NEET Physics Semiconducting Devices NEET PYQ-Semiconducting Devices

  • question_answer
    In a common emitter (CE) amplifier having a voltage gain G, the transistor used has trans conductance 0.03 mho and current gain 25. If the above transistor is replaced with another one with trans conductance 0.02 mho and current gain 20, the voltage gain will                                                                                                                         [NEET 2013]

    A)  \[\frac{2}{3}G\]                       

    B)  \[1.5\,G\]

    C)       \[\frac{1}{3}G\]                       

    D)       \[\frac{5}{4}G\]

    Correct Answer: A

    Solution :

    As         \[{{A}_{v}}=\beta \frac{{{R}_{L}}}{{{R}_{i}}}\]\[\left[ \because {{g}_{m}}=\frac{\Delta {{l}_{c}}}{\Delta {{V}_{B}}}=\frac{\Delta {{l}_{c}}}{\Delta {{l}_{B}}{{R}_{i}}} \right]\]
    or         \[G=\left( \frac{\beta }{{{R}_{i}}} \right){{R}_{L}}\]      \[\left[ \because {{g}_{m}}=\frac{\beta }{{{R}_{i}}} \right]\]
    \[\Rightarrow \]   \[G={{g}_{m}}{{R}_{L}}\]
    \[\Rightarrow \]   \[G\propto {{g}_{m}}\]
    \[\therefore \]      \[\frac{{{G}_{2}}}{{{G}_{1}}}=\frac{{{g}_{{{m}_{1}}}}}{{{g}_{{{m}_{2}}}}}\]
    \[\Rightarrow \]   \[{{G}_{2}}=\frac{0.02}{0.03}\times G\]
    \[\therefore \] Voltage gain \[{{G}_{2}}=\frac{2}{3}G\]


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