when we study the o/p curves for common emitter configuration, we find that for particular iB, curves for iC go slightly upwards for various values of vCE.Curves are treated to be at constant value of iC for particular iB.

My question is: we have iC=B x iB
So for particuar iB we have constant iC
Now vCE = vCC - iC x Rc in active mode

so ultimately we get constant vCE for that particular value of iB. MY QUESTION IS Why we draw collector current curves for various vCE (ON x AXIS)?

 

Because, in many cases for many transistors the variation of Ic with VCE can be important. There are a lot of H parameters displayed on that family of curves. At one time even the best transistors departed significantly from the ideal of a constant Beta for even small variations in operating point. This display also shows any breakdown and issues with linearity. With today's better construction techniques, you don't see as much variation in parameters, but you still design transistor circuits to be independent of the normal range of parameters because you still can't depend on an exact B or VBE or VCE(sat), The family of curves shows, at a glance how close to ideal a particular device is. Along with an Ib/Vce plot you have nearly completely characterized the device, assuming leakages normal for today's devices. And that the device is actually capable of amplification.

Regards,

cww - Who has hundreds, if not thousands of hours on a curve tracer.