# input and output impedance why we should find them in the way told ! any specific reason

Discussion in 'General Electronics Chat' started by srikar sana, Jul 6, 2016.

1. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
i was revising the hybrid parameters in the transistor ac analysis . i was reading boylstead and my question is not related to the book but it was given that h11 or hie= Vi/Ii | Vo=0 i.e., input resistance is equal to input voltage by input current when output is shorted why should we do that i mean why should we short output when finding input resistance and why should we open input when finding output resistance is there any specific reason for doing so ...........?

i know that there is a reason for everything and some one pls tell me that thank you .

2. ### DGElder Member

Apr 3, 2016
347
87
How do you determine the impedance of a voltage source and resistor in series? How do you determine the impedance of a current source and resistor in parallel? Now look at your H-parameter model.

There you have it.

Last edited: Jul 6, 2016
3. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
sir DGElder could you pls be more specific because i am a slow learner thank you

4. ### DGElder Member

Apr 3, 2016
347
87
If you post the drawing of the transistor model to which you refer - with labels, I will walk you through it.

Last edited: Jul 6, 2016

Feb 19, 2010
3,500
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Homework?

6. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
haha home work now there is no one to give me home work because i have completed my BE but there are some doubts and said as i am slow learner i trying to learn

7. ### DGElder Member

Apr 3, 2016
347
87
"Homework?"

It's not a problem, it's a theory question

8. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
DGElder here is the diagram forget about the capacitance values

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9. ### DGElder Member

Apr 3, 2016
347
87
You were asking about the hybrid parameters in the transistor ac analysis. That is a common emitter amplifier, it is not an H-model of a transistor, e.g. where is hie and hfe?

Last edited: Jul 6, 2016
10. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
could you do that for the above circuit sir pls because that way it helps me better and pls note what you are going to say is recorder into a word document for my future use

or else you it self take a hybrid parameter circuit for a CE amp and pls expalin me these

11. ### srikar sana Thread Starter New Member

Feb 3, 2016
18
0
dgelder sir pls put an answer sir why did you remain silent sir ? or else tell the things that are to be done by me so that i can figure it out in the way you told

12. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
The 6 two-port parameters (z, y, h, g, a, b) define relations between currents and voltages at one port to currents and voltages at the other port. The choice of which variables to consider independent and which to consider dependent leads to the conditions imposed at the output port when evaluating the parameters.

See this for full details: https://en.wikipedia.org/wiki/Two-port_network

h parameters were originally chosen to use for transistor circuit analysis because transistors in the common emitter configuration generally have a low impedance at the input and high impedance at the output at low frequencies. Because of that, it's easier to set up the terminal conditions for accurate measurement of h-parameters of a transistor then one of the other parameter sets.