Monday, July 24, 2017

BF2040W Amp prototype

A couple of TRB members have told me the ground plane is very important. This week end I was listening to a lecture from a man who owns a company that makes and sells electronic gadgets. He suggest a layered construction. The RF on top with a ground plane under it and the power connects under the ground plane. He has the boards made using 4 layer methods. While it is a great method it doesn't lend itself to a build and modify person like me. So I adopted a different approach which may work for a galoot like me. I cut a thin slice from the end of a 2X4 board. It is about 1/8". I drilled the four corner holes of the board and prepared to mount it.

I put copper tape on the bottom of the board to form a ground plane and a Vcc bus.

The black line helps me know where to find a ground or Vcc. I tinned four pads to take the chip.

Position the chip an apply a little heat to mount it to the board.

The method presented was to create the DC circuit first so i soldered a 100k and a 10k resistor to produce the voltage for biasing gate 2. A second 100k resistor biases gate1.

I put a capacitor on the input and output.

A bypass capacitor on gate 2 to prevent cross modulation.

Now for the test.  Dip meter feeds  a signal.

It is giving a gain and the signal looks good. I attach an 8 foot test lead for an antenna. I put my headphone on the output. Nothing. This is good. Put a diode and capacitor across the headphones and hear my local station. No signal without the diode tells me it is not oscillating, if it was I'd hear the beat.

This amp will produce audio output with my test lead antenna attached. The capacitor with full length leads is causing it to oscillate. The bare copper wires are ground and Vcc. I  wonder if it would be more tame with the layered construction? Another project for another day!
One inductor, three capacitors, 4 resistors and 1 BF2040W is all it took to build this one active device radio. Now all it needs is a tuning circuit. The voltage divider sets the gain so it could be replace with a pot and would act as a gain/volume control.

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