What is the relationship between BJT characteristic curves and load lines?

[Bassalisk]

Hello,

I have a quick question about Characteristic curves.

[PLAIN]http://pokit.org/get/1958a855486487230cd4e3c0a1cc0908.jpg First: Do these curves go to infinity, i mean in theory? If I had a steeper load line, I would hit saturation later.

And what about that portion of load line between Ic axis and that curve? It doesn't intersect anything. Is this saturation? Or is saturation only that point on Ic axis that is mainly limited by the collector resistor? Saturation is really giving me a headache, even after countless PF posts about it.
And should I look at this load line in couple with output characteristic? It confuses me that at 0 V you have Ic max but there are no lines from output characteristic to intersect.

 

[vk6kro]

If you had a steeper load line, that would represent a lower resistance load which would mean more current at saturation. So there would be a greater saturation voltage.
That is why the curves tilt to the right as they rise from zero.

If you made suitable assumptions, you could assume the lines go on forever.
But why would you?
This is a practical subject and the lines shown represent the useful area of operation of the transistor.

The area you have marked does represent saturation. The saturation current would be about 4.6 mA because this is where the load line crosses the curves. You couldn't actually get 5 mA.
The voltage at this point could also be read off the horizontal scale if it was shown.
It would be something less than half a volt.

The point you have marked as saturation is not correct, though. It should be where the loadline crosses the curves.

And should I look at this load line in couple with output characteristic? It confuses me that at 0 V you have Ic max but there are no lines from output characteristic to intersect.

This isn't true. At zero volts you have zero current. That is why all the curves converge at the 0 V, 0 mA point.
You can't get zero volts on a loadline because the loadline has to be drawn from the supply voltage to the current through the load if it was across the supply voltage.
The transistor can't become a short circuit no matter how much base current you feed into it. It always has some voltage across it.

 

[Bassalisk]

If you had a steeper load line, that would represent a lower resistance load which would mean more current at saturation. So there would be a greater saturation voltage.
That is why the curves tilt to the right as they rise from zero.

If you made suitable assumptions, you could assume the lines go on forever.
But why would you?
This is a practical subject and the lines shown represent the useful area of operation of the transistor.

The area you have marked does represent saturation. The saturation current would be about 4.6 mA because this is where the load line crosses the curves. You couldn't actually get 5 mA.
The voltage at this point could also be read off the horizontal scale if it was shown.
It would be something less than half a volt.

The point you have marked as saturation is not correct, though. It should be where the loadline crosses the curves.

And should I look at this load line in couple with output characteristic? It confuses me that at 0 V you have Ic max but there are no lines from output characteristic to intersect.

This isn't true. At zero volts you have zero current. That is why all the curves converge at the 0 V, 0 mA point.
You can't get zero volts on a loadline because the loadline has to be drawn from the supply voltage to the current through the load if it was across the supply voltage.
The transistor can't become a short circuit no matter how much base current you feed into it. It always has some voltage across it.

Well thank you ! See, nobody told me, or wrote or made a statement, that the saturation current is where the load line intersects with characteristic curves. Everybody on the internet and my material said that the saturation current is that current where load line ends.


So actually, in realty load line ends at the last intersected point with characteristic curves? That gives me, maximum current, and minimum voltage?


And one more small question, why is this shifting to the right? I mean these curves?

 

[vk6kro]

So actually, in realty load line ends at the last intersected point with characteristic curves? That gives me, maximum current, and minimum voltage?


Yes, that is right. The load line represents actual values of collector voltage and current you could achieve with varying base current.
With a proper graph like the ones you find in data sheets, you can read off the saturation voltage and current quite easily.