Simple RF Preamp

I threw together this RF preamp for 88–108 MHz from junkbox parts. I had intended to use a low-noise GaAsFET, but I discovered that the ones I had were bad. I settled for a 3N204. (Dual-gate MOSFETs are so obsolete that I had to create my own schematic symbol.) Thinking I might mount it at the antenna some day to overcome feedline loss, I designed the preamp to be remotely powered. I found the Scientific Atlanta DA-PI power inserter at a garage sale.

The coil self-capacitance plus the 3N204 input capacitance resonate the input circuit in the FM band. The trimmer capacitor, which adjusts the input match, also affects resonance. I adjusted the trimmer and the coil shape until the gain peaked near the center of the band. For biasing I started with pots in the 3N204 source and G2 circuits, but I found that the arrangement shown worked best. It certainly minimized the parts count. R1 sets the drain current via the G2 voltage. I used 1kΩ with a 17.5-V supply, which yielded 13.5 mA at 4 V. Gain increased little at higher current.

The upper trace is the preamp response from 88–108 MHz. The lower trace is with a barrel connector substituted for the preamp and power inserter. The vertical scale is 2 dB/div. I tweaked the input trimmer for equal gain at the band edges. Midband gain is 10 dB, which is just what I wanted. This is sufficient to swamp a tuner's input mismatch loss while minimizing RF overload.

The noise figure of a 3N204 isn't nearly as low as that for a modern semiconductor. Still, the preamp improved the 50-dB quieting sensitivity of a factory-aligned Sony XDR-F1HD by 2.5 dB. I measured the preamp's third-order input intercept as 125 dBf. This implies that 87-dBf input signals will generate 11-dBf spurs, and 90-dBf signals 20-dBf spurs. Since this range covers the 50-dB quieting level for the preamp with any tuner, the RF intermod spec of the combination will be 87-90 dBf, or 10 dB less than the tuner's spec alone, whichever is lower. The latter figure should dominate. RF intermod for my Technics ST-9030 is 98 dBf, while my other tuners measure in the mid- to high-80s.

This shows how the preamp lowers modulation-induced noise in a Sony XDR-F1HD. The audio spectrum goes to 20 kHz and the vertical scale is 10 dB/div. The upper trace is without the preamp. The 13-dBf monophonic signal, at approximately the tuner's 50-dB quieting level, is 100%-modulated with 1 kHz. Modulation greatly increases broadband noise for a signal this weak. The preamp's gain counters the effect. I equalized the tone levels to undo the tuner's soft muting. Although the dramatic difference in noise is easily audible in this test, I have yet to notice it on the air.

I built the preamp in a box that I salvaged from a defective RF slide switch (another garage sale find). 82Ω was as close as I could come to the 75Ω drain load. The response curve rose a couple MHz when I attached the press-fit box cover.

More is here.

Updated September 4, 2008