What is the primary purpose of a screen grid in a vacuum tube?
The correct answer is A: To reduce grid-to-plate capacitance. The primary purpose of a screen grid in a vacuum tube is to reduce grid-to-plate capacitance. The screen grid acts as an electrostatic shield between control grid and plate, reducing unwanted capacitance. For amateur radio operators, this improves tube performance. Understanding this helps when using tube amplifiers.
Exam Tip
Screen grid purpose = reduce grid-to-plate capacitance. Think 'S'creen 'G'rid = 'S'hields 'G'rid from plate. Acts as electrostatic shield between control grid and plate, reducing unwanted capacitance. Not efficiency, not grid resistance, not plate resistance - just capacitance reduction.
Memory Aid
"Screen grid purpose = reduce grid-to-plate capacitance. Think 'S'creen 'G'rid = 'S'hields 'G'rid. Acts as electrostatic shield between control grid and plate, reducing unwanted capacitance. Primary purpose of screen grid."
Real-World Application
A vacuum tube has a screen grid between the control grid and plate. The screen grid acts as an electrostatic shield, reducing the capacitance between control grid and plate. This prevents unwanted feedback and improves tube performance. Reducing grid-to-plate capacitance is the screen grid's primary purpose.
Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Screen grid doesn't directly increase efficiency - it reduces capacitance, which may improve performance, but efficiency increase isn't the primary purpose.
Option C: Incorrect. Screen grid doesn't increase control grid resistance - it reduces capacitance, not resistance. Resistance isn't the issue.
Option D: Incorrect. Screen grid doesn't decrease plate resistance - it reduces capacitance, not resistance. Plate resistance isn't affected.
题目解析
The correct answer is A: To reduce grid-to-plate capacitance. The primary purpose of a screen grid in a vacuum tube is to reduce grid-to-plate capacitance. The screen grid acts as an electrostatic shield between control grid and plate, reducing unwanted capacitance. For amateur radio operators, this improves tube performance. Understanding this helps when using tube amplifiers.
考试技巧
Screen grid purpose = reduce grid-to-plate capacitance. Think 'S'creen 'G'rid = 'S'hields 'G'rid from plate. Acts as electrostatic shield between control grid and plate, reducing unwanted capacitance. Not efficiency, not grid resistance, not plate resistance - just capacitance reduction.
记忆口诀
Screen grid purpose = reduce grid-to-plate capacitance. Think 'S'creen 'G'rid = 'S'hields 'G'rid. Acts as electrostatic shield between control grid and plate, reducing unwanted capacitance. Primary purpose of screen grid.
实际应用示例
A vacuum tube has a screen grid between the control grid and plate. The screen grid acts as an electrostatic shield, reducing the capacitance between control grid and plate. This prevents unwanted feedback and improves tube performance. Reducing grid-to-plate capacitance is the screen grid's primary purpose.
错误选项分析
Option B: Incorrect. Screen grid doesn't directly increase efficiency - it reduces capacitance, which may improve performance, but efficiency increase isn't the primary purpose. Option C: Incorrect. Screen grid doesn't increase control grid resistance - it reduces capacitance, not resistance. Resistance isn't the issue. Option D: Incorrect. Screen grid doesn't decrease plate resistance - it reduces capacitance, not resistance. Plate resistance isn't affected.
知识点
Screen grid, Grid-to-plate capacitance, Electrostatic shield, Vacuum tube
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.