Low noise preamplifier OZ2OE 21 – 30 MHz

The author of the following connection is Ole OZ2OE. You can find the original description in English at http://hjem.get2net.dk/ole_nykjaer/oz2oe/hfpreamp/hfpreamp.html. we thank you!

Preamplifier I broadcast on VHF “deaf” HF receiver. Suitable when you need extra sensitivity to 15, 12 a 10 m – for example due to satellites.


This simple little preamplifier uses the advantages of the MMIC MAR-6 monolithic amplifier from MiniCircuits. It achieves a gain of approximately 20dB with a noise figure of 3dB in the band 21 – 30 MHz. Selectivity is determined by the band pass at the input.

Mechanical design

The preamplifier is constructed on a double-sided printed circuit board with dimensions of 32×60 mm. The copper surface serves as the ground and all the components are in the air. The circuit board is screwed into a small metal box terminated with BNC connectors.


The diagram is below. The input consists of a non-tunable bandpass made of conventional components and two coils.
The bandpass is designed for impedance 50 ohms and the output impedance is directly at the junction of the two tuned circuits.
The monolithic amplifier is powered from +12V through a collision resistor 560 ohms. The output is broadband.

L1, L3: 6,5 thread with a copper wire with a diameter of 0.5 mm on the diameter 4,5 mm
L2: 1,5 uH RFC


The parts are mounted like this, as the picture shows. The entrance is on the left.


(There are also two parts in the picture, which you will not find on the diagram. They are 4.7uH RFC and 3.3uF, which are used according to the catalog connection in the power supply. However – 560 ohmic resistor works great, but the choke was still connected to the PCB.)



It is not necessary. The amplifier is broadband. I just measured the current consumption - 16 mA is fine.


The properties of this preamplifier are mainly determined by the properties of the monolithic amplifier. The catalog data MAR-6 refers to the noise figure 3 dB and IP 3rd order +14,5 dBm. The bandpass reduces the overall gain as can be seen in the figure.


It's very important, that the gain drops significantly at lower frequencies, otherwise, strong signals from frequencies below 15 MHz could penetrate the amplifier and create interfering products in the bands on the nonlinearities 21 – 30 MHz.
The bandpass has a slight ripple at lower frequencies.

Possible changes

The preamplifier can also be equipped with another monolithic circuit. They are suitable for preamplifiers with medium gain (15 – 25 dB), low noise and IP 3rd order at least +10 dBm. Pay attention to the different housings and the layout of the terminals.

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