MRF101 - How to modify the reference circuits?

[SaaifK]

Hello everyone,

I am a junior engineer who is completely new to the world of RF. I am trying to use the MRF101 to replace an obsolete transistor in a circuit used for plasma generation. The current system works in the FM band, but instead of having a range of 87.5Mhz to 108Mhz, it uses the Japanese system which begins at 76Mhz.

I am trying to modify a reference circuit of the MRF101 in order to have it operate in the range from 76Mhz to 108Mhz. If anyone can give me a starting point or some tips on how to do this, I would really appreciate it. The MRF101 seems to have quite a large operating range, so I believe this should be possible.

If anyone has any ideas, please let me know!

[LPP · Accepted solution]

Sure, the NXP circuit can be tuned to extend frequency low bound. Current circuit has maximum input return loss at 87MHz. It can be shifted to 76MHz by modifying L1=36nH to L1=47nH.

Best Regards,

Pavel

NXP CAS

[SaaifK]

Thank you so much Pavel! I really appreciate it! I will definitely give this a try and report back how well it works. Does lowering the inductance further reduce the low frequency cutoff? Any insight you can give me on this will be immensely helpful!

[LPP]

The original design was optimized to provide flat gain curve over the target frequency range 87.5-108MHz. It was achieved by matching output load at the upper frequency bound and tuning input matching at lower frequency. See the simulated input/output return loss curve in these graphs:

I modified design to match input impedance at lower bound of your frequency range and left output matching unchanged:

Increasing (!) inductance will reduce frequency cutoff. This inductance compensates reactance of input capacitance of the transistor at the target frequency.

Note, further increasing the inductance will worsen gain flatness over the frequency band.

Regards,
Pavel
NXP CAS