# Help with two port network question

Discussion in 'Homework Help' started by csw123, Feb 27, 2014.

1. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
hi everyone, the question is to find the Hybrid parameters (h-parameters) of this circuit, i tried and i have no idea how to do that.
thx

the circuit attach as image.

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2. ### WBahn Moderator

Mar 31, 2012
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So since you tried, you should be able to post your best attempt (doesn't matter that it was unsuccessful) to solve the problem. That will give us a starting point to work from.

3. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
well its not really something that can help, the way to solve this its from those equations (image), and someone also told me to try and convert this circule into a Parallel circuit (but i have no idea how to do that).
thx for the quick respone

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4. ### LvW Active Member

Jun 13, 2013
674
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csw123,

at first, you should know how the hybrid parameters are defined. Do you?

5. ### csw123 Thread Starter New Member

Feb 27, 2014
16
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yes i do, but its still kinda diffrent then the normal hybrid circuit

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Why is is different?

The whole point of a two port network is that is doesn't matter what is in the box. Everything is defined in terms of the voltage and current that appear at the two pairs of terminals.

LvW is quite right, Do you understand what the conditions mean that you have posted in your post#3?

7. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
well i understand the general idea and ive done that with z-parameters.
i know that i need to make a voltage and current equations and work from there but i just dont manage to do it on this circule.
if you think i dont fully understand the idea i will be happy to learn
thx again

8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515

You have noted that V2=0 and I1=0 as conditions.

what does this mean for the conditions at the two port terminals.

You must have covered this in Z parameters as well.

9. ### LvW Active Member

Jun 13, 2013
674
100
That means: How do you realize these conditiones?

10. ### csw123 Thread Starter New Member

Feb 27, 2014
16
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ha ok, now i understand your question, i got the "normal" hybrid circuit (image) and from that i got 2 equations : v1=h11*i1+h12*v2 and i2=h21*i1+h22*v2.
to find h11 i need i1=0 and etc. but i dont know how to do with the question circle

• ###### Capture.PNG
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Last edited: Feb 28, 2014
11. ### studiot AAC Fanatic!

Nov 9, 2007
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If I1 = 0 is the input open circuit or shorted?

If V2 = 0 is the output open circuit or shorted?

12. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
I1=0, both are shorted.
V2=0, both are open.
thats for the normal h circuit, the question circuit just look diffrent, i always get the equations for the circuit givning at the question and get those terms from there, its just hard to figure out what to do in here.
can you give me a lead of how to solve this?
and again thank you

Last edited: Feb 28, 2014
13. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
and i have another question
the circuit is the upper one, he divided this into 2 ports
the upper one and the T one.
my question is about the first one (the square)
i want to know if this matrix it is the correct answer

thank you

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14. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515

I have attached a very quick sketch of the basics, including how they apply to a bipolar transistor, where they are normally used.

Given the four quantities that appear at the external pairs of terminals (input voltage and current, output voltage and current), six sets of equations can be derived connecting these.

Impedance, Admittance, Hybrid, Inverse Hybrid, Transfer, Inverse transfer.

They all work in the same way being a pair of simultaneous equations connecting one of the quantities with two of the others via parameters.

Note that the parameters do not necessarily have the same units in any set
So

h11 is an impedance (resistance)
h12 a pure number (scaling factor)
h21 is a pure number

Each equation in the pair defines one variable in terms of the sum of two others times an appropriate h parameter.

If we can choose external conditions such as a short circuit or open circuit we can eliminate one of these variables from the equations, so enabling the second to be defined in terms of the input or output voltage (current) alone.

We do not simultaneously short or open both input and output.

So in my first equation if we short the output v2 becomes zero, but I2 does not vanish because h22 still exists.
However we can now find a relationship between V1 and I1 and solve for h11.

and so on for the others.

Now can you apply this to your circuit (with transformations if required) ?

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15. ### csw123 Thread Starter New Member

Feb 27, 2014
16
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ok first thank you so much!
and secend i understand that i need to find V1 and I2 equations
so my equations is : v1=(R1*alpha) I1-(R1/R2(R+1))V2
I2=(alpha-1) I1 + beta/(R2(beta+1)) V2
thats correct?

*the main question was to find whats alpha equal so the matrix will be reverse, so i need to Determinant that so it will be equal to 1, so i know my Determinant cant contain beta and i still got beta in there so something have to be wrong with my equations, can u point for me whats the problem?
thx

Last edited: Mar 1, 2014
16. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
*actually im not sure if they said anything about R1 and R2 (it was a question from test)
so ignore the previous *
but i just want to know if my equations are true
thx

17. ### csw123 Thread Starter New Member

Feb 27, 2014
16
0
I uploaded my answer, can u take a look and tell me if thats correct?
thank you

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18. ### studiot AAC Fanatic!

Nov 9, 2007
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Your Z matrix is your post#13 is a little short on algebra.

Since this is a passive network and easier than one with sources I have worked through some algebra and left you some to finish off.

You must use your knowledge of basic circuits (ohms law, series, parallel resistors, voltage dividers etc) to deduce the expressions for the z parameters in the matrix.
At least they are all impedances (resistances in this case)

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• ###### zmatrix2.jpg
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19. ### csw123 Thread Starter New Member

Feb 27, 2014
16
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the Z matrix i asked about its not mine its something i had in my friend textbook and i thought its wrong matrix, i guess i was right
thank you so much

and my H matrix at post#17 for the orginaly question its correct?

Last edited: Mar 2, 2014
20. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
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But can you follow the derivation of the simple z matrix?

And can you calculate the final z parameter I left for you?