Simple RF Preamp

I built this RF preamp from junkbox parts. The common-gate circuit provides an input return loss of > 18 dB from 88 to 108 MHz. The corresponding mismatch loss is < 0.07 dB. I could not find a recommended source impedance for lowest noise figure. I went for a good match instead since input mismatch loss directly degrades the noise figure.

The untuned input inductor, 25 turns on a T-25-7 core, provides a DC path. As long as its reactance is high, its value is noncritical. The output inductor is 15 turns on a T-37-10 core tapped 3 turns from the cold end. It resonates with the J309 drain/gate capacitance. I adjusted the turns spacing until the gain was about equal at the band edges. Output return loss is poor, but this lowers the midband gain peak and has almost no effect on noise figure. It does complicate measurement since the output impedance is nowhere near 75Ω. To avoid inconsistencies, I measured everything without reconnecting the preamp.

The high input return loss is due to the J309 transconductance, which is near 13 mmho. Invert that and you get an input impedance close to 75Ω. Transconductance and noise vary with current. With additional parts you could make the current adjustable to minimize overall noise figure. But the transistors are so cheap that it is simpler to just select one for best in-circuit performance. The J309 IDSS spec is 12–30 mA. My circuit drew 17 mA.

At midband the third-order input intercept was 132 dBf. I measured the following gain and noise figure values:

 Freq    Gain     NF
  MHz      dB     dB
 88.0    12.6    3.0
 96.9    14.8    3.1
107.7    12.2    3.8

I built an even simpler preamp to mount at the antenna. The output inductor forms an L-network with the J309 drain/gate capacitance. I used 16 turns on a T-37-10 core and spread them to adjust resonance. Gain was about 16 dB midband and 12 dB at the band edges. The input coil is a Wee-ductor. I found the Scientific Atlanta DA-PI power inserter at a garage sale. 220Ω for R1 yielded 13 V at the drain from a wall wart that delivered about 18 V with no load. Current was 20 mA.

It's easy to build your own preamp, but don't duplicate these designs. Instead, use a modern, low-noise device and follow the manufacturer's datasheet. If the gain is too high, you can attenuate the output without significantly degrading the noise figure.


December 28, 201188108 MHz