Choosing a MOSFET for class-A linear operation - Page 1
Forum section: electronics / rf-microwave

author: vu2tve
author_rank: Contributor
timestamp: March 16, 2022, 10:15:50 am
content: Hi,I've posted before on this forum concerning a class-A R.F. PA that I've been building for a multi-carrier FM broadcast project. The current status of the design is that I'm using a NXP MRF300AN for my design, as it has a lower case-to-junction thermal resistance than my previous choice of MRF101AN (0.55 C/W as opposed to 1.1C/W).Now, the design is performing reasonably well at this point, but I'm exploring options for pushing the power envelope.Because this is class-A, thermal management is one of the prime concerns. I was exploring some NXP transistors and found this device.MRFE6VP5150N - 150W CW, 1.8-600MHz.https://www.nxp.com/products/radio-frequency/rf-industrial-scientific-and-medical/1-600-mhz-broadcast-and-ism/1-8-600-mhz-150-w-cw-50-v-wideband-rf-power-ldmos-transistors:MRFE6VP5150N?tab=Design_Support_TabWhile it's max rated power of 150W is lower than the MRF300AN's 300W it has much better thermal specs. Case to junction thermal resistance is 0.21 C/W (as opposed to 0.55 C/W) and max operating junction temperature is 225C as opposed to 175C.However, since there are two MOSFETS on one die, this chip was designed with class-AB operation in mind, it seems.I was wondering what if I just parallel up the two transistors and run them both in the linear region. Thoughts? Has anyone have experience with using these transistors in the linear region?Thank you!

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2022, 10:53:50 am
content: Typical power FETs are specifically for SMPSand poor performance linearity in linear application especially subject matter UHF parasitic oscillation.BJT may be a better choiceJon

author: vu2tve
author_rank: Contributor
timestamp: March 16, 2022, 04:07:38 pm
content: If you refer the link in the original post, you'll see these are specifically RF LVDMOS technology MOSFETS.

author: RoV
author_rank: Regular Contributor
timestamp: March 16, 2022, 04:41:04 pm
content: Quote from: vu2tve on March 16, 2022, 10:15:50 amYou may have difficulty at stabilizing the operating point (two devices, one temperature...). I also suspect you'll have to apply some derating, because the individual cells in each MOSFET may have some degree of umbalance (my opinion only).But why don't you consider a push-pull in class AB? Distortion would be more or less the same, but with a much lower power consumption for the same PEP power if you have several FM carriers.

author: vu2tve
author_rank: Contributor
timestamp: March 16, 2022, 05:12:39 pm
content: I do want to consider class-AB push pull as well, but because it's a multicarrier setup, having class-A linear operation reduces the in-band imd. Atleast that's the theory. The current MRF300AN is working quite well in linear region. Efficiency is not really a concern at this point, but thermal management definitely is a challenge that would be much easier to manage in class-AB.Perhaps I'm too afraidbut class-A feels like a safer bet for this application, and once I have one design nailed down, then go and explore class-AB.I do also have a MRF101AN and MRF101BN lying around, so could in theory try a design. Need to search for reference designs for class-AB, and learn about matching baluns