JEE Main & Advanced Physics Semiconducting Devices Question Bank Critical Thinking

  • question_answer
    The plate current in a triode is given by \[{{I}_{p}}=0.004\ {{({{V}_{p}}+10{{V}_{g}})}^{3/2}}mA\]where Ip, Vp and Vg are the values of plate current, plate voltage and grid voltage, respectively. What are the triode parametersm, rp and gm for the operating point at \[{{V}_{p}}=120\,volt\] and \[{{V}_{g}}=-\,2\,volt\]?

    A)            10, 16.7 kW, 0.6 m mho    

    B)            15, 16.7 kW, 0.06 m mho

    C)            20, 6 kW, 16.7 m mho       

    D)            None of these

    Correct Answer: A

    Solution :

                       \[{{I}_{p}}=0.004\,{{({{V}_{p}}+10{{V}_{g}})}^{3/2}}\] Þ \[\frac{\Delta {{I}_{p}}}{\Delta {{V}_{g}}}=0.004\,\left[ \frac{3}{2}\,{{({{V}_{p}}+10{{V}_{g}})}^{1/2}}\times 10 \right]\] Þ \[{{g}_{m}}=0.004\times \frac{3}{2}\,{{(120+10\times -2)}^{1/2}}\times 10\] Þ \[{{g}_{m}}=6\times {{10}^{-4}}mho=0.6\,m\,mho\] Comparing the given equation of Ip with standard equation \[{{I}_{p}}=K\,{{({{V}_{p}}+\mu {{V}_{g}})}^{3/2}}\] we get m = 10 Also from m = rp ´ gm Þ \[{{r}_{p}}=\frac{\mu }{{{g}_{m}}}=\frac{10}{0.6\times {{10}^{-3}}}\] Þ \[{{r}_{p}}=16.67\,\times {{10}^{3}}\Omega =16.67\,k\Omega .\]


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