# help with finding Y parameters in a two port network circuit

#### Idan Mizrahi

Joined Jul 23, 2018
4
Hi there
My name is Idan, I'm an Electrical engineering student and I'm a new member here.

I struggle for a solution in this two port network exercise, im trying to find Y parameters,, being more specificly - Y21 and Y12.

here is the exercise: I'll be glad for any kind of assistance
Thanks

#### WBahn

Joined Mar 31, 2012
26,131
MOD NOTE: Moved to Homework Help.

#### WBahn

Joined Mar 31, 2012
26,131
You have the definitions of the Y parameters. But your diagram doesn't define the quantities (both location and polarity) involved. So first draw the various voltages and currents on your diagram.

Then pick one of them, say y21, and draw a modified circuit that imposes the given constraint, which is v2 = 0 in this case. How will you do that?

Then apply a voltage to v1 and analyze the circuit to find i2.

Show your best attempt to work the problem so that we can see where you are struggling and help you get back on track.

#### Idan Mizrahi

Joined Jul 23, 2018
4
You have the definitions of the Y parameters. But your diagram doesn't define the quantities (both location and polarity) involved. So first draw the various voltages and currents on your diagram.

Then pick one of them, say y21, and draw a modified circuit that imposes the given constraint, which is v2 = 0 in this case. How will you do that?

Then apply a voltage to v1 and analyze the circuit to find i2.

Show your best attempt to work the problem so that we can see where you are struggling and help you get back on track.
Hi, thanks for the quick reply!
I'm not sure if I understand exactly what you say

I did something but im pretty sure my calculations were wrong: #### WBahn

Joined Mar 31, 2012
26,131
Where is V2 on the circuit? Is it forced to be zero?

How do you force the voltage between two points to be zero?

You are on the right track, but in your diagram I2 must be zero because it is an open circuit. While you write the notation V2 = 0 above the controlled source, you haven't modified the circuit to force that voltage to be zero. The mechanism that you need to use to make that happen results in a path for I2 to flow so that it can be non-zero.

Your work is not consistent with your diagram because your diagram forces I2 to be zero but you don't impose this in your work.

Your work also has a problem because you thread Vπ as being the result of a voltage divider, but that equation is only valid if the current flowing from V1 can only go through rx and rπ, but some of that current can go through ru.

#### Idan Mizrahi

Joined Jul 23, 2018
4
Where is V2 on the circuit? Is it forced to be zero?

How do you force the voltage between two points to be zero?

You are on the right track, but in your diagram I2 must be zero because it is an open circuit. While you write the notation V2 = 0 above the controlled source, you haven't modified the circuit to force that voltage to be zero. The mechanism that you need to use to make that happen results in a path for I2 to flow so that it can be non-zero.

Your work is not consistent with your diagram because your diagram forces I2 to be zero but you don't impose this in your work.

Your work also has a problem because you thread Vπ as being the result of a voltage divider, but that equation is only valid if the current flowing from V1 can only go through rx and rπ, but some of that current can go through ru.
Hi
well, V2 equals zero, that's a must be because this is the first condition when solving Y21 parameter.

I know that I2 must be zero too according to my diagram, but i'v tried this method already with a similar exercise and I got the righ results.
I'm pretty confused, you probably understand way better than me about the correct way to figure this out.
please, if you can, show me a solution for Y21 parameter and I'll figure out the rest.

#### WBahn

Joined Mar 31, 2012
26,131
No, I will not just work your homework for you, not even the first problem. That will defeat the purpose. You've already seen examples worked for you in your text, in lecture, on YouTube, or whatever. Something isn't clicking and seeing one more problem worked FOR you is unlikely to change that. It will change as you struggle with problems worked BY you. It's an ugly, frustrating, and painful process, but that's how you learn best.

If I were to give you that circuit, contained in a black box with only the two two-pin terminals accessible and tell you to force V2 to be zero, what would you need to do to make that happen?

You can't just SAY that V2 equals zero. You must impose that constraint by modifying the circuit (in an allowed way, which here means working with the port connections) to FORCE the voltage at the V2 port to be zero.

You only have two options. You can leave a port unconnected (open circuit), in which case the port current is forced to be zero but it can have whatever voltage it happens to have across that open, or you can short-circuit the port in which case the voltage across the port is forced to be zero, but the port current can be whatever current it happens to be flowing through that short.

#### Idan Mizrahi

Joined Jul 23, 2018
4
Ok listen
If I short-circut the port the voltage is forced to be zero and the value of the current can vary.
I chose the cross section where I2 and Iru come in and right now what I need is a way to express Vπ with V1. I think if I find any relation between them I manage to solve this.
But this is exactly where I'm stuck, I can't seem to find the right equation.
By the way these are not my homework, I'm preparing myself for an oncoming exam and I seek for some sort of a guidance.

#### WBahn

Joined Mar 31, 2012
26,131
Ok listen
If I short-circut the port the voltage is forced to be zero and the value of the current can vary.
So draw the original circuit with the modification that the V2 port is short circuited and you have a test voltage applied to V1. Make ONLY those modifications.

Then analyze the circuit, using any technique you like, to find I2. The standard candidates are Mesh Current Analysis, Node Voltage Analysis, and Superposition. You have a variety of other options as well, including Thevenin/Norton equivalents, various transform techniques, and the old fallback of KVL/KCL.

I chose the cross section where I2 and Iru come in and right now what I need is a way to express Vπ with V1. I think if I find any relation between them I manage to solve this.
But this is exactly where I'm stuck, I can't seem to find the right equation.
This could be just a language issue, but maybe not. Saying that you are trying to "find" the right equation implies that you are trying to regurgitate some equation from your memory and hoping that it helps -- sort of like the voltage divider equation you misapplied earlier. What you want to do is "derive" an equation that describes the behavior of that circuit.

By the way these are not my homework, I'm preparing myself for an oncoming exam and I seek for some sort of a guidance.
All the more important that you fight through it and not just be shown how to do it. No one is going to be there to show you how to do it on the exam. If I just show you how to do it, what will happen is that you will be convinced that you have learned how to do it (and you will honestly believe that to be the case) right up to the point where you have to actually do it on the exam, at which point you will discover that you are still stuck because you can't find the right equation.