Ex. 1-13 Sedra Smith

Thread Starter


Joined Aug 22, 2007

here I am with another question:

Given the attached circuit, the exercise says. "An amplifier with a voltage gain of 40 dB, and inpur resistance of 10 kohm and an output resistance of 1kohm is used to drive a 1kohm load. What is the value or Avo?"

If I understand correctly, Av=Vo/Vi, Vo is the V on RL, so Vo= (RL * Avo* Vi)/(RL+Ro), so Av= (RL * Avo)/(RL+Ro), therefore Avo=((RL+Ro)*Av)/RL.

With the given data that makes twice of Av. Av= 40dB= 100 so Avo should be 200.

The answer says 100!

As usual, I trust answers, but I thought that, since in a coupled amplifier, not all the voltage supplied gets transferred to the load, but only a fraction due to output resistance that must be kept lower than load resistance, i.e. Ro << RL, so how can Av and Avo have the same value?

Or, (since english is not my native language), does "An amplifier with a voltage gain of 40 dB" means "An amplifier with Avo=40dB", since the Av is the overall gain, i.e. when the load is applied, so if for example RL were 1 Mohm, Av = 99% Avo?




Joined Apr 26, 2005
Look at that right hand schematic. You set the Avo to 200 ... what is the voltage across the Load resistor?

Then change the load resistor to 1 M ohm and answer the same question ... what is the voltage across the load resistor.