# Transistor Hybrid Parameter Model

#### mentaaal

Joined Oct 17, 2005
451
We are learning the hybrid parameter model for transistors in school and i dont think that i have ever been so confused in my life!

Like say for example to determine the input resistance for a simple Common Emitter amplifier circuit the lecturer started the formula like so:
Rin = Vbe/Ib (Rin = resistance in, Vbe = voltage between base and emitter, Ib = base current)

How come Rin isn't something like: Rin = Rs + Vbe/Ib (Rs input resistor)

And its the same thing with the output resistor, the load resistor doesn't get taken into account for the output resistance????

#### thingmaker3

Joined May 16, 2005
5,084
This is only Ohm's Law, but with a fresh coat of paint. E=I*R, therefore R=E/I. R is the input resistance of the transistor. E is the voltage across said input. I is the input current.

The base resistor can limit the base current, of course, but is not needed for computation of Rin. Base current will depend on both the base resistor value and the voltage applied to the base resistor.

#### mentaaal

Joined Oct 17, 2005
451
Ok that is what i was thinking... but the reason why i wasn't understanding this is because i thought that Rin was the resistance in of the circuit, i.e. if you stuck in a multimeter you would read Rin. Isnt Rin of the transistor Hie(for common emmitter) And input resistance has to be something else like Rs + Hie??

#### thingmaker3

Joined May 16, 2005
5,084
My bad, I got my initials out of order again. Sorry.

Rin = Hie + Hfe + Re

This is in parallel with any bias resistors.

What's worse, Hie and Hfe are frequency dependent. Life gets tedious, don't it?

#### mentaaal

Joined Oct 17, 2005
451
I hope i am not becoming irritating at this stage by not getting this but well i have been stuck on this for days now, surely the base resistor Rs should be taken into account for Rin? I mean its in series with Hie so howcome its neglected in the Rin formula? Am i missing something here or what?

In parallel? But they are in series because they share the same current?

*Cant sleep, confusion will eat me!*

Thanks for bearing with me!

#### omnispace

Joined Jul 25, 2007
27
I don't remember the details (I promptly forgot everything from Electronics II over the summer), but I think there might be a different Rin depending on whether you are looking at the transistor by itself, or the entire amplifier.

#### mentaaal

Joined Oct 17, 2005
451
Well i was told that Rin is the resistance as if you were looking in from the perspective of the Vs (Signal voltage or input voltage)???

#### mentaaal

Joined Oct 17, 2005
451
Ok so just to clarify: The input resistance is the resistance from the perspective of the signal source right? And the base resistor doesnt factor into this amount whatsoever even though its in series with Hie (transistor input resistance)