2026-09-01
The Armstrong oscillator — patented by Edwin Armstrong in 1914 — was the first practical vacuum-tube oscillator, and it still shows up in JFET/BJT designs where you want an LC tank without the capacitive divider of a Colpitts or the tapped inductor of a Hartley. Instead, it uses a separate feedback winding (the "tickler coil") magnetically coupled to the tank inductor. That physical isolation between tank and feedback path is what makes the topology distinctive.
How it works: The tank (L1 ∥ C) sets the frequency. A second winding L2, wound on the same core as L1, samples the tank current and feeds it back to the active device's input. Coupling coefficient k (typically 0.1–0.5) and turns ratio n = N2/N1 together set the feedback fraction. For sustained oscillation, the Barkhausen criterion demands loop gain ≥ 1 and 360° total phase shift — the transformer's winding polarity (dot convention) supplies the 180° needed to complement the active device's inversion.
Design rule of thumb: For a JFET common-source Armstrong with device transconductance gm and tank parallel resistance Rp (=Q·ω·L), you need:
Worked example: Build a 455 kHz Armstrong for an IF-alignment signal generator. Pick L1 = 240 µH, then C = 1/((2π·455k)²·240µ) ≈ 510 pF. If your ferrite bobbin gives Q = 100, then Rp = 100·2π·455k·240µ ≈ 68 kΩ. A J310 JFET has gm ≈ 10 mS at typical bias, so gm·Rp ≈ 680 — enormous margin. You need k·n ≥ 1/680 ≈ 0.0015, so even a lightly-coupled 5-turn tickler on a 100-turn tank oscillates enthusiastically. In practice you'd reduce coupling to keep waveform clean and avoid clamping the JFET into gate conduction.
Real-world use: Regenerative receivers (the classic "one-tube wonder" shortwave sets) exploit the Armstrong's adjustable tickler — a variable coupling knob puts the stage right at the edge of oscillation, dramatically boosting Q and sensitivity. Modern uses include RFID reader front-ends, grid-dip meters, and low-cost signal generators where you want a mechanically-tuned single-band oscillator without a tapped coil.
Gotchas: Winding polarity is everything — reverse the tickler and you get a stable amplifier instead of an oscillator. Over-coupling causes squegging (motorboating) as the device drives itself into cutoff between bursts. And unlike Colpitts/Hartley, harmonic content is highly dependent on transformer parasitics, so it's a poor choice when spectral purity matters.