Modern Physics 6 Question 28
31. For a given plate voltage, the plate current in a triode valve is maximum when the potential of
$(1985,2 M)$
(a) the grid is positive and plate is negative
(b) the grid is zero and plate is positive
(c) the grid is negative and plate is positive
(d) the grid is positive and plate is positive
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Solution:
$$ \begin{array}{rlrl} \Rightarrow & & I _b+I _c & =I _e \\ \Rightarrow & \quad \frac{I _b}{I _c}+1 & =\frac{I _e}{I _c} \\ \Rightarrow & \frac{1}{\beta}+1 & =\frac{1}{\alpha} \\ \Rightarrow & \frac{1}{\alpha} & =\frac{1+\beta}{\beta} \Rightarrow \alpha=\frac{\beta}{1+\beta} \end{array} $$
