Guys;

I'm doing derivations H parameters stuff for circuit's take this for example: -



Now according to my notes RB1 and RB2 are in parallel and that's probably true. But why?

Some common definitions for series and parallel: -

Series - All current leaving R1 enters R2.
Parallel - same voltage across both resistors.

But let's have a look at these two... Ok so we short the 5 volts for Thevenin - ok that's good.

Now both have at least 1 common voltage source - the node at the bottom of RB1 and the top of RB2.
But RB2 is grounded while RB1 is referenced to whatever the voltage is at the top of RC...

So are they in parallel truly?

 

hi Kevin,
Consider that the +5Vs and 0V are 'common' nodes with reference to the transistor Base, what does that tell you.?
E

 

Ok bare with me here. Complex things I can understand. Sometimes I struggle with the foundations...

To answer the question. I think that's what i'm saying - if we call that N1 the node at the transistor base. Say it 2.5V... ahhh ok wait that must be that voltage there.... k I think I get it... maybe

 

hi,
You are thinking in DC, consider an AC signal.
E

 

So it's going to fluctuate between v1 and -v1...

 

hi K,
Look thru this AAC Article.
https://www.allaboutcircuits.com/technical-articles/bjts-after-biasing-the-small-signal-model/
E

 

 

hi Kevin,
Pleased to see that you have figured it out OK.

BTW: there are lots of of Articles in the AAC files.
E

 

Ahhh but it's the knowing where to look. Our lectures in this, are more an exercise in math, than there are an explanation of what we're doing or why we're doing it for. But thanks v much this has cleared it up Eric.

 

Here you have an example:
https://forum.allaboutcircuits.com/...-vs-schematic-pnp-or-npn.145186/#post-1235468And if you have time try to read this:
http://www.ittc.ku.edu/~jstiles/412...Small Signal Operation and Models lecture.pdf

 

Indeed "mad professors" is what it is Jony

Can't wait till I finish this year, till I get back to my real job... Sigh... 8 months and counting...