With a change of load resistance of a triode, used as an amplifier, from $50$ kilo ohms to $100$ kilo ohms, its voltage amplification changes from $25$ to $30.$ Plate resistance of the triode is
A$25 k \Omega$
B$75 k \Omega$
C$7.5 k \Omega$
D$2.5 k \Omega$
[MP PET 1986]
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A$25 k \Omega$
$ \text { Voltage amplification } A_v=\frac{\mu}{1+\frac{r_p}{R_L}}$
$ \Rightarrow 25=\frac{\mu}{1+\frac{r_p}{50 \times 10^3}}$
and $30=\frac{\mu}{1+\frac{r_p}{100 \times 10^3}}$an solving equation (i) and (ii), $r_p=25 \mathrm{k} \Omega$.
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