Garage Sale Preamps

At garage sales I find wideband cable-TV amplifiers like this one. They work fine as a preamp for less sensitive FM tuners. The internal power supply with captive line cord is convenient, although it precludes mounting the amplifier outdoors at the antenna. This particular amplifier had 13.7 dB of gain in the FM band and a surprisingly high third-order input intercept (IIP3) of 135 dBf. Saturated output was 136 dBf (40 mW). Return loss was 14.5 dB input and 12 dB output. The noise figure was 4.8 dB. This is not a low value, but the amplifier still improved the 50 dB quieting sensitivity of an aligned Technics ST-9030 from 18.1 to 15.4 dBf.

I traced out this circuit. The emitter voltage is −12 V and total current is 15 mA. Rated power consumption is 2 W. The feedback improves linearity but degrades the noise figure. Breaking the feedback loop increased the gain 5 dB and decreased the noise figure 1.4 dB in the FM band. I didn't check any other parameters, including stability. I didn't try bypassing the emitter degeneration.

A similar amplifier with four outputs had 7.0 dB of gain. The circuit was the same as that above minus the input diodes and with tiny ferrite transformers to divide the output four ways. In this unit the transistor was a 2SC3777. IIP3 was 134 dBf. The power division degrades the noise figure just 0.2 dB.

This amplifier has adjustable gain. In the FM band I measured 18.5 to 29.5 dB. Unless you're feeding hundreds of feet of lossy coax, this is too much gain for all but the most bulletproof of tuners in a benign signal environment. In addition, reducing the gain to minimum degraded the noise figure so much that preamp+tuner sensitivity dropped below that of the tuner alone.

June 16, 201488108 MHz