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One of the reasons why vacuum tubes eventually fail at microwave frequencies is that their

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  1. Noise figure increases
  2. Transit time becomes too short
  3. Shunt capacitive reactances become too large
  4. Series inductive reactances become too small

Answer (Detailed Solution Below)

Option 1 : Noise figure increases
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For vacuum tubes, the noise figure increases as the frequency of operation increases. Thus, at microwave frequencies, vacuum tubes eventually fail to provide minimum required SNR.
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