Hi

Can someone explain to me how they got the answer? I don't understand why they have only used a few of the values to find the gain.



P.S

 

Try to draw this two-port amplifiers in series (cascase amplifier).
And then you will see that the output resistance of the first stage form voltage divider whit input resistance of a second stage. And we have similarly situation when we connect second stage output resistance to the third stage input resistance.
http://forum.allaboutcircuits.com/showthread.php?p=153702#post153702 ( post 8)

 

And then you will see that the output resistance of the first stage form voltage divider whit input resistance of a second stage. And we have similarly situation when we connect second stage output resistance to the third stage input resistance.

ok i understand this part, however i don't understand why isn't the first input resistance and the third output resistance not included in the gain calculation.

 

That is because they don't form any voltage divider. They are just resistances "hanging in the air" that don't drop any voltage on them and consequently don't suppress the signal level.

If you connect a load on the output of that system, or a source on its input, then further calculations about the gain are needed again.

Is that clear?

 

thanks i understand now