250W FM planar pallet amplifier

Description

Since single-ended designs with MRF300 transistors don’t perform well across entire FM band we designed this innovative planar 250W amplifier that does much better. It also has temperature compensation which the previous version did not. Designed for FM radio transmitters, this amplifier uses the new inexpensive plastic easy to replace NXP LDMOS to produce a maximum of 250-300W! In a design where easy serviceability was top priority the cost of this product remains very reasonable. Finals are very easy to replace. Please keep in mind that more expensive ceramic devices are still more efficient and more robust than low cost plastic. Taking into account filter losses we recommend this pallet for 200W transmitters, not more. It is very easy to replace the output transistor in this amplifier.

Technical specifications:
– Frequency band: FM band 87.5MHz to 108MHz,
– Pout max: >270W @ 99MHz (@50V)
– Pout typ: 250W
– Drive power required: 3-4W
– Input reflectivity: -20dB
– Requires 28-50Vdc (requires 48-50V for full power)
– No bias adjustments or RF circuit tuning is required
– Input and output impedance: 50 ohms
– Size: 50x82mm, 15-20mm high
– Aluminum base is included

You can find suitable filters here.
Suitable mains power supply are here.
A heatsink used for this pallet can be used as well, scroll to the bottom.

Warning: Do not exceed maximum supply voltage of 50V or damage will occur.

Note 1: Please use PTFE coaxial cable for RF input  and output (RG178 is perfect) connections, this makes it possible to have very short leads without burning the coax. Keep the coax leads as short as possible (few mm at most). Also keep leads short on connectors, this  N female flange for RG142 or .141 semirigid – easy install is perfect!

Note 2: A heatsink is definitely needed!

Note 3: Ensure output transistor is fully in contact with heatsink. It must be completely flat and when purchased without heatsink you must install the pallet and re-heat the three pads on the transistors to make it fully ease down to the heatsink. After that re-tighten the transistor. Repeat, if necessary until transistor is completely flat on heatsink. Poor contact here will cause wild oscillations which will damage the output transistor.