VHF preamplifier with BFY91

Posted on Jun 12, 2012

When the start switch is pushed, the output of the charger goes to 14.5 V. As the batttery approaches full charge, the charging current decreases and the output voltage is reduced form 14.5V to about 12.5V, terminating the charging process. Transistor Q1 then lights the led as a visual indication of a full charge. The actual preamplifier in this circuit is a practical wideband amplfier that gives about 15 - 20dB of gain from 1.8 MHz through to well over 200 MHz. You will no-doubt replace this circuit for your own favorite amplifier for the band you are interested in. The 2.2mH chokes should have an absolute minimum reactance of 5000 ohms at the lowest frequency to be used. For 145 MHz these may simply consist of a few turns of wire. I personally used a 10-turn 4mm Dia. coil in series with a 2.2mH torroidal inductor.

VHF preamplifier  with BFY91
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VHF preamplifier  with BFY91 - image 1
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The relay should have a coil resistance in excess of 1000 ohms and have two-pole changeover contacts. When I built this circuit I used two 5v PCB mounting relays, each having a single pole changeover contact. The relays each had a 560 ohm coil. If the DC power is switched OFF the preamplifier will become inactive and the relay will pypass the amplifier. In this manner it may be used with a low-power transmitter (typically up to 10 watts). My first switch was built inside the balun housing of a 2-metre beam antenna. Note that the DC 12 volts should NEVER be applied whilst transmitting or the poor little BFY91 will be blown to bits. There is also a short delay between removing the DC and the preamp dropping out into a safe condition for transmitting. I should have used a pair of back-to-back diodes across the input to the preamp (before the 100p) to protect the preamp, but I got away with it for a couple of years without a problem.

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