Lubomir73 wrote: ↑Sat Apr 27, 2019 8:56 pm
Hello all. This is my first post as you can see... so go easy on me. I found this forum a couple days ago by searching for any suggestions on MRF9180/gate/protection. I know I'm a little late on this subject but I believe there is something else that needs consideration. A couple of months ago, I started a project using an MRF9180 to replace a MRF186 in the UHF version of this Chinese power amp board. After pondering the spec's and costs for various FET's, I realized that there is one huge, glaring difference between the MRF9180 and the MRF186 and many other devices... it's inability to tolerate negative gate voltage swing. Maximum Vgs for the MRF186 is -20/+20 volts. Maximum Vgs for the MRF9180 is -0.5 to +15 volts. That's a maximum of only minus one-half volt! Otherwise, Poof! The 9180 can't take a negative swing of the same magnitude as the positive swing needed to drive it in a Class B or AB configuration.
The diagrams of some FET's show a schottky diode drawn in the transistor case. I believe the 9180 lacks this gate diode and that maybe if you put one in at the gate connection, the input will be resistant to overdriving or as one commenter put it, "Breathing on it wrong." I also suspect that possibly Motorola, NXP Freescale and others, may have sneaked a schottky into their designs to make them more resilient. I would have done so. If, (and that's a Big IF) my assumption is correct you can bulletproof the 9180's input easily, by adding a diode.
I am still looking, but so far I think I have found a schottky barrier diode for this job. Panasonic DB2J50100L. Only 1.6 ns from reverse to forward conduction...suitable for UHF. Forward voltage (Vf) under 0.5 V, reverse voltage (Vr) of +50 V, and forward current max. 200 ma. Also a capacitance when reverse biased that is only 6 pf at a Vr of around +3 volts. You can buy them through Mouser, DigiKey or Newark for 10 cents a piece or less. For use at 440 mhz, I may need to reduce the FET's input capacitance to accommodate this diode. But for use at around 100 mhz, 6 pf may not be a big factor.
All kinda makes sense when I read an earlier post that he has to keep the drive less than 4 watts. If Pwr still equals E²/ R, then driving an input impedance of say 5 ohms with 4 watts gives an RMS voltage of 4.5 volts RF, with peaks at 6 volts. Take a volt or two away for input matching loss and you have about 4 volt peaks. Combining the RF with +3 volts gate bias gives you peaks of plus 9 volts and minus 1 volt going to the FET gates. I may be wrong, but I think that right there... that negative voltage swing is part of the reason why the 9180 FET's are dying so easily.
Also, remember the diode turns on when the input goes negative, and is reverse biased when the FET is forward biased. So the cathode connects to the FET's gate. I apologize for not having any proof of this concept. Anyone want to try this out? I am still working and lack the time and equipment. Any other ideas? All suggestions are welcome. Who's perfect? Not me!