Transistor characteristic understanding

[anhnha]

Hi, this is not a homework. I need your help to understand basic concepts. I am learning by myself.

In textbooks, I usually see the circuit used to make Ic -Vce characteristics.
I see that there are no resistors at base and collector/emitter.
What will happen if VBE is larger than 1V or more?
Is the PN juction will break down?
How about if the transistor in saturation? Then if this is ideal transistor Vce = 0 but it is also connected to a external battery VBE ≠ 0. I am totally confused. Please help.

 

[ehild]

Hi, this is not a homework. I need your help to understand basic concepts. I am learning by myself.

In textbooks, I usually see the circuit used to make Ic -Vce characteristics.
I see that there are no resistors at base and collector/emitter.
What will happen if VBE is larger than 1V or more?
Is the PN juction will break down?
How about if the transistor in saturation? Then if this is ideal transistor Vce = 0 but it is also connected to a external battery VBE ≠ 0. I am totally confused. Please help.

That is not a real circuit to measure transistor characteristics. In a real one, there are meters (voltmeters and ammeters) and there are resistances. If nothing else, those of the bulk of the semiconductor and the wires.

Read http://www.electronic-factory.co.uk/transistor-operation/ and http://www.electronic-factory.co.uk/transistor-characteristics/, for example.

ehild

 

[anhnha]

Thanks,
can you answer my questions with the circuit above? I am confused.
What will happen if VBE is larger than 1V or more?
Is the PN juction will break down?
How about if the transistor in saturation? Then if this is ideal transistor Vce = 0 but it is also connected to a external battery VBE ≠ 0.

 

[ehild]

You cannot increase the BE voltage without getting extremely high current and breaking down the transistor.
V_CE=0 does not mean the transistor is ideal. You do not get collector current at zero CE bias. See about transistor characteristics http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/bpcur.html

Are you sure you study from a proper book?

ehild

 

[anhnha]

Are you sure you study from a proper book?

Yes, but the image above is in a lecture slide I got from the net.
Here is a set up in the lecture to illustrate early effect:

In this circuit both Vbe and Vce can be changed.
What is the limits that I can apply for Vbe and Vce that don't break down the transistor?
For example, Vbe has to be in 0- 5V?

 

[CWatters]

The path Base->Emitter behaves like a forward biased diode.

The emitter is connected to 0V so the base voltage will be limited to roughly 0.7V. If you try and force it higher the transistor might well be damaged by excess current.

Personally I don't like the drawing you posted. You can control a transistor by varying the base voltage BUT it's much better to think of a transistor as a current controlled device. eg It's the base Current contols the Collector->Emitter Current.

This is because Vbe is temperature sensitive. If you try and control the base voltage the circuit can become temperature sensitive.

See this diagram which looks nearly identical but note the curves are labeled Ib rather than Vbe.