Transistor characteristics

How to determine output and onput characteristics of transistor in given mode? I am confused about what versus what graph displays output characteristics of given particular transistor (same confusion about input characteristics). For example in common emitter npn transistor I[SUB]C[/SUB] Vs V[SUB]CE[/SUB] graph shows output characteristics, but why I[SUB]C[/SUB] Vs V[SUB]CE[/SUB]? Please explain how to determine that for both input and output characteristics?

Rohitchampion, a lot of basic questions. Detailed answers you certainly will find in relevant textbooks.

Some short explanations:

1.) The BJT needs a certain voltage Vce to work in the active region. Here, the BJT acts as a (not ideal) current source. That means: The output current depends somehow on the voltage Vce.
Because the gain determining resistor Rc causes a change in Vce (during amplification) the output resistance of the BJT alone is Delta(Vce)/Delta(Ic). This is a rather large value in the range of (20..100)kohms.
This value can be found directly by evaluating the SLOPE of the output characteristic Ic=f(Vce).

2.) More than that, this output characteristic (as a set of curves for various base currents) is a very illustrative and versatile tool to find a suitable bias point.

3.) The input characteristic Ib=f(Vbe) is a graphical illustration of the bias point dependent input resistance rbe. This resistance is important for calculating the allowable load for the driving signal source.

4.) There is another characteristic, which is very important: The transfer curve Ic=f(Vbe).
This curve shows how the transistor can amplify. It demonstrates how the output current changes with the input voltage.
As the most important transistor parameter, the SLOPE of this curve gives the transconductance g=Delta(Ic)/Delta(Vbe).

Hope I could help.

Winterstone, yeah question is basic (sometimes you go advanced and still confused about basics ). I tried to find in many books but almost every book explains in same way i.e. for example it is given like this - this vs that is output characteristic graph in this mode(CE CB etc) and they start explaining about graph (this region is active because ... that region is cutoff because ...). I got all that explanation but still confused why Ic vs Vce is output characteristic graph gor CE npn? why Ic Vs Vbc for CB pnp? why Ib vs Vbe is input characteristic curve for CE npn? and all other modes.

Rohitchampion said:

....... still confused why Ic vs Vce is output characteristic graph gor CE npn? why Ic Vs Vbc for CB pnp? why Ib vs Vbe is input characteristic curve for CE npn? and all other modes.

Click to expand...

* You are asking WHY Ic vs. Vce is output characteristic?
Because in CE mode, the collector node is the output node and Ic is the output signal (current). The output voltage is created across the collector resistor Rc, which is in parallel to the internal
output resistance rce=delta (Vce)/delta (Ic)=vce/ice (very large). That`s the explanation.

* In CE operation, the base node is the input and the corresponding pair Vbe resp. Ib forms the input characteristic. Seems to be logical, does it not?

W.

Now bit cleared..