need help on understanding two port network

Discussion in 'General Electronics Chat' started by kougou, Mar 28, 2013.

  1. kougou

    Thread Starter New Member

    Oct 20, 2012
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    [​IMG]


    Hi Guys

    I have trouble on understanding two port network(with no independent source).
    First of all, they say that I1 could be viewed as a supperposition of V1 and V2, therefore, I1=y11*V1+y12*v2.
    How should I view V1 and V2 ? could I view them as they are like a independent voltage source?

    second, they say y11 could be find by shor-circuiting v2 so that y11=V1/I1. I understand that when you are short-circuiting some thing, you are making the voltage difference to become zero, so that's how you get y11=V1/I1 equation.
    But when you are circuiting something, are you also alter the nature of the two port net-work (containing no independent source) so that the current to pass through that short-circuited branch instead of passing through that two port network?

    Sorry, I still on the single port black box page. In a single port system, V=IR, everything is not that complicate. How do I understand more using what we have known about single port system?

    Thank you
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    When you short circuit something outside of a black box you might change how it is responding right now, but you are not changing anything inside the box itself. The two-port parameters describe what is in the box, not what it is doing right now. Short circuiting and open circuiting things are simply tools to probe what is in the box (i.e., get at the values of the two-port parameters) without opening the box.

    Think about a flashlight. If you hand be a flashlight that is off, I have no idea if the batteries and bulb are good or not. But if I turn it on and light comes out, I have learned something about the internals without opening it up. Did turning it on change what is going on? Sure (at least I hope it did). But turning it on did not change what is inside the thing.

    In general, V1 and V2 are not independent voltage sources. You have four parameters, I1, I2, V1, and V2. The admittance matrix simply gives you two equations that describe the relationships between them.

    Just like a resistor has two parameters, I and V, and the resistance is a 1x1 matrix that describes the relationship between I and V. By itself, it is not sufficient to tell you either I or V. For that, the resistor has to interact with one other thing (because you have two variables and only one equation. With a two port network, you have four variables and two equations, so the network has to interact with two other things to fully determine all four variables. Those two things are whatever are attached to the two ports.
     
  3. kougou

    Thread Starter New Member

    Oct 20, 2012
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    Thank you for your reply.
    But I have one more question. Regarding the input port R1n, a1, and output port Rout, a2. How should I view them?
    Should I view them as if they are independent source or independent voltage?

    Or they are just the voltage and current applied at that point?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Rin and Rout are effectve resistances, so asking if they should be viewed as any kind of source is nonsensical.

    It appears that a1, a2, b1, b2 are simply the port currents. KCL should immediately lead you to the conclusion that a1=b1 and a2=b2.

    Rin is simply the ratio of the voltage across the port to the current into the port. Similarly, Rout is simply the ratio of the voltage across that port to the current into that port. Don't make it more complicated than it is.
     
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    The image in the first post is somewhat confused.

    It is customary when explaining scattering parameters to use the lower case a to represent the incident wave and lower case b to represent the reflected wave. See the figure under the heading "Two-port S-Parameters" at this page:

    http://en.wikipedia.org/wiki/Scattering_parameters

    [​IMG]

    Thus I am led to believe that a1, b1, a2, b2 are not port currents, but wave power.

    Also, in the post #1 image, there appears to be a designation for an input impedance, Rin, but the character I have given here as "R" looks like an upper case Greek letter gamma in the image, commonly used for reflection coefficient in connection with S parameters:

    http://en.wikipedia.org/wiki/Reflection_Coefficient

    So even though the image seems to involve S parameters, the OP is asking about Y parameters.

    Furthermore, the OP doesn't seem to have a specific question; he just seems to be asking for help understanding two-port parameters.

    Perhaps he would be helped by referring to various tutorials to be found on the web concerning two-ports, such as:

    http://fourier.eng.hmc.edu/e84/lectures/ch2/node4.html
     
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