|MadSci Network: Engineering|
Hello, 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. Regards, Bruno
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