# How to find the input resistance of an amplifier?

## 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 VL (RL/(RL + Ro))

## 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?

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Exercise 1.21.

gneill
Mentor
Write KVL for the left hand loop. Note that ix is the same as ib.

Write KVL for the left hand loop. Note that ix is the same as ib.
Ok, that worked out pretty nicely! Thank you very much. So just to clarify, when Im doing this type of exercises I need to use KVL's all the time or does it depend? Im really new to amplifiers so Im trying to get where all this is coming from.

gneill
Mentor
Ok, that worked out pretty nicely! Thank you very much. So just to clarify, when Im doing this type of exercises I need to use KVL's all the time or does it depend? Im really new to amplifiers so Im 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 vx/ix, 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 vx to find ix, and then finally determine the ratio vx/ix, but it seems like more work to me.

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 vx/ix, 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 vx to find ix, and then finally determine the ratio vx/ix, 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
Mentor
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.