HF 1-30MHz linear amplifier

Posted on Mar 13, 2013

This project was a particular surprise for me in that the BC547 (equiv 2N2222) can be used to build a 500mW linear amplifer covering the entire HF band with excelent spectral purity and no neutralising at all. Ugly-bug construction was used but I dare say that the good results are partly to do with the method of construction. The circuit is fairly straight-forward and does not even need any form of RF neutralising. Two pairs of BC547 transistors are used in a push-pull type of output stage, biassed by a single diode and resistor. The driver is also very conventional using T1 to transform the drive impedance to a very low value for the output pairs.

HF 1-30MHz linear amplifier
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The amplifier is constructed on a piece of copper-clad board 45mm long by 17mm wide. Superglue a 44mm long by 3mm wide strip of copper-clad board along the center. This will become the battery supply rail. Using a sharp knife, remove some copper to form a 3mm x 3mm pad at one end of the battery rail to form the RF output terminal. Next fit the 10n and 33n decoupling capacitors; one pair at either end. These should lay flat on the board. The rest is easy after you see the photographs. T1 primary is 14-turns of very thin wire (0.1mm Dia.) and the secondary is 1+1 turn of thin wire (0.2mm Dia.). T1 former is two of the smallest ferrite beads I could find. You can just see it in the left-hand photograph above. T1 is composed of two grey ferrite beads. The right-hand photograph shows T2 and the mounting of the two output pairs of BC547 transistors. T2 is a little special. I found two small ferrite rings in the junk-box and decided to give them a try. The windings are 11-turns triflar wound using thin wire (0.2mm Dia.): Twist together three 1-metre lengths of thin enamelled wire. Wind 11-turns through the ferrite rings (1-turn is passed through both rings). Do NOT cut off the surplus yet. Identify A1-A2, B1-B2, C1-C2 using an ohm-meter. Thread each end of C1 and C2 back through the ferrite rings to add 2-1/2 extra turns to each end. Winding C should now have a total of 16-turns. Twist together A2 and B1 and connect to the...

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