What are two-port networks all about?

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Discussion Overview

The discussion revolves around the concept of two-port networks, exploring their usefulness, underlying principles, and the conditions under which they operate. Participants express confusion about the definitions and applications of two-port networks, particularly regarding the relationships between currents and voltages at the ports.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question the usefulness of two-port networks and seek clarification on why incoming and outgoing currents must be equal at each port.
  • Others argue that two-port networks can be beneficial for analyzing circuits, especially when assembled in cascade, and can be represented by simultaneous linear equations in matrix form.
  • A participant mentions that the equations for voltage and current at the ports can be derived without solving the entire circuit, suggesting a focus on the parameters involved.
  • There is a discussion about the conditions under which the relationships hold, including the need for linear circuits and the absence of independent sources.
  • Some participants express skepticism about the textbook definitions and emphasize the importance of understanding the context and assumptions behind the statements made in the book.
  • Participants discuss the physical meanings of the z-parameters and mention that there are multiple ways to express the relationships, including admittance and mixed parameters.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions and applications of two-port networks. There are multiple competing views regarding the conditions necessary for the equations to hold and the interpretations of the textbook statements.

Contextual Notes

Limitations include the need for specific conditions (e.g., linearity, absence of independent sources) for the equations to be valid, which are not universally applicable. The discussion reflects varying interpretations of the textbook material.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in circuit analysis, particularly those exploring the theoretical foundations and practical applications of two-port networks.

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I do not understand two-port networks. Why are they so useful?
My book says that for a two port network current incoming and outgoing at each port must be equal. Why is that? Okay, even if they are equal how does that help us? How are we able to neglect all the inner circuit and just focus on the ports and still get the values of voltage and current so easily?
 
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Two port network can be very useful. But there are many two port networks depend which one you are referring to. Without knowing what two port network your are referring to, there is no way I can comment on that.
 
I do not understand two-port networks. Why are they so useful?
My book says that for a two port network current incoming and outgoing at each port must be equal. Why is that? Okay, even if they are equal how does that help us? How are we able to neglect all the inner circuit and just focus on the ports and still get the values of voltage and current so easily?

Are you quite sure? Under what circumstances does your book say this?

Why are they so useful?

Because
Two port networks can be assembled in cascade to analyse circuits.
The offer a format that can be represented by a pair of simultaneous linear equations that can be written as a matrix and the matrices multiplied in cascade.

Further by choosing one of the ports as common to input and output they can be used to represent many three terminal devices such as transistors.


for example

v1 = z11i1 + z12i2
v2 = z21i1 + z22i2

subscript 1 refers to input and 2 refers to output

This is the impedance version and clearly shows different input and output currents and voltages.
 
Studiot said:
Are you quite sure? Under what circumstances does your book say this?
Yes we can write voltage and current at the ports in terms of the parameters. We don't need to solve the whole circuit right?

Studiot said:
v1 = z11i1 + z12i2
v2 = z21i1 + z22i2

subscript 1 refers to input and 2 refers to output

This is the impedance version and clearly shows different input and output currents and voltages.
That's what I want to know. How do we get these equations?
 
Quote by Studiot
Are you quite sure? Under what circumstances does your book say this?

Yes we can write voltage and current at the ports in terms of the parameters. We don't need to solve the whole circuit right?

I am sorry how is this an answer to my question?

If your book genuinely says this (yes) for all circumstances, change your book soonest.
 
Studiot said:
I am sorry how is this an answer to my question?

If your book genuinely says this (yes) for all circumstances, change your book soonest.
It doesn't say for all conditions. It has some restrictions:-
(1) Incoming and outgoing current must be equal at each port
(2) Circuit must be linear
(3) There should be no independent sources present

So if the circuit meets all these conditions how can we derive the following equations:-
v1 = z11i1 + z12i2
v2 = z21i1 + z22i2

Thank you
 
Well then pleaase tell us what circumstances you (your book) is talking about.

What circuit is it considering?
 
Hmm could you just tell me what all circumstances this concept is valid and how do we get those equations
Sorry for the confusion
Thanks a lot
 
I don't like to contradict a textbook; they can be wrong, but in my experience most issues arise through misreading or misreporting what they say.

A two port network is the classic 'blackbox' theory.
You do not need to know what goes on inside the box.
The blackbox contains two pairs of terminals labelled input and output and unknown works inside.
Each pair of terminals forms a port.
All its properties are characterised by the relationships between variables of interest measurable at the two terminal pairs or ports.
There may be (or may not be) interaction between the input and output terminals. That is what the current at the input may or may not affect the current at the output.
A blackbox may (but does not have to) contain an energy source.

All this leads to the the equations I wrote.
The first term of each describes the interaction between curennt and voltage at each port, without regard to the other port.
The second term of each describes the interaction of the input port with the output port.
 
  • #10
Okay, so z11, z21 and others are all just constants. Do they have a physical meaning?

And why so many parameters, apart from the impedance parameters there are y (admittance), t, g and other parameters.
 
  • #11
Yes the z parameters are impedances.

z11 is the input impedance

z22 is the output impedance

z12 is the forward transfer impedance

z21 is the reverse transfer impedance, which may or may not be equal to z12

You can write the equations is 5 other ways with admittances, and with what are known as mixed parameters.
 
  • #12
Well thanks a lot - I got the basic why, how questions all cleared up from you.
No idea where would I be without PF!
 
  • #13
:wink:

Time for tea and chocolate biscuit.
 

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