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The voltage transfer function of the network shown in the figure below is :

Vin(s) across a 4 ohm resistor, a 2 H inductor in series from its upper terminal to the output, and a 1 F capacitor across the output where Vout(s) is taken

  • A.\(1 + 4s\)
  • B.\(6 - s\)
  • C.\(\frac{1}{1 + 2s^{2}}\)
  • D.\(\frac{1}{1 + 2s}\)

Answer: C

The transfer function is \(\frac{1}{1 + 2s^{2}}\).

The 4 Ω resistor is connected directly across the input, so it carries its own current but does not change the voltage applied to the rest of the network. The output is then a voltage divider of the 2 H inductor and the 1 F capacitor:

\[ \begin{aligned} \frac{V_{out}}{V_{in}} &= \frac{1/sC}{sL + 1/sC} = \frac{1}{1 + s^{2}LC} \\ &= \frac{1}{1 + 2s^{2}} \end{aligned} \]

\(\frac{1}{1 + 2s}\) would come from a resistor and capacitor, not an inductor and capacitor. \(1 + 4s\) and \(6 - s\) grow with frequency, which a passive divider of this kind cannot do.

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