JCECE Medical JCECE Medical Solved Paper-2013

  • question_answer
    Two triodes having amplification factors 30 and 21 and plate resistances \[5\,k\Omega \] and \[4\,k\Omega \] respectively are connected in parallel. The composite amplification factor of the system is

    A)  25                

    B)  50

    C)  75                

    D)  100

    Correct Answer: A

    Solution :

     Since, \[\mu ={{r}_{p}}.{{g}_{m}}\] \[\therefore \] \[{{g}_{{{m}_{1}}}}=\frac{{{\mu }_{1}}}{{{r}_{{{p}_{1}}}}}\] \[=\frac{30}{5}=6\] and \[{{g}_{{{m}_{2}}}}=\frac{{{\mu }_{2}}}{{{r}_{{{p}_{2}}}}}\] \[=\frac{21}{4}\] Since, triodes are connected in parallel, hence effective plate resistance, \[{{r}_{p}}=\frac{{{r}_{{{p}_{1}}}}{{r}_{{{p}_{2}}}}}{{{r}_{{{p}_{1}}}}+{{r}_{{{p}_{2}}}}}\] \[=\frac{5\times 4}{5+4}\] \[=\frac{20}{9}k\Omega \] and transconductance, \[{{g}_{m}}={{g}_{{{m}_{1}}}}+{{g}_{{{m}_{2}}}}\] \[=\left( 6+\frac{21}{4} \right){{(k\Omega )}^{-1}}\] \[\therefore \] Amplification factor \[\mu ={{r}_{p}}.{{g}_{m}}\] \[=\frac{20}{9}\left( 6+\frac{21}{4} \right)\] \[=25\]


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