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I think for ease, Series LC is the way to go, but how to deal with a very low impedance that the amp will see, maybe a series resistor could be used, we know the current the inductor see's is common for the resistor, capacitor and inductor.

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Hi Peter,

No need for using a series resistor if you meant it for easing the matching of the very low impedance, it can be transformed up by a matching network.

I attached the schema again with the addition of a possible matching network to transform a small (2-3 Ohm at resonance, indicated in the blue block) real impedance to the drain output circuit of the MOSFET. It is called an L matching network and needs a coil (L) and a capacitor (C) only. The C capacitor should include the drain-source output capacitance of the MOSFET plus a trimmer to make up for the needed value. For the coil a toroidal winding would be the best with ample core cross section but an air core one could also be used. See this link on calculating the L and C values:

https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html Source resistance 50 Ohm (this would be the up-transformed impedance the drain circuit will see as a load)

Load resistance 2 Ohm (this is either your "current amplified resonant" or a simple series LC resonant circuit)

For the Source and Load Reactances use zero

For the Desired Q use say 5

For Frequency use 45.25e6

The L and C values you need will appear under the

**LOWPASS Hi-Low MATCHING NETWORK** circuit schematic (first row on the right), I received L=34.46 nH and C=344.6 pF.

You can use a normal series LC resonant circuit within the blue block too, for simplicity, if you wish, instead of the current amplified resonant circuit.

The tuning procedure (to use first a normal series resonant LC as the load for the power amplifier output) would be advisable like this (fully separate the coil mentioned below from the circuit) :

1) fill in the iron powder for the coil assigned for exciting it. (you have already that plastic coil holder for this right?)

2) try to measure the inductance of this coil with the L meter. perhaps first without the iron powder, then with it.

3) then try to find a capacitor which when connected in parallel with this coil (that filled with the iron powder) gives a parallel resonance around 45 MHz, checked with your grid dip meter. IF the capacitor needed for this comes to be a very small value like under 10 pF, then reduce the number of turns of the coil. Beware: if the iron powder can move inside the coil, as you handle the coil holder with your hand, its inductance will surely change!

4) aim for a capacitor of at least 15-20 pF to give resonance around 45 MHz with the coil.

5) when done, you now have a simple series resonant LC circuit if you connect the capacitor in series with the coil, giving a very low value real impedance (any value like 1-3 Ohm) which will be stepped up towards 50 Ohm by the LC matching network.

Before you build and switch on this total circuit, I advise to go through the procedure first with the 50 Ohm dummy load connected into the drain circuit as I wrote in the previous post.

When that is done and seems ok, there may come placing the choke coil into the drain with the matching LC circuit as calculated from the link and the L coil is wound and the C cap value is considered with the output cap of the MOSFET (C

_{DSS}=55 pF at 25 V supply), and also the pretuned output series resonant circuit could be connected. Start with 12 V again first, only then raise the supply voltage higher and higher to 50 V. The main tuning may involve the trimmer cap adjustment across the drain-source for voltage maximum indicated by the scope probe with a sniffer coil. The L coil may also be tuned by pulling away the turns or pushing the turns closer slowly and carefully, it can greatly influence the up-transformed impedance towards the drain, hence the output power too. Notice if you increase this coil value, then the C cap would need to be decreased to keep the network at resonance (but the loaded Q is around 5 only, so bandwidth will surely be wide).

Gyula

ADDITION: here is shorter link to the ebay offer,

https://www.ebay.co.uk/itm/151274234687 you may wish to use it in your above post, so the horizontal width of the page will be restored to normal (for those who do not use wide screen).

By the way, the

**same** FM Linear Amplifier is available as a DIY Kit here:

https://dutchrfshop.nl/en/diy-kits-pcb-s/589-diy-kit-15-watt-amplifier-87-108mhz-rd01mus-rd15hvf1.html The schematics, the Bill of Materials are also included to see. All the L and C values used for matching and in the filters are valid for the FM band which is roughly twice as high as the 45 MHz needed here. Also, the output impedance is surely matched to a 50 Ohm load, as usual.

The kit does not include the coils but the wire for winding them...

By mentioning this kit, I do not hint to buy it... you decide.