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A physic theory problem!

user4416462
Prodigy 50 points
Other Parts Discussed in Thread: THS3091

Hi!　Recently I have confused by a question. 

When I designed the circuit of power amplifer THS9031 in datasheet. It used 3 THS9031 parallel and I thought it maybe output higher voltage than 1 THS9031.

But it didn't behavior what I expected. Then I analysised the circuit and found that I didn't notice the internal resistance.

As a new source , THS9031 has its internal resistance. In that circuit it will output lower voltage than only use 1 THS9031.

But , a new question comes:

There are two microwave in same frequency , phase and amplitude. I call them V1 = sin(wt)   and V2 = sin(wt). 

When they have same propagation-direction, they will add together, and the result maybe is V3 = V1 + V2 = 2sin(wt).

Then the problem comes!  Before they are added, the total energy is  1^2/R + 1^2/R = 2/R.

But after they add, the energy is 2^2/R = 4/R.

4/R > 2/R ;

Where the energy comes?

  • 0
    TI__Genius 12631 points
    I assume you mean THS3091. First lets discuss the amplifier question. You cannot directly short the outputs of two amplifiers together because each amplifier may have a slightly different offset voltage. When you short the two outputs together the amplifiers will try and overdrive each other with the strongest one winning. Usually what happens is that they both find some middle ground but there is a large load current flowing between the opamp outputs. The internal impedance of the opamp actually helps to reduce that load current. Usually when we parallel multiple amplifiers we add a small external series resistance between the outputs of the amplifiers.

    In case 1 (individual sine waves) the individual voltage is say 1V and if R=1ohm then the current in each resistor is 1A and the total power burnt is 1W+1W = 2W

    In case 2 you have doubled the voltage (lets say you stacked 2 1V batteries in series) and you are only assuming a single resistor of 1ohm, so the power is 4W. However if you did the same with the resistors as you did with the batteries, i.e stack them in series then you would come back to 2W.
  • 0 in reply to Samir Cherian
    Prodigy 50 points

    Hi! Thank you for you answer!

    I think you got a point there  and  I want to say THS3091.

    In the last question, I calculated the energy by using the formula P = (U^2)/R.  While the R is unsure and I also ignore the energy of the internal resistance.

    So it is really interesting that the energy is decided by the load rather than the source.

    So, maybe it is meaningless to discuss the energy of a propagating microwave in free space?

    How about this question:

     Two microwave --"V1 = sin(wt) and  V2 = sin(wt+pi)" --spread along the same direction in the free space and the synthesis wave is V3 = 0."

    The energy of the microwave seems like to  vanishing.

    If this question is an pseudo-proposition or it has some explanation?

  • 0 in reply to user4416462
    TI__Genius 12631 points
    Hello Hao,
    Power dissipated will always require some kind of load, whether it is an intentional load or parasitic impedance.

    With regards to two sine waves adding out of phase, it is possible. That is how noise cancelling and pre-distortion works.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 50 points
    Thank you very much! I think the law of conservation of energy is correct. I take a part for the whole. It's meaningless to calculate the energy of a local energy.