Question
What is the primary purpose of a screen grid in a vacuum tube?
Answer Options
- A) To reduce grid-to-plate capacitance
- B) To increase efficiency
- C) To increase the control grid resistance
- D) To decrease plate resistance
Correct Answer: A
Explanation
Tubes with only a control grid (triodes) suffer from high internal capacitance between the control grid and the plate. At high frequencies (RF), this capacitance allows the output signal to feed back to the input, causing unwanted oscillation and instability.
The screen grid is a second grid placed between the control grid and the plate. Its primary purpose is to act as an electrostatic shield, effectively reducing the grid-to-plate capacitance to very low levels. This shielding allows the tube to operate stably as an amplifier at much higher frequencies than a triode, which is why it is used in tetrodes and pentodes.
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