Can a transistor limit it self?

[vlad777]

Here's a newbie question:

Can and under what circumstances a transistor
limit (the current trough) it self?? (shut it self off)

If we connect base to collector and polarize CE at some point
saturation voltage should fall bellow 0.7V BE threshold and limit
CE current.

This is of course in [ my ] theory.

I know this kind of wiring is used in op amps
to make current mirror (?).
Don't understand this also...


Many thanks for any insight.

 

[vlad777]

Some experimental data:

~1:

U=11.05V
R=19.7 k Ohm

I measured =U/R= 556 uA

It measured = 524 uA

~2:

U=11.05 V
R=1476 Ohm

I measured =U/R= 7.47 mA

It measured = 6.99 mA

---

So the current trough the resistor and transistor in serial
is always about 94% which in first case makes the transistor
resistance 1182 Ohm and in second 89 Ohm.

 

[Dragon Tamer]

I'm not a hundred percent sure I understand what you're asking. If you are talking about just connecting a resistor from the base to the collector, then the transistor will not limit the current. If you have a voltage devider on the base, that will be more likely to limit the current for what ever you are trying to do.

If you could post a schematic of what you are trying to do, that would be a big help.

 

[vlad777]

I'm not a hundred percent sure I understand what you're asking.

Sorry.
This is what I mean:

 

[colin55]

In the diagram above, the transistor will be equal to placing a diode in the circuit to replace the transistor. A voltage of 0.7v will appear across the diode and current will flow through the resistor according to ohms law. The voltage across the resistor will be the battery voltage minus 0.7v.

 

[BrownOut]

A transistor is already a constant current device, and does not need any special configuration to limit current, aside from being operated in the active region. Transistors connected as in your diagram are operating at the edge of saturation, and not limiting current so much as the resistor is in those cases. This connection is really only useful when matched transistors are paired together. I've never seen a useful circuit as such, that only has a single transistor.

 

[Gary B]

By connecting the base to the collector, you have turned the transistor on to the point of saturation. Second, the resister is limiting both the base and collector currents and therefore also providing negative feedback to the base. I have used such arrangements in the past but, with a resister between the base and collector to keep the transistor in its linier range over a wide temperature and supply voltage range.