, not zero.

You should study the VI curves of a typical transistor, say a 2N2222 and read some articles about transistor bias.

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