The KA7OEI FT-817 RF Power Amplifier


Posted on Feb 5, 2014

The 2SK2975 was reportedly been discontinued by the manufacturer and the replacement part was the RD07MVS1 manufactured by Mitsubishi. This device is very similar - but it seems to have slightly higher gain than the `2975 (10 db typ. at 520 MHz versus 8. 4 db at 450 MHz. ) Note that this newer device is not a drop-in replacement and it requires some changes to associated components.


The KA7OEI FT-817 RF Power Amplifier
Click here to download the full size of the above Circuit.

While a modification of the original board is possible, it is often more cost-effective to simply replace the entire board. (A data sheet for the newer device may be seen here. ) Since first revision of the circuit, other changes were made to the PA board but reports indicate that the newest PA board can be installed in the oldest FT-817 with good results. Taking a look at the schematic, one can see that the power amplifier is optimized for VHF/UHF operation and has been "de-optimized" somewhat (in terms of gain) for HF/6 meter operation. Now, before one gets worried about this, one must realize that owing to the way the physics works, if you make a broadband RF amplifier with plenty of gain at VHF/UHF, it will have way too much gain at lower frequencies. Because of this, degenerative feedback is employed to reduce the gain at HF and 6 meter frequencies to a "reasonable" level - otherwise the gain would go to astronomically high values (making for an unstable circuit) at the lower frequencies. How much power is the amplifier capable of It all depends on what you are looking for. At 70cm, the RF amplifier chain as a whole isn`t capable of too much more power than its ratings - 7 or 8 watts maximum at 15 volts is the most one could expect and have reasonable linearity on SSB. Most likely this is due to a lack of excess gain in RF stages prior to the final amplifier section At HF the story is different: There is enough gain...




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