Discussion
One of the reasons why vacuum tubes eventually fail at microwave frequencies in that their
Answer: D
The answer is that the noise figure increases. At microwave frequencies conventional tubes fail because of transit time (which becomes comparable to the period, not too short), inter-electrode capacitance and lead inductance, and because their noise figure increases.
Option A says the transit time becomes too short, but it is too long. Option B says the shunt capacitive reactance becomes too large, but at high frequency it becomes small and shunts the signal away. Option C says the series inductive reactance becomes too small, but it grows with frequency.
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