# maximum power transfer

Discussion in 'General Electronics Discussion' started by bhuvanesh, Feb 15, 2014.

1. ### bhuvanesh

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0
Aug 29, 2013
if u have 8 ohm load resistance(speaker) in series (voltage divider circuit) with another resistor(?)

what resistance do u choose same 8 ohm as load or less or high
i have heard by many to choose same but i dont know reason.plz explain me detially.thank you in advance

2. ### Laplace

1,252
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Apr 4, 2010
Have you calculated the power transferred in your circuit for a range of resistor values and plotted the result on a graph? Or better yet, derive the power transfer expression as a function of the resistor value then take the derivative with respect to the resistance and look for the point of inflection where dP/dr=0. Does the differential calculus answer your question?

3. ### bhuvanesh

201
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Aug 29, 2013
thanks for your response.first clear one thing,in voltage divider bias.if you want to transfer maximum power to load resistance(simply a resistor)what you do.does you choose the source resistance small or same as load resistance(simply resistor).thanks in advance

4. ### BobK

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Jan 5, 2010
Putting a resistor in series with a speaker will always reduce the power to the speaker. So what resistance will reduce it the least?

Bob

5. ### Electrobrains

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Jan 2, 2012
Does this trace belong in the "Homework section"...?
I think Laplace has given the perfect hint. Try the derivation of the Power function!
Basically it doesn't matter if the resistors are seen to be outside or inside the source.

6. ### BobK

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1,686
Jan 5, 2010
Finding a zero in the derivative will not help in this case. The max will be at an endpoint.

I think the question is phrased wrongly. The way it is phrased, putting a resistor in series with a speaker, the answer of what resistor will result in the most power transfer (to the speaker presumably) is obvious.

Bob

5,165
1,087
Dec 18, 2013
For optimum power transfer to a load you have to match the source resistance with the load resistance this is 50% power. If you change the load resistance up or down this reduces the power in the load and either increase it or decrease the power in the source resistance. SqrtRs/ SqrtRL = 1 so Rs must equal RL.

8. ### BobK

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Jan 5, 2010
But that is not what he asked. He is asking about putting a resistor in series with the speaker, then presumably connecting it to an amp. You will get the most power to the speaker with 0 Ohms as a series resistor. I think he really wanted to ask what you are answering, but phrased it badly.

The only way the equal resistance is optimal is if you have a voltage source with a given output impedance that you cannot change. For most real amps, this will be a fraction of an Ohm. With is why you get more power into a 4 Ohm speaker than an 8 Ohm one on a typical amp.

Bob

Last edited: Feb 15, 2014

5,165
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Dec 18, 2013
Yes Bob but I read between the lines as I think I know what he meant.

10. ### bhuvanesh

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Aug 29, 2013
thanks bobk.u meant to use active device instead of resistor.so the load does not bend .could u explain a little bit how it will be optimal if we use op amp.please.and really thanks bob

Last edited: Feb 16, 2014
11. ### BobK

7,682
1,686
Jan 5, 2010
Arouse,

I hate to belabor the point, but I can't see that the answer (an equal resistor) is possible for the question he asked. He is talking about a speaker of fixed impedance in series with a resistor that you can choose. If you can choose the resistor, the answer is always 0. The answer you give is for a different question. Given a voltage source with a fixed impedance, what load resistor would result in the maximum power in the load. This might indeed be the question his professor was asking, but perhaps it is not and I cannot see that from the way the OP was phrased.

Bob