Discussion
Is it necessary to separately define Z and Y parameters as Z can be obtained from Y and vice a versa ?
I. Not necessary to define Y and Z separately as Y = 1/Z.
II. Necessary as Z-parameters cannot be defined at high frequency.
III. Y = 1/Z is applicable only at low frequency, so conversion is not valid under all conditions.
IV. Hybrid \(\pi\)-model enables Z to be determined using Z = 1/Y.
Answer: B
The key marks II and IV as the correct statements. The reasoning behind the key is that the two sets of parameters are not interchangeable in every case: the Z-parameters are defined by open-circuit conditions, which cannot be realised at high frequency, where the open-circuited port has stray capacitance and the test condition fails, and the hybrid pi model gives a direct way to obtain Z from the admittance form of the device. The statement that Y and Z need not be defined separately because Y = 1/Z is not accepted, since for a two-port the matrices are inverses only if the inverse exists, and the statement that conversion is not valid at low frequency is the reverse of what applies. I could not verify every step of the key's argument from a standard text, so the answer rests on the official key. The answer is that options II and IV are correct.
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