Hybrid PI Model BJT: V0 + or - | Is It Important?

[Bassalisk]

Hello,

I am trying to figure out at some level(not fully) Hybrid PI model BJT. Only one thing bothers me. Does it matter if the v0 is + or -.

Is for npn transistor v0 -ive and for pnp +ive? How do I figure that out and does it really matter, and if not why?

 

[jegues]

Hello,

I am trying to figure out at some level(not fully) Hybrid PI model BJT. Only one thing bothers me. Does it matter if the v0 is + or -.

Is for npn transistor v0 -ive and for pnp +ive? How do I figure that out and does it really matter, and if not why?

The Hybrid PI model for BJT's is the same regardless whether the transistor is NPN or PNP.

The output will be,

$$V_{o} = -gmv_{be}R_{L}$$

The output voltage is simply the current across the load times the resistance of the load.

What is the current in the branch containing the load? Well it's simply the current of the current source, but it's going in the "wrong" direction.

To obtain Vo we would like the current to travel in the other direction across the load hence the negative sign.