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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

RF preamplifier for world-band receivers

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Synthesised world-band receivers are very popular but they do suffer generally from poor reception with the standard telescopic aerial. The radios are very sensitive by themselves but the aerial is far too short on the lower shortwave bands to supply any reasonable signal for good reception, the result being hiss and noise.

Wideband RF stages do not lend themselves to being connected to large antennas and usually suffer from overloading and cross modulation problems. The circuit described here combines modest gain with a tuned RF stage to help reduce out-of-band signals.

Inductor L1 and plastic dielectric variable capacitor VC1 comprise a tuned circuit, resonant to the band of interest. Dual-gate MOSFET Q1 then buffers the high impedance of the tuned circuit and also provides some gain which is limited by its unbypassed 470Ω source resistor. Transistor Q2 functions as an emitter follower to buffer the drain output of the MOSFET before it is fed into the antenna input of the radio.

Q3 functions as a switch, automatically turning the preamplifier on when the radio is activated. The Sony ICF7600GR, the radio for which this circuit was developed, has approximately 3.5V present at the external antenna input when switched on. This voltage will turn Q3 fully on via its 10kΩ bias resistor, thereby connecting the 9V internal supply of the preamp.

The overall gain is adjusted by VR1, allowing approximately +6dB gain when wound fully clockwise. When rotated fully anticlockwise, VR1 will attenuate the signal by around the same amount, which is very useful with strong signals.

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