Why do we change output voltage for plotting input characteristic of a transistor and change input current for plotting output characteristics of the transistor?

I am self reading the material and I have no teacher.

  • \$\begingroup\$ What do you mean by input/output characteristics? What voltages are you changing? \$\endgroup\$
    – DavideM
    Nov 5, 2016 at 10:47
  • \$\begingroup\$ electrical4u.com/transistor-characteristics \$\endgroup\$ Nov 5, 2016 at 10:49
  • \$\begingroup\$ I am reading the above. So I hope that explains what I am trying to ask. \$\endgroup\$ Nov 5, 2016 at 10:50
  • \$\begingroup\$ It says that input characteristics are measured by keeping output voltage constant (not by changing it); output characteristics by keeping input voltage constants (again, not by changing it). \$\endgroup\$
    – DavideM
    Nov 5, 2016 at 10:57
  • 2
    \$\begingroup\$ Please, can you re-formulate your question? No one (me included) is going to read all of that in order to understand your question. \$\endgroup\$
    – DavideM
    Nov 5, 2016 at 11:07

1 Answer 1


Karan, the bipolar transistor is a two-port device that is not free of return effects. That means:

  • While measuring input characteristics (input port) , the conditions at the output port (voltage, current) play a certain role and, therefore, should be kept constant.

  • Likewise: While measuring output data (at the output port), the conditions at the input port must be kept constant.

  • In practice, we require to keep all the DC conditions (voltage, current) at the respective ports constant. This is identical to the following requirements (which sometimes are misunderstood):

(1) DC voltage constant means: ac voltage zero, short circuit in the small-signal equivalent diagram.

(2) DC current constant means: ac current zero, open circuit in the small-signal equivalent diagram.

I hope this helps.

  • \$\begingroup\$ Okay so as you said, while measuring output characteristics, input is kept constant. The plots that we make are by taking the readings at different input current (in almost every book). So, if we want, we can also change the input voltage and then make graphs of it right? \$\endgroup\$ Nov 5, 2016 at 14:30
  • \$\begingroup\$ Yes - that is correct. As a consequence, we have different sets of values. For example, have a look on the output curves Ic=f(Vce). As you can see, the slope of these curves is different for the various Ib values. Therefore, the data sheet gives small signal parameters (like hfe and hie) for one fixed pair of DC parametrers (Ic, Vce) only. \$\endgroup\$
    – LvW
    Nov 5, 2016 at 14:58

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