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Asked in CBSE AIPMT 1990 · Vacuum tubes and triode amplifier
Idea: follow what a rise in grid voltage does to the plate voltage.
A more positive grid lets more electrons through, so the plate current increases.
A larger current through the load resistor drops more voltage across it, leaving LESS voltage at the plate.
The output therefore falls as the input rises, which is a phase difference of π, and the gain expression -(μ R_L)/(R_L+rₚ) carries the same minus sign.
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