Low noise preamplifier on 144 MHz OZ2OE
Introduction:
Maybe someone will ask a question: why preamplifier with dvjbase MOSFET? It is of course possible to choose from several connections with GaAs FETs, but for 2m tropo operation there is no noticeable difference, while the MOSFET is much more durable – and cheaper!
Mechanical construction:
The preamplifier is constructed on a double-sided printed circuit board with dimensions 25 x 50 mm. The upper side serves as the ground. The mounted printed circuit board is in a small metal box terminated with a BNC connector.
It can be seen from the diagram, that the input circuit forms a series tuned circuit, outgoing “normal” parallel with capacitive coupling. Because the preamplifier has too much gain, there is an attenuation element at the output. The bias voltage for the transistor is realized by voltage dividers in both G1 and G2. In this way, it is possible to precisely set the DC parameters of the preamplifier.
L2, L3: 5,25 thread with a Cu wire with a diameter 0,25 mm on Neosid K3312 body, F100 a pod.
Connection details:
The input part is solved rather unconventionally with series inductance L2, which achieves impedance conversion from 50 ohm and asi 1600 ohm, which is the optimal impedance for noise matching. L2 resonates with C4 and G1 transistor capacitance.
This method minimizes the loaded Q of the circuit - important for low losses - and the correct impedance (noisy) adaptation.
The output parallel circuit consists of L3, C6 a C8. Capacitance divider C6 – C8 sets the impedance to approx 2 kohm-y. This also affects the profit.
Reviving begins by first checking the drawn current, which should be about 20 mA. After that, you only need to set L2 and L3 to the maximum signal.
The most advantageous is a weak unmodulated signal directly from the band, which we bring to the receiver in FM mode. Set L2 to the greatest noise reduction.
With an FM signal at noise level, the receiver's FM detector will amplify any improvement in the signal-to-noise ratio, that is, your own ears are sufficient for optimal tuning! Any change in preamplifier gain is eliminated by the FM receiver's limiter.
Attenuator:
You can choose it according to your requirements - which should not be “more than necessary”. The following table offers several options:
I've built and measured a number of these preamps and most had identical characteristics. Typical characteristics with a –3 dB attenuator are in the graph:
Circuit board and components:
When I manage to convert the printed circuit diagram into a suitable format, I will place it here. Until then, the best I can offer is a scan of the original. if you are interested, contact me.
I can also help with the mounting of the printed circuit board and special bones from Neosid.
Ole, OZ2OE
