To get this event, i must tell you about something. Now, suppose that we have a circuit, N circuit as you can see in the picture. It may contain more than one inputs or outputs, for instance, this circuit has 2 inputs and 2 outputs, this is called as "with 2 gates circuit".
For this circuit (and also for all of the circuits has more than 1 gate) there are 6 six parameters, these are : 1-Empedance parameters [Z matrice], 2-Admitance parameters [Y matrice], 3-Hybrid parameters [h matrice], 4-Transmission parameters [g matrice], 5-Chain parameters and 6- Reverse chain parameters.
1-Empedance parameters (these are matrices) :
| V1 | = | Z11 Z12 | . | I1 |
| V2 | | Z21 Z22 | | I2 |
2- Admitance parameters :
| I1 | = | Y11 Y12 | . | V1 |
| I2 | | Y21 Y22 | | V2 |
3-Hybrid (h) parameters :
| V1 | = | h11 h12 | . | I1 |
| I2 | | h21 h22 | | V2 |
4-Transmission (g) parameters :
| I1 | = | g11 g12 | . | V1 |
| V2 | | g21 g22 | | I1 |
In matrices "1" indicates the first gate (1st input and 1st output) and "2" indicates the second gate (2nd input and 2nd output).
In some circuits the empedance values may be important for you, or Y parameters, or h parameters may be. You can use one of them for the circuit you're analysing, it depends on what you want to do.
For instance, a transistor is modelled as h parameters, cause it has 2 gates (input circuit and the output circuit). You can see it in the 2nd picture. This is a common emitter transistor AC model (or equivalent circuit) for small and middle frequency signals. In here hie's unit is ohm, it's called as "input empedance" [Vbe/Ib, like V1/I1]. hoe's unit is siemens (1/ohm), and it's called as "output admitance" [Ic/Vce, like I2/V2]. hfe and hre are constant, hfe is "forward current gain" [Ic/Ib, like I2/I1]. hre is "reverse voltage gain" [Vbe/Vce, like V1/V2]. These are the specific parameters for a transistor (hie, hoe, hfe, hre) like as "h11,h12,h21,h22".
Also you can consider the capacitance values for the transistor (espacially in higher frequencies). These are inner capacitance of the transistor. But for that (if you consider the capacitance values) you must model the transistor with another model, for instance "pi model". There will be series and shunts capacitors in the circuit equivalent. Here the capacitance (reactance or suspedance) values must be considered.
I wish you understood better now.
