Maximum Power Transfer??

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aryajur

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I have a very basic question. Why are we concerned with Maximum Power transfer ? If we do maximum Voltage Transfer, like having infinite input impedance to a amplifier for voltage amplification, how would this system be inferior to a system having a matched input impedance to the source???

I hope I will get many responses to this because its a very basic query...
 

Terms such as "Maximum Power Transfer" and "Matched Impedence" are used in RF design rather than for amplifier circuits, where one of the objectives is to achieve maximum radiated power for a given input power. That's why we aim for maximum power transfer, which will occur when impedences are matched. If impendences are not matched, most of the input power would be reflected back to power source rather than be radiated via the antenna, which would simply be a waste of power.

For amplifier circuits, the aim is still to achieve high input impedence and low output impedence.
 

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MPT is also widely used in acoustic systems. For example, the impedance of the output speaker is matched with the output impedance of the amplifier in order to get maximum power output from the speakers.
 

This is just what I am saying. It makes sense to match impedance when you are running an appliance from a battery or driving a speaker from an amplifier so that you get the maximum power from te source. But like in RF systems where the signal is received from the antenna, that received signal hardly has any power and if we can get the characteristic waveform of the signal and amplify it and at the output we can add as much power to the output as we want. Why do we loose half of this characteristic waveform just to get the power which finally we are going to supply anyway????
 

I can see the term power max. is more accurate than max. current or max. voltage trems as the basic sense of any communication system is the power transfere . (eg. if we have empty battary it has voltage measured but there is no delivered power from it)
 

Let me frame the question in another way.
When we want to run a appliance from a battery, or when we drive the transmitting antenna from the amplifier output it makes sense to match the source impedance to the load impedance to transfer maximum power from the source to the load since the source is mainly supplying power to the load.
Now when we have a receiving antenna, which receives a signal and then gives it to the input filter or amplifier, why do we need to have the input impedance of this filter or amplifier matched to the antenna? The main thing we want from this signal is the waveform of the signal, not its power, isn't it? So why bother for maximum power transfer in this case, why not only maximum voltage transfer???
 

It has to do with the fact that amplifier on either end must remain biased under a changing load condition. You do this with low value resistors. Remember that resistor to ground at the base of a transistor that makes the input independent of the transistors change in beta. This is the reason for the low value resistors. But instead of having 30 20 40 you have 50 50 50. And this is relatively close enough to equal magnitude.
 


thats not true.. power is important. the input signal will ultimately drive some load which in turn needs power. the amplifier amplifies the power of the input signal.. so to gain maximum power output from the amplifier, input power has to be maximum which in turn means : Maximum Power Transfer Theorem!
 



This will be the case if we give the received signal to a power amplifier. But like if we lets say have a common source or a common emitter amplifier to which the signal is fed, then we amplify its voltage signal and then at the output we can drive the load easily if the output impedance of the amplifier is matched to the load impedance.
So my question is why is this system inferior to having a power amplifier which has the input impedance matched to the source?
 

I understand your question, and I think it has to do with this:
Max power transfer is beneficial because in RF more the strength of your radiated signal lesser the complexity of your receiver. Note- unlike RF applications many others dont operate in the region of max power transfer as the efficiency is very bad in this region. But in RF we sacrifice efficiency for more accuracy which is more important in communications. So in your argument you are totally ignoring the noise embedded in the transmission, so to have a high signal to noise ratio we need a stronger signal output of the transmitter. without this the noise would overwhelm the signal and you certainly wont get a neat sinusoid as you expect at the receiver. Max power output at the transmitter ensures that the signal can be reproduced at the receiver without much interference by noise.
 

yes...in analog ckts also it will apply...for example take an loud speaker..its nothing but a multi stage cascoded amplifier..the internal stages except i/p and o/p stages r designed for max power r else say max gain so as to get the required volume using less no. of stages...but the o/p stage is designed for max o/p volt w/o signal loss..
 

In radio, transmission lines, and other electronics, there is often a requirement to match the source impedance (such as a transmitter) to the load impedance (such as an antenna) to avoid reflections in the transmission line. That is why source and load impedance are made equal.
 


U can have max voltage by having high input impedance. But current will be zero at that time. For working power i srequired, for power we need to have voltage and current. So to get the optimum value of power transfer there should be some voltage and current, this will heppen by matching impedance. that is at max power transfer condition...........
 

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