Understanding PNP and NPN Transistors

[logearav]

Homework Statement

revered members,
in the following attachment, i could work out the path of current in pnp transistor, that is, from base through R_B AND THE output is taken across R_L.
but i can't figure out the path of current in npn transistor. could u please help?

Homework Equations

The Attempt at a Solution

 

[phyzguy]

Why are you having difficulty? The two circuits are completely symmetric. Just replace I in the PNP circuit with -I in the NPN circuit. The currents will be the same, just in the opposite direction.

 

[logearav]

in pnp circuit, current flows from emitter to base and it reaches the collector. but in npn circuit current flows from base to emitter and to ground. where is the starting point for current in npn circuit. that is the problem for me

 

[phyzguy]

Both transistors work exactly the same. In both, charge carriers are emitted by the emitter, pass through the base (where a few recombine), and are collected by the collector. The only difference is that in the NPN device the charge carriers (electrons) have a negative charge. So while in the PNP device, most of the current flows from the emitter to the collector, with a small amount going out the base, in the NPN device, current flows from the collector to the emitter, with a small amount coming in from the base.