Vacuum tube 真空管/电子管

真空管/电子管的发明

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

真空管的结构 - Diode [ˈdaɪoʊd], Triode ['traɪoʊd], Tetrode ['tetroʊd], Pentode ['pentoʊd]

Anode [ˈænoʊd]: 阳极， Grid 栅极， Heated cathode [ˈkæθoʊd]: 被加热的阴极, Heater 电热丝

阴极用来放射电子, 分为氧化物阴极和碳化钍钨阴极。一般来说氧化物阴极是旁热式的, 它是利用专门的灯丝对涂有氧化钡等阴极体加热, 进行热电子放射。寿命一般在1000~3000小时。碳化钍钨阴极一般都是直热式的,通过加热即可产生热电子放射, 所以它既是灯丝又是阴极。理论上碳化钍钨阴极比氧化物阴极寿命长得多, 一般在2000~10000 小时以上。大功率发射管应用最为广泛的是碳化钍钨阴极, 氧化物阴极一般在输出功率为1kW 以下的发射管中应用

栅极根据它们在管中所起的作用不同分为一栅、二栅, 有时也称为控制栅、帘栅。第一栅的主要作用是控制阴极电流, 二栅的作用是屏蔽板极对第一栅的影响。栅极结构关系到本身的机械强度和散热效果, 关系到管子可否稳定工作。为了减小电子的渡越时间, 栅阴间距作的很短甚至不到1mm , 因此厂商多采用机械强度高、导热系数高、辐射系数好以及溶点高的材料来做栅极, 以闭免在很小的间距下发生热碰极。一栅和二栅应严格对栅, 这样帘栅对电子截获小, 可减小帘栅耗, 改善电流分配提高性线

阳极是收集阴极发射出来的大部分电子的电极。电子管工作时, 由于电子管轰击板极表面, 以及其它电极的热辐射, 在板极产生大量热能, 因其板极的耗散功率密度是每平方厘米几十瓦到几百瓦, 这样大的功率密度采用自然辐射或传导的冷却已不能胜任。故须采用强制冷却方式。常用的有风冷、水冷和蒸发冷却等

真空管的原理

真空管具有发射电子的阴极, 和工作时通常加上高压的阳极或称屏极. 灯丝通电后产生光和热, 激发阴极放射电子. 栅极置于阴极与屏极之间, 栅极加电压后抑制电子通过栅极, 所以能够在阴极和阳极之间对电流起到控制作用.

为保持管内的真空状态, 真空管添加了除气剂, 一般为钡铝镁等活泼金属合金. 在抽出管中空气后, 将管加热至红热, 这样就可以吸收管内气体. 利用围绕管子的高频电磁场而使除气剂迅速升华, 除气剂就会吸收管子中的气体. 反应过后玻璃管内壁会积存银色的除气剂披覆层. 若管体漏气, 内壁积存银色的除气剂便会褪色, 也表示该真空管已损坏.

电子在放射过程中会与空气中的分子相撞而产生阻力, 比在真空状态中迁移困难, 所以需要将管体内部抽成真空状态. 若真空度不足，阴极射出的电子会击打空气, 令空气的原子被激发至激态发出红光, 严重影响真空管的正常工作. 另一方面, 电子打到玻璃也会产生蓝光并产生二次电子反射噪音.

真空管的优点

高度线性 Highly linear without negative feedback, specially some small-signal types
Clipping is smooth, which is widely considered more musical than transistors
对过载和电压峰刺的容忍度很高, 过载挂掉需要花几秒钟时间. Tolerant of overloads and voltage spikes
Characteristics highly independent of temperature, greatly simplifies biasing
大的动态范围 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

真空管的缺点

笨重 Bulky, hence less suitable for portable products
需要高压 High operating voltages required
高能耗 High power consumption, needs heater supply
产生高热量 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
使用寿命不长 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

对于大功率放大（如百万瓦电台）以及大功率微波而言，大功率真空管及行波管仍是唯一的选择, 对于高频电焊机及X射线机仍是主流器件.