On Monday 24 July 2006 09:56, Vasile Surducan wrote: > On 7/24/06, palani wrote: > > On 7/23/06, Dave Tweed wrote: > > > Palaniappan C wrote: > > > > Maximum power transfer theorem states that maximum power > > > > is supplied to the load when load resistance is equal to output > > > > impedance. so in that case , efficiency will be only 50% . > > > > > > > > But class C ,D amplifiers claim more than 50% efficiency. > > > > If we match impedance , will it reduce their efficiency ? > > > > > > Yes. > > > > > > But be careful about what "efficiency" you're referring to in each > > > case. > > > > One more question regarding output impedance, How to find out > > output impedance of transistor/MOSFET output stage ? > > Have you heard about "bootstraping" ? (a DC/AC feedback between the > amplifier output and an intermediate stage of the amplifier) > The output impedance of an amplifier it's a function of the amplifier > design and not of the output stage transistors. > So your question is incomplete. > > Vasile Thanks Vasile, I missed to mention the circuit configuration. I am working on common Emitter configuration (transistor) or Common source configuration (for MOSFET). and i trying to amplify 14Mhz 100mw input to 4 W output. Class A configuration. The common thumb rule used for output impedance is (Vcc * Vcc )/(2 * Po). (Vcc - supply voltage , Po - output power ) How people arrived at this formulae, and how output impedance depends on output power ? If the amplifier has variable power output (by varying input power), output impedance will also vary ? Its hard to understand. regards, palani -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist