Hi guys.
I take first fight with differential amplifiers based on single transistors and some things are not sure for me.

On output charasteristics is not included one part shown below via red line:


What is reason of this behaviour ? When input voltage reaches Up ? What about leaning of this line ?

Thank you in advance

 

Is there a schematic that you didn't include?

 

Common solution like that:

With Vb2 grounded we may take into account just green plot

 

What is reason of this behaviour ?

Did you ever hear about saturation?

When Q1 enters the saturation region (and Vin is still rising) the Q1 emitter voltge is Ve = Vin - Vbe and Vc = Ve +Vce_sat.