2026-06-01
You've seen gyrators turn capacitors into simulated inductors. But sometimes you need to boost an actual physical inductor — make a 10 µH coil behave like 1 mH for a specific filter or oscillator application. That's the inductance multiplier, and it's especially valuable in audio crossovers, low-frequency filters, and tunable LC circuits where wire-wound inductors get prohibitively large, heavy, and expensive.
The classic topology uses an op-amp in a positive-feedback arrangement around a small inductor. The op-amp senses the voltage across the real inductor L, then drives current back into the node such that the apparent impedance seen at the input is multiplied by the gain factor. Effectively, L_apparent = L × (1 + R2/R1), where R1 and R2 set the feedback ratio.
Concrete example: Suppose you're building a 100 Hz low-pass filter and the math demands a 500 mH inductor. A physical 500 mH inductor at meaningful current rating is enormous — think shielded chokes, $30+, palm-sized. Instead, grab a tiny 5 mH SMD inductor and wrap it in an inductance multiplier with R1 = 1 kΩ, R2 = 99 kΩ. Now L_apparent = 5 mH × 100 = 500 mH. The op-amp does the heavy lifting, your BOM cost drops, and the board shrinks dramatically.
Rule of thumb: Keep the multiplication factor under about 100×. Beyond that, the op-amp's finite gain-bandwidth product makes the synthesized inductance roll off well below your target frequency, and parasitic capacitance starts dominating. Also, the apparent Q can be very high — sometimes too high — leading to ringing or instability in resonant applications. Add a small damping resistor in series if you see peaking.
This trick shines anywhere you need bulky low-frequency inductance in a small-signal path: subwoofer crossover prototyping, seismic sensor filters, low-frequency notch filters for power-line interference rejection, and tunable analog synth filters.