NEET Physics Semiconducting Devices NEET PYQ-Semiconducting Devices

  • question_answer
    A transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produce oscillations of frequency f. If L is doubled and C is changed to 4C, the frequency will be:     [AIPMT (S) 2006]

    A)  \[f/4\]              

    B)  \[8f\]

    C)  \[f/2\sqrt{2}\]   

    D)       \[f/2\]

    Correct Answer: C

    Solution :

    In a series LC circuit, frequency of LC oscillations is given by
                            \[f=\frac{1}{2\pi \,\sqrt{LC}}\]
                or         \[f\,\propto \,\,\frac{1}{\sqrt{LC}}\]
    \[\Rightarrow \]   \[\frac{{{f}_{1}}}{{{f}_{2}}}=\sqrt{\frac{{{L}_{2}}{{C}_{2}}}{{{L}_{1}}{{C}_{1}}}}\]
    Given,   \[{{L}_{1}}=L,\,{{C}_{1}}=C,\,{{L}_{2}}=2L,\,{{C}_{2}}=4C,\,\,{{f}_{1}}=f\]
    \[\therefore \]      \[\frac{f}{{{f}_{2}}}=\sqrt{\frac{2L\times 4C}{LC}}=\sqrt{8}\]
                \[\Rightarrow \]   \[{{f}_{2}}=\frac{f}{2\sqrt{2}}\]


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