Single chip amplifier

A single chip active broadband magnetic loop HF receiving antenna.

When you like to try such a loop the easy way, you can use the new AD8099 chip (abt $3) from Analog Devices. The AD8099 is an ultra low distortion low noise high speed opamp. With this chip one can make an acceptable broadband active magnetic loop for shortwave reception. Instead of the broadband amplifier (see link above) you can try this opamp.

The schematics looks a lot simpler, it takes also less effort to build.

Schematics of the single chip amplifier (57kB)

Picture of the single chip amplifier (353kB)

However it still needs VHF building experience. The fat lines in the schematics have to be as short as possible. Surface mount devices (especially the capacitors) are preferred. See also the datasheet of the AD8099 on the site of Analog Devices. The trimmer capacitor (10pF) is used to disable oscillations. The amplifier has to be stable for all powersupply voltages. When changing the supply voltage from 5V to 12V the amplifier must stay stable.

Of course one can see the oscillation on the oscilloscope, but because this will be a VHF oscillation, you will need a high bandwidth scoop. Sometimes there is also a much lower frequency oscillation (1MHz hiccup). When you have a strong noise signal (S9+) on the receiver, you also know that the amplifier oscillates.

Noise behaviour.

Both amplifiers give about the same noise results.

Large signal behaviour

The AD8099 is not as good as the broadband amplifier (see link above). In the evening several whistles can be heard, but during the rest of the day its large signal behaviour is very acceptable. The AD8099 is not very useful on the long and medium wave. When you want to receive the medium and the long wave also, you have to go for the broadband amplifier (see link above).

Large signal behaviour can be improved by making Rgain 4.7k. The 6dB lower output signal is not a problem for most receivers, may be even an advantage.

Last updated: September 24, 2006