EC-1.5 Two-Port Networks
You can describe a network by any of the standard two-port parameter sets, convert between them, and combine two networks by choosing the set that makes the combination an addition.
A two-port description throws away everything inside a network and keeps only the relationship between what happens at its two ends, which is exactly what you need when the network is a building block inside something larger. There are several parameter sets because each one makes a different kind of connection trivial: some add when networks are stacked, one multiplies when they are put end to end. Choosing the set to match the connection is the entire skill, and the conversions between them are lookup rather than insight.
Work through these
What a port is, and what a two-port description deliberately forgets
A port is a pair of terminals where the current in equals the current out, and the description keeps only the terminal behaviour. Everything inside becomes invisible, which is what makes the block reusable.
NPTEL: Circuit Theory · CourseImpedance and admittance parameters, and measuring them by opening or shorting a port
Each parameter is a ratio measured with one port open or shorted, which is also how you would find them on a bench. Knowing which condition defines which parameter is what stops the sets being confused.
NPTEL: Circuit Theory · CourseHybrid and transmission parameters, and the connection each one is built for
Hybrid parameters mix an open-circuit and a short-circuit condition, which is why transistor models use them. Transmission parameters relate the output end to the input end, so putting networks end to end becomes a matrix product.
Interconnection: which parameter set adds for stacked, side-by-side and end-to-end
Impedance parameters add when networks are stacked, admittance parameters add when they sit side by side, and transmission matrices multiply end to end. Picking the wrong set turns a one-line combination into an hour.
Conditions for a reciprocal and for a symmetric two-port
Reciprocity shows up as an equality between two off-diagonal parameters, and symmetry as an equality between the two diagonal ones. Both are quick checks that a set of measured parameters is self-consistent.
The wye and delta forms, and converting between them
Three impedances in a star and three in a triangle can each be turned into the other, which unlocks networks that no series or parallel reduction will simplify. The formulas are worth deriving once and then looking up.
MIT OpenCourseWare 6.002: Circuits and Electronics · Course
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