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Author Topic: 100W Linear PA for the FM Band  (Read 480 times)

Offline VU2TVE

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100W Linear PA for the FM Band
« on: October 09, 2020, 0737 UTC »
Hi folks. This is my very first post to this forum. I am a radio HAM based in the mountains in North India.

I am working on a project that involves using a SDR Radio to output multiple FM signals, and researching the power stage part of the design.

I need about 1W average per signal, and there may be upto 10 carriers, so 10W average Power. So this probably means an amplifier capable of upto 100W of output power. It also needs to be a linear amplifier.

I was wondering if anyone had pointers where I can start looking for designs I can build or purchase. I came across these reference designs based on the MRF101AN and wonder if they can do the job
https://www.nxp.com/products/rf/rf-power/rf-ism-and-broadcast/1-600-mhz-broadcast-and-ism/mrf101an-reference-circuits:MRF101AN-REFCirc

Thoughts?

73 // VU2TVE

Offline redhat

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Re: 100W Linear PA for the FM Band
« Reply #1 on: October 09, 2020, 1754 UTC »
Although the datasheet specifies linear operation, there is no recommended operating conditions for such, and none of the reference designs are specified for linear operation either.  The problem with doing something like this is the intermodulation products can become very large.  While this may not be a problem for the levels you are proposing, it still can cause a lot of interference to other band users.  I assume the idea here is to broadcast multiple stations across the dial with one transmitter?  I would probably look at a larger device, perhaps the MRF300AN, possibly in push pull.

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Offline VU2TVE

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Re: 100W Linear PA for the FM Band
« Reply #2 on: October 10, 2020, 0146 UTC »
Thanks for the reply. At this point, given the scope and place where this will be deployed, interference to other users won't be an issue, I have checked that.

What characteristics do you think their reference circuit has ..
https://www.nxp.com/products/rf/rf-power/rf-ism-and-broadcast/1-600-mhz-broadcast-and-ism/mrf101an-reference-circuits:MRF101AN-REFCirc

I see a place for Vgs bias, so perhaps there is some control over linearity?

Or maybe I should hunt for push-pull designs as you say (ClassA/ClassAB), but I would rather begin with experimenting with off the shelf available components given my limited RF design experience.

Any more pointers would be quite helpful!

Offline ThaDood

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Re: 100W Linear PA for the FM Band?
« Reply #3 on: October 10, 2020, 1716 UTC »
I certainly can see the VHF-High version as the perfect 2M FM HT amp. Duty cycle would be right for two-way, than full bore for broadcasting.
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline VU2TVE

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Re: 100W Linear PA for the FM Band
« Reply #4 on: October 11, 2020, 0405 UTC »
Could you point me to a link where such an amp is being sold. Most of what I can see online is in the 2m band. Thanks :)

Offline redhat

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Re: 100W Linear PA for the FM Band
« Reply #5 on: October 15, 2020, 2228 UTC »
Sorry for the delay, I'm on the road this week.
 Finding a commercial amp in that frequency range and power level is one thing, something rated or capable of linear operation is near impossible.  You could try a conventional class B/C amp and turn up the bias and see what happens.  I looked at some of the TV fets, and they recommend somewhere around 1.5-2A per fet.  With 50V of drain, that's about 200W of static dissipation!  With a push pull circuit, I'd try between 0.5-1A of bias (1-2A total) and see what the intermod looks like.  I'm not aware of any COTS amplifiers for this level, although a SD2942 pallet would be a good place to start.  Not cheap, about $400 US, but I do know that particular fet is used in Digital FM band transmitters in class AB linear.

+-RH
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WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
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Offline VU2TVE

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Re: 100W Linear PA for the FM Band
« Reply #6 on: October 16, 2020, 0353 UTC »
Thanks for the reply, and no worries on the delay. I am grateful for this conversation. I think, to begin experimenting, I'll tone down my expectations. Instead of 10 carriers at 1W, lets say there are 6 carriers at 0.5W, so a total avg power of 3W, and a peak power of 9W.

I'll begin by looking at 10W class A designs, and then raising expectations from there. This would allow be to perhaps build a circuit of two of my own without spending many many $$$, and learn a thing or two about RF circuit design.

Offline Stretchyman

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Re: 100W Linear PA for the FM Band
« Reply #7 on: October 17, 2020, 1228 UTC »
Plenty on ebay, $20 for a kit WO FET.

Str.
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Offline VU2TVE

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Re: 100W Linear PA for the FM Band
« Reply #8 on: October 23, 2020, 0947 UTC »
Could you point to one or two which you know are class A/AB/B. I guess the bias on Class AB/B could be tweaked to run it in Class A.

Thanks!! :)

Offline Stretchyman

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Re: 100W Linear PA for the FM Band
« Reply #9 on: October 23, 2020, 1631 UTC »
They are all class C, forget class A as it would be massive, AB is fine. Your going to have to do some work and design your own. The kits will be a good starting point, just alter the bias and fit a Massive heatsink.

Str.
« Last Edit: October 23, 2020, 1732 UTC by Stretchyman »
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)