真空管/電子管的發明

1904, John Ambrose Fleming invented the two-electrode vacuum-tube rectifier, which he called the oscillation valve. It was also called a thermionic valve, vacuum diode, kenotron, thermionic tube, or Fleming valve.
1906, Lee de Forest invented the first successful three-element (triode) vacuum tube, and the first device which could amplify electrical signals.
1913, Harold Arnold developed high-vacuum tubes which were tested on AT&T's long distance network. The high-vacuum tubes could operate at high plate voltages without a blue glow.
1919, The physicist Walter H. Schottky invented the tetrode tube to solve stability problems and limited voltage gain due to the Miller effect.
1926, The three-grid tube which is called pentode was invented by Bernard D. H. Tellegen, and became generally favored over the simple tetrode

真空管的結構 - Diode?[?da?o?d], Triode?['tra?o?d], Tetrode?['tetro?d], Pentode?['pento?d]

Anode?[??no?d]: 陽極， Grid 柵極， Heated cathode?[?k?θo?d]: 被加熱的陰極, Heater 電熱絲

陰極用來放射電子, 分為氧化物陰極和碳化釷鎢陰極。一般來說氧化物陰極是旁熱式的, 它是利用專門的燈絲對涂有氧化鋇等陰極體加熱, 進行熱電子放射。壽命一般在1000~3000小時。碳化釷鎢陰極一般都是直熱式的,通過加熱即可產生熱電子放射, 所以它既是燈絲又是陰極。理論上碳化釷鎢陰極比氧化物陰極壽命長得多, 一般在2000~10000 小時以上。大功率發射管應用最為廣泛的是碳化釷鎢陰極, 氧化物陰極一般在輸出功率為1kW 以下的發射管中應用

柵極根據它們在管中所起的作用不同分為一柵、二柵, 有時也稱為控制柵、簾柵。第一柵的主要作用是控制陰極電流, 二柵的作用是屏蔽板極對第一柵的影響。柵極結構關系到本身的機械強度和散熱效果, 關系到管子可否穩定工作。為了減小電子的渡越時間, 柵陰間距作的很短甚至不到1mm , 因此廠商多采用機械強度高、導熱系數高、輻射系數好以及溶點高的材料來做柵極, 以閉免在很小的間距下發生熱碰極。一柵和二柵應嚴格對柵, 這樣簾柵對電子截獲小, 可減小簾柵耗, 改善電流分配提高性線

陽極是收集陰極發射出來的大部分電子的電極。電子管工作時, 由于電子管轟擊板極表面, 以及其它電極的熱輻射, 在板極產生大量熱能, 因其板極的耗散功率密度是每平方厘米幾十瓦到幾百瓦, 這樣大的功率密度采用自然輻射或傳導的冷卻已不能勝任。故須采用強制冷卻方式。常用的有風冷、水冷和蒸發冷卻等

真空管的原理

真空管具有發射電子的陰極, 和工作時通常加上高壓的陽極或稱屏極. 燈絲通電后產生光和熱, 激發陰極放射電子. 柵極置于陰極與屏極之間, 柵極加電壓后抑制電子通過柵極, 所以能夠在陰極和陽極之間對電流起到控制作用.

為保持管內的真空狀態, 真空管添加了除氣劑, 一般為鋇鋁鎂等活潑金屬合金. 在抽出管中空氣后, 將管加熱至紅熱, 這樣就可以吸收管內氣體. 利用圍繞管子的高頻電磁場而使除氣劑迅速升華, 除氣劑就會吸收管子中的氣體. 反應過后玻璃管內壁會積存銀色的除氣劑披覆層. 若管體漏氣, 內壁積存銀色的除氣劑便會褪色, 也表示該真空管已損壞.

電子在放射過程中會與空氣中的分子相撞而產生阻力, 比在真空狀態中遷移困難, 所以需要將管體內部抽成真空狀態. 若真空度不足，陰極射出的電子會擊打空氣, 令空氣的原子被激發至激態發出紅光, 嚴重影響真空管的正常工作. 另一方面, 電子打到玻璃也會產生藍光并產生二次電子反射噪音.

真空管的優點

高度線性 Highly linear without negative feedback, specially some small-signal types
Clipping is smooth, which is widely considered more musical than transistors
對過載和電壓峰刺的容忍度很高,?過載掛掉需要花幾秒鐘時間.?Tolerant of overloads and voltage spikes
Characteristics highly independent of temperature, greatly simplifies biasing
大的動態范圍 Wider dynamic range than typical transistor circuits, thanks to higher operating voltages
Device capacitances vary only slightly with signal voltages
Capacitive coupling can be done with low-value, high-quality film capacitors
Circuit designs tend to be simpler than semiconductor equivalents
Operation is usually in Class A or AB, which minimizes crossover distortion
Output transformer in power amp protects speaker from tube failure
Maintenance tends to be easier because user can replace tubes

真空管的缺點

笨重 Bulky, hence less suitable for portable products
需要高壓 High operating voltages required
高能耗 High power consumption, needs heater supply
產生高熱量 Generate lots of waste heat
Lower power efficiency than transistors in small-signal circuits
易碎 Low-cost glass tubes are physically fragile
More prone to microphonics than semiconductors, especially in low-level stages
使用壽命不長 Cathode electron-emitting materials are used up in operation, resulting in shorter lifetimes (typically 1-5 years for power tubes)
High-impedance devices that usually need a matching transformer for low impedance loads, like speakers
高成本 Usually higher cost than equivalent transistors

對于大功率放大(如百萬瓦電臺)以及大功率微波而言，大功率真空管及行波管仍是唯一的選擇, 對于高頻電焊機及X射線機仍是主流器件.

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