哈特萊振盪器

哈特萊振盪器（Hartley oscillator），又稱赫特利振盪器，電感三點式振盪器，是一種由電容和電感的調諧電路（即LC振盪器）決定振盪頻率的電子振盪器電路。該電路是美國工程師雷夫·哈特萊於1915年發明的。Hartley振盪器的特點是調諧電路由一個電容器與串聯的兩個電感（或單抽頭電感）並聯，振盪所需的反饋信號取自兩電感連接的中心。

## 操作

Hartley振盪器的突出特點是諧振電路包含兩個串聯線圈（或者通常用抽頭線圈）與電容器並聯，在相對高阻態的LC槽電路和線圈間，相對低電壓/高電流點之間有一個放大器。

1905年的原始版本使用一個三極體作為屏極接地（陰極跟隨器）組態中的放大器件，含有三節電池，以及獨立可調線圈。右圖顯示的簡化電路使用一個JFET（共漏極組態）、一個LC諧振電路（這裡的單繞組是抽頭的）和一個單電池。該電路說明了Hartley振盪器的操作：Circuit operation

- JFET的源極（若使用BJT則是發射極；三極體則是陰極）的輸出與它的柵極（基極）的信號同相位，電壓與它的輸入（整個槽電路兩端的電壓）粗略相等，但電流被放大了，即它是作為電流緩衝器或Sallen–Key拓撲。
- 然後這個低阻態輸出送入到線圈抽頭中，有效地進入一個可以提高電壓的自耦變壓器，需要比較高的電流（相對於線圈頂部的電流來說）。
- with the capacitor-coil resonance, all frequencies other than the tuned frequency will tend to be absorbed (the tank will appear as nearly 0Ω near DC due to the inductor's low reactance at low frequencies, and low again at very high frequencies due to the capacitor); they will also shift the phase of the feedback from the 0° needed for oscillation at all but the tuned frequency.

Variations on the simple circuit often include ways to automatically reduce the amplifier gain to maintain a constant output voltage at a level below overload; the simple circuit above will limit the output voltage due to the gate conducting on positive peaks, effectively damping oscillations but not before significant distortion (spurious harmonics) may result. Changing the tapped coil to two separate coils, as in the original patent schematic, still results in a working oscillator but now that the two coils are not magnetically coupled the inductance, and so frequency, calculation has to be modified (see below), and the explanation of the voltage increase mechanism is more complicated than the autotransformer scenario.

A quite different implementation using a tapped coil in an LC tank feedback arrangement, still called a Hartley oscillator (or sometimes "the" Hartley Oscillator circuit) is to employ a common-grid (or common-gate or common-base) amplifier stage, which is still non-inverting but provides voltage gain instead of current gain; the coil tapping is still connected to the cathode (or source or emitter), but this is now the (low impedance) input to the amplifier; the split tank circuit is now dropping the impedance from the relatively high output impedance of the plate (or drain or collector).

The Hartley oscillator is the dual of the Colpitts oscillator which uses a voltage divider made of two capacitors rather than two inductors. Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented using a tapped coil, with the feedback taken from the tap, as shown here. The optimal tapping point (or ratio of coil inductances) depends on the amplifying device used, which may be a bipolar junction transistor, FET, triode, or amplifier of almost any type (non-inverting in this case, although variations of the circuit with an earthed centre-point and feedback from an inverting amplifier or the collector/drain of a transistor are also common), but a junction FET (shown) or triode is often employed as a good degree of amplitude stability (and thus distortion reduction) can be achieved with a simple grid leak resistor-capacitor combination in series with the gate or grid (see the Scott circuit below) thanks to diode conduction on signal peaks building up enough negative bias to limit amplification.

振盪頻率約為槽電路的諧振頻率。若槽路電容器的電容為 C 而抽頭電感的總電感是 L，則

$$f = {1 \over 2 \pi \sqrt {LC}} \,$$

若使用了電感為 L1 和 L2 兩個非耦合的線圈

$$L = L_1 + L_2 \,$$

然而，如果兩個線圈磁耦合，總電感會因互感 k 而增大

$$L = L_1 + L_2 + k \sqrt{L_1 L_2} \,$$

由於線圈中的寄生電容和電晶體的負載，實際振盪頻率將略低於上述。

Hartley振盪器的優點包括：

- 頻率可以使用一個可變電容器進行調整，電容器的一邊可接地
- 輸出幅度保持恆定的頻率範圍內
- 需要一個抽頭線圈或兩個固定電感，以及很少的其他組件
- Easy to create an accurate fixed-frequency crystal oscillator variation by replacing the capacitor with a (parallel-resonant) quartz crystal or replacing the top half of the tank circuit with a crystal and grid-leak resistor (as in the Tri-tet oscillator).

缺點包括：

- 如果從放大器，而不是直接從LC電路（除非採用振幅穩定電路），則會得到諧波豐富的輸出。

## 外部連結

- Hartley oscillator, Integrated Publishing