Hello All, I attempted to build VHF power amplifier (2M band) using MRF184 device. Not a great success, i got 60 watt output with 1 watt drive level on 24 volt supply. BUT it dropped to 25 watt after few seconds, looks like the mosfet is toasted. some has Drain-source DC resistance low (hundreds ohm), others has gate-source broke down. I burnt all mrf184 spares i have, 5-6 pcs . Need advise/comments what i have done wrong. thank you
Did you use an adequate heat sink and make sure the fet wasn't overheating? I'd suspect either too much gate bias or RF drive voltage. Something had to exceed the parts specifications. Variable power supply with a current meter is the bare essential for fixing it since you will need to ramp up voltage and monitor current and see where things get overheated, too much current, or failure. Scope helps too since parasitic oscillations can kill an RF amp from poor design but I know few of us have access to a 200MHz or faster one to visually see what's going on Might want to test all of the capacitors off the drain pin too if nothing else solved it. If you purchased the fets off of ebay there is always the chance you got fakes. Some of these scammers are wiping off numbers on cheap transistors and ICs and printing a more expensive part number on them and occasionally transistor fakes will work temporarily until it inevitably goes up in smoke.
Helo Kage, Yes, heatsink is good. I was also thinking that i over drive the amplifier (according to the datasheet, it requires 2 watt input to get 60 watt @960mhz) thats the reason I add 6 db pi attenuator network in circuit I checked parasitic oscillation with 1 Ghz Spectrum analyzer, I see nothing from 0 to 1 Ghz but i couldnt verify beyond 1 Ghz. DC bias is good I believe (200mA quiescent current, 3.5 A when RF input applied) You probably right i got fake transistor or MRF184 simply not good for lower frequency. It designed for UHF/sub Ghz freq band
Helo Kage, Yes, heatsink is good. I was also thinking that i over drive the amplifier (according to the datasheet, it requires 2 watt input to get 60 watt @960mhz) thats the reason I add 6 db pi attenuator network in circuit I checked parasitic oscillation with 1 Ghz Spectrum analyzer, I see nothing from 0 to 1 Ghz but i couldnt verify beyond 1 Ghz. DC bias is good I believe (200mA quiescent current, 3.5 A when RF input applied) You probably right i got fake transistor or MRF184 simply not good for lower frequency. It designed for UHF/sub Ghz freq band
regards
I suspect Kage is right about the fakes. I've personally had problems with the MRF186, but they could very well be doing the same thing with the 184. It is an odd device package however and I don't think there is much demand unlike the 186, so that goes against the argument, but I still think it's true.
Hello....i would suggest Always test your circuit in rfsim99 or ltspice to ensure it has the right characteristics and is unconditionally stable. You can download S parameters for all of these transistors and import them into rfsim99. Some will also have spice models. Generally, use rfsim99 to check small signal gain, frequency response, and stability; use ltspice to check for large signal (transient) response and linearity.
. Need advise/comments what i have done wrong.
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