Topic: Two-Port Network

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Two Port Network

A two-port network (a kind of four-terminal network or quadripole) is an electrical circuit or device with two pairs of terminals connected together internally by an electrical network. Two terminals constitute a port if they satisfy the essential requirement known as the port condition: the same current must enter and leave a port.

Examples include small-signal models for transistors (such as the hybrid-pi model), filters and matching networks. The analysis of passive two-port networks is an outgrowth of reciprocity theorems first derived by Lorentz.

A two-port network makes possible the isolation of either a complete circuit or part of it and replacing it by its characteristic parameters. Once this is done, the isolated part of the circuit becomes a "black box" with a set of distinctive properties, enabling us to abstract away its specific physical buildup, thus simplifying analysis. Any linear circuit with four terminals can be transformed into a two-port network provided that it does not contain an independent source and satisfies the port conditions.

There are a number of alternative sets of parameters that can be used to describe a linear two-port network, the usual sets are respectively called z, y, h, g, and ABCD parameters, each described individually below. These are all limited to linear networks since an underlying assumption of their derivation is that any given cicuit condition is a linear superposition of various short-circuit and open circuit conditions. They are usually expressed in matrix notation, and they establish relations between the variables

 Input voltage
 Output voltage
 Input current
 Output current

which are shown in Figure 1. These current and voltage variables are most useful at low-to-moderate frequencies. At high frequencies (e.g., microwave frequencies), the use of power and energy variables is more appropriate, and the two-port current–voltage approach is replaced by an approach based upon scattering parameters.

The terms four-terminal network and quadripole (not to be confused with quadrupole) are also used, the latter particularly in more mathematical treatments although the term is becoming archaic. However, a pair of terminals can be called a port only if the current entering one terminal is equal to the current leaving the other; this definition is called the port condition. A four-terminal network can only be properly called a two-port when the terminals are connected to the external circuitry in two pairs both meeting the port condition.

Author	Message
divyalaks Senior Member Joined: 22Jun2009 Online Status: Offline Posts: 630	Topic: Two-Port Network Posted: 08Dec2009 at 12:09am
	Two Port Network A two-port network (a kind of four-terminal network or quadripole) is an electrical circuit or device with two pairs of terminals connected together internally by an electrical network. Two terminals constitute a port if they satisfy the essential requirement known as the port condition: the same current must enter and leave a port. Examples include small-signal models for transistors (such as the hybrid-pi model), filters and matching networks. The analysis of passive two-port networks is an outgrowth of reciprocity theorems first derived by Lorentz. A two-port network makes possible the isolation of either a complete circuit or part of it and replacing it by its characteristic parameters. Once this is done, the isolated part of the circuit becomes a "black box" with a set of distinctive properties, enabling us to abstract away its specific physical buildup, thus simplifying analysis. Any linear circuit with four terminals can be transformed into a two-port network provided that it does not contain an independent source and satisfies the port conditions. There are a number of alternative sets of parameters that can be used to describe a linear two-port network, the usual sets are respectively called z, y, h, g, and ABCD parameters, each described individually below. These are all limited to linear networks since an underlying assumption of their derivation is that any given cicuit condition is a linear superposition of various short-circuit and open circuit conditions. They are usually expressed in matrix notation, and they establish relations between the variables $V_%7b1%7d \,\stackrel%7b\text%7bdef%7d%7d%7b=%7d\, %7b%7d$ Input voltage $V_%7b2%7d \,\stackrel%7b\text%7bdef%7d%7d%7b=%7d\, %7b%7d$ Output voltage $I_%7b1%7d \,\stackrel%7b\text%7bdef%7d%7d%7b=%7d\, %7b%7d$ Input current $I_%7b2%7d \,\stackrel%7b\text%7bdef%7d%7d%7b=%7d\, %7b%7d$ Output current which are shown in Figure 1. These current and voltage variables are most useful at low-to-moderate frequencies. At high frequencies (e.g., microwave frequencies), the use of power and energy variables is more appropriate, and the two-port current–voltage approach is replaced by an approach based upon scattering parameters. The terms four-terminal network and quadripole (not to be confused with quadrupole) are also used, the latter particularly in more mathematical treatments although the term is becoming archaic. However, a pair of terminals can be called a port only if the current entering one terminal is equal to the current leaving the other; this definition is called the port condition. A four-terminal network can only be properly called a two-port when the terminals are connected to the external circuitry in two pairs both meeting the port condition.
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