MadSci Network: Engineering

Re: Pertaining to amplfiers- what is power added efficiency

Date: Wed Apr 4 03:40:43 2001
Posted By: Bruno Putzeys, Staff, Electroacoustics and Analog Electronics, Philips ITCL
Area of science: Engineering
ID: 985749775.Eg


In low-frequency (power) amplifiers it is common to operate them in 
voltage-in voltage-out mode, because they have a very high input impedance 
and a very low output impedance. This means the output is capable of 
delivering power into about any kind of load whilst the input needs 
virtually no power at all. There the term "power efficiency" is simply the 
ratio of power delivered (to the load connected to the output) to power 
taken from the power supply.

In RF amplifiers it is not feasible to have this volts-in-volts-out 
operation as both output have to be interfaced with transmission lines 
(cables or circuit board traces long w.r.t. wavelength). In this case 
input and output have impedances matching the respective line connecting 
to them. This means the input actually requires power, and gain is 
specified as power gain. Now, if an amplifier has a power gain of 10, 
its "power added" is 9 times input power, as input power is considered to 
be supplied to the device too. Efficiency is calculated based not on total 
output power but on output power minus input power. Efficiency is then 
calculated as before.



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