Ex. 1.9 Sedra Smith

Thread Starter


Joined Aug 22, 2007

I'm studying sedra smith's microelectronic circuits, 5th ed.

I'm trying exercise 1.9, but I don't agree with results.

Exercise says:"An amplifier operating from a single 15 V supply provides a 12 V peak to peak sine wave signal to a 1 kohm load and draws negligible input current from signal source.. The DC current drawn from the 15V supply is 8 mA. What is the power dissipated in the amplifier and what is the ampifier efficiency?

I said:

Pdc= Vdc * Idc = 12 * 0.08 = 120 mW
Pload = (Vo /sqrt(2))^2/Rl = 72 mW

Pdiss + Pl = Pdc + Pi

Since no current is drawn from source, Pi=0 =>

Pdiss = Pdc-Pl = 48 mW

efficiency=Pl/Pdc = 60%.

Answer says: 102mw, 15%.

I trust prof Sedra & Smith, :D, but what's wrong in my thoughts?:confused::confused::confused:

Thanks a lot!!!!

Thread Starter


Joined Aug 22, 2007
Look carefully at the question concerning the voltage at the load and your calculation of the power in the load.
I'm a double chump!!!! I haven't seen your attachment, and I've not thought that 12Vpp means 12 v between the positive slope and the negative!!!!!!!

Thanks for your help!

One more question: what do you use to type the formulas?

panic mode

Joined Oct 10, 2011
hello troublesome,

welcome to the forum. i suggest you should start your own tread and ask the question (completely) along with your attempt to deal with the problem. notice how sandman provided all mentioned. since we don't know where you are going wrong, we cannot help. part of the helping is to identify and break bad habits (like not following forum policies, hijacking treads, not providing info on question, not providing info on own attempt etc.). this is an online forum and members here (other than few exceptions) are not taking your course and likely don't have book you are referring to. we don't know what is a and b part of the question. i have Sedra book (ed.6 instead of 5) but for question like yours i would not bother walking across the room to reach it. i am willing to help but if you don't show effort (in posting question), why should anyone care?