Question

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

• A. $25$
• B. $50$
• C. $75$
• D. $100$

Answer 1

The correct option is A $25$

Since, $\mu =r_p\cdot g_m$
$\therefore g_{m_1}=\frac{{\mu }_1}{r_{p_1}}=\frac{30}{5}=6$
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,
$g_m=g_{m_1}+g_{m_2}=(6+\frac{21}{4})(k\Omega {)}^{-1}$
$\therefore$ Amplification factor
$\mu =r_p\cdot g_m=\frac{20}{9}(6+\frac{21}{4})=25$.