If you do not match impedances at the RF frequencies, you will get reflections due to the mismatch (known as gamma). These reflections can be troublesome, and can destroy equipment. If you get good with a smith chart, you can even determine lengths of coax that when shorted, look like an open circuit, or an open coax look like a dead short! Its really pretty interesting stuff. Google will give you a treasure trove of info on this matter. EDN had a great article on this as well. (see attached).

Attached Files

EDN lossy lines.pdf (169.0 KB, 7 views)

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"Everything that is done in the world is done by hope." -Martin Luther
"There are two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle."-Albert Einstein

 

Your presumption is quite correct. In fact, most amplifiers are designed to deliver a certain performance when driving a rated load, such as the popular 50 ohms used in RF work. They are not designed to have an output impedance of 50 ohms. They are designed to drive a load of 50 ohms, that's all. There are many other characteristics that must be traded off to achieve optimum performance and the best tradeoff does not occur when the amplifier's output Z is 50 ohms.

Maximum conversion efficiency is usually achieved by setting the amplifier's output impedance low relative to the load impedance. At times like that, you can get DC to RF conversion efficiencies of up to 90% even at RF frequencies (ref: "The Electronics of Radio", David Rutledge, pp 180-196). This is certainely not the point of maximum power transfer. It is common to find power amplifiers for GSM cellphones that do indeed deliver efficiencies of 60%, at frequencies like 900 or 1800 Mhz. These amplifiers do not have 50 ohm output impedance, and they are not linear but instead operate in Class C.

Another point to make is to reiterate the maximum power theorem, which says that with a given thevenin voltage source with fixed output impedance you will deliver the maximum power to a load if the load impedance is the complex conjugate of the source impedance. However, and this is the key point, if you can vary the source impedance as well as the load impedance you can have better efficiency. Consider taking the source impedance to the extreme of 0 ohms. In this case, all the power available from the source is delivered to the load since there are no internal source losses at all. So the most efficient setup is to set the output Z as close to zero as you can.

The reason this isn't done in some RF circuits is because is impossible to get the RF amp down to zero ohms of output Z while making all the other critical things as good as possible (things like gain, stability, distortion, efficiency etc.). But they do try.

Conversely, the efficiency also tends towards 100% when the load R tends towards infinity but this time the total power also tends towards zero.

A good explanation of this with further references is here:
http://en.wikipedia.org/wiki/Maximum_power_theorem

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RadioRon

 

Thanks for the information.

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hi,

the pdf attachment may help u for ur question.


gangadhar

Attached Files

why_50_Ohms.pdf (471.5 KB, 6 views)