How to find the input resistance of an amplifier?

[jean28]

Homework Statement

Here is a picture of the problem

http://i1226.photobucket.com/albums...-8A04-C5855FF971D8-87001-000004D91E955189.jpg

Homework Equations

Im guessing Vo = Avo V_L (R_L/(R_L + R_o))

The Attempt at a Solution

The solution is already beneath the problem. I just can't seem to figure out how to get to it. Could someone please explain?

 

[jean28]

Exercise 1.21.

 

[gneill]

Write KVL for the left hand loop. Note that i_x is the same as i_b.

 

[jean28]

Write KVL for the left hand loop. Note that i_x is the same as i_b.

Ok, that worked out pretty nicely! Thank you very much. So just to clarify, when I am doing this type of exercises I need to use KVL's all the time or does it depend? I am really new to amplifiers so I am trying to get where all this is coming from.

 

[gneill]

Ok, that worked out pretty nicely! Thank you very much. So just to clarify, when I am doing this type of exercises I need to use KVL's all the time or does it depend? I am really new to amplifiers so I am trying to get where all this is coming from.

You use whatever circuit analysis method suits the given circuit arrangement -- choosing which method is simplest to use comes from practice. Here you wanted to find the ratio v_x/i_x, and those would be principle variables in the KVL for that loop.

I suppose you could apply KCL at node E to find the node voltage there, and then use that and v_x to find i_x, and then finally determine the ratio v_x/i_x, but it seems like more work to me.

 

[jean28]

You use whatever circuit analysis method suits the given circuit arrangement -- choosing which method is simplest to use comes from practice. Here you wanted to find the ratio v_x/i_x, and those would be principle variables in the KVL for that loop.

I suppose you could apply KCL at node E to find the node voltage there, and then use that and v_x to find i_x, and then finally determine the ratio v_x/i_x, but it seems like more work to me.

So I treat it as if I was analyzing any other circuit. Then what exactly does the blue highlighted area mean/represent?

 

[gneill]

So I treat it as if I was analyzing any other circuit. Then what exactly does the blue highlighted area mean/represent?

It happens to be a simple equivalent circuit (or model) of a transistor.