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An inexpensive HB mosfet linear please?




 
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Author Topic: An inexpensive HB mosfet linear please?  (Read 1734 times)
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W2BX
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« on: May 23, 2020, 10:40:05 AM »

Has anyone had any success building a reliable, inexpensive homebrew mosfet linear amp?

I see there are Ldmos kits and assemblies available from various sources, not really inexpensive.. What about the mosfets you folks are using on class E transmitters?

Any comments, suggestions, would be greatly appreciated. Just starting to research this. 

Thanks

Glenn W2BX


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KD6VXI
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« Reply #1 on: May 23, 2020, 10:56:49 AM »

I built a Class E 500 Watt pep amp with the arf448.

I'm buying stuff to build a MRF300 based pair.  They have siamese parts for push pull and make 600 watts easy.

The dollars are spent in the output filtering as much as the amp. 

--Shane
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W2BX
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« Reply #2 on: May 23, 2020, 11:08:26 AM »

This looks very interesting...

https://qrpblog.com/product/a600-hf-vhf-600w-linear-amplifier-kit-v2-1/

I wonder how difficult it would be to combine two of these for 1200 watts?
(to provide enough headroom to be used as a 300w carrier AM linear)
I'm sure there are simple to build combiners around.
You would need 40 amps at 44 volts... not a problem.

Have no idea regarding IMD performance on these..  Roll Eyes


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M0VRF
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« Reply #3 on: May 23, 2020, 11:34:47 AM »

MOSFETs used in class "Generators" are switching and NOT in Linear service. Completely different animals!

Linear wise look no further than the BLF188XR.

JB.
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W2BX
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« Reply #4 on: May 29, 2020, 10:00:44 AM »

Sorry for the delayed reply....

Here's a circuit that uses a switching mosfet in a class A amplifier.



MOSFETs used in class "Generators" are switching and NOT in Linear service. Completely different animals!

Linear wise look no further than the BLF188XR.

JB.

* RFDRIVE.pdf (42.6 KB - downloaded 74 times.)
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KD6VXI
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« Reply #5 on: May 29, 2020, 12:02:14 PM »

This looks very interesting...

https://qrpblog.com/product/a600-hf-vhf-600w-linear-amplifier-kit-v2-1/

I wonder how difficult it would be to combine two of these for 1200 watts?
(to provide enough headroom to be used as a 300w carrier AM linear)
I'm sure there are simple to build combiners around.
You would need 40 amps at 44 volts... not a problem.

Have no idea regarding IMD performance on these..  Roll Eyes


That is sold as a kit on eBay.

Needs output filtering, as the kit doesn't have it.

Also available off eBay.

--Shane
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steve_qix
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« Reply #6 on: June 03, 2020, 10:30:42 PM »

At one point in the past, I built a 1500 watt + (PEP) linear amplifier using 6 FQA11N90 devices.  It worked VERY well.

This is a standard class B design with an output network - much like you would find in a tube linear amplifier.  Dirt simple design and worked quite well.  I did not measure the IMD or anything like that, nor did I ever receive any complaints.

May build another one some day.  The reason for the construction was to amplify a DX-60 that I had modified for high fidelity audio.  The MOSFET linear brought the 6 watts that I got out of the DX-60 (leaving MUCH headroom) up to about 250 watts (carrier) output.
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KD6VXI
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« Reply #7 on: June 03, 2020, 11:26:33 PM »

Steve,

Have a schematic?

--Shane
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W2BX
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« Reply #8 on: June 05, 2020, 06:35:31 AM »

At one point in the past, I built a 1500 watt + (PEP) linear amplifier using 6 FQA11N90 devices.  It worked VERY well.

This is a standard class B design with an output network - much like you would find in a tube linear amplifier.  Dirt simple design and worked quite well.  I did not measure the IMD or anything like that, nor did I ever receive any complaints.

May build another one some day.  The reason for the construction was to amplify a DX-60 that I had modified for high fidelity audio.  The MOSFET linear brought the 6 watts that I got out of the DX-60 (leaving MUCH headroom) up to about 250 watts (carrier) output.

Hi Steve and thanks very much for the reply.
I would very much like to have your schematic for that amp as that's exactly what I'm interested in building. I love dirt simple! Cool
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ki4nr
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« Reply #9 on: June 05, 2020, 08:00:44 AM »

That's just what I'm looking for too. I have rebuilt SB200 with fresh Cetron's for my SSB station and really don't want to use it on AM. I just need a simple amp using inexpensive Mosfet to give me 250 watts carrier and 1000 peak just for my FT-990 AM rig on 3.885.  That's the only place the amp with be used, well if it could be used on 40 meters that would be a plus.
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« Reply #10 on: June 05, 2020, 01:27:14 PM »

Another good application is as an IPA driver after a ricebox for a big AM plated modulated rig. IE, even if the 11N90s have IMD problems in linear service, when used as a carrier driver IMD problems will not show up.


Instead of abusing a ricebox for a dead carrier, the ricebox can put of a few watts and drive the 11N90 amp for 20-30 watts, or whatever.


I'd like to try one out too with a schematic. I have a (two device) 11N90 linear amp here but never measured it for IMD.  Even the expensive and popular MRF-150s are not all that clean. I tried the kit version and was disappointed. It's not easy to build a clean SS amp that is much better than -25 DB 3rd order without spending big bux on devices OR using pre-distortion technology..


T
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« Reply #11 on: June 23, 2020, 09:18:28 PM »

Unfortunately, I never had a schematic of the liner.  I just built it out of my head.

However, I remember some of what I did.

Each MOSFET had a .5 ohm 5 watt (metal film) resistor from the source to ground.  This was to balance the gain of the MOSFETs (6 in parallel).  Also provides degenerative feedback, which linearizes the devices.

The gates and drains were in parallel.

Drive power to the gates was fed through an RF transformer made from a single FB-43-1020 ferrite bead.  The gates required forward bias to bring the MOSFETs into conduction (barely).

The input was more or less broadband.

The output had a resonant circuit for tuning and loading.  I don't remember the particulars - it was nothing special.

I do remember it didn't take too much to put thing thing together.

The drain voltage was around 70VDC.

If I remember more I'll post it here... or if I ever build another one.
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W2BX
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« Reply #12 on: June 26, 2020, 08:11:20 AM »

Unfortunately, I never had a schematic of the liner.  I just built it out of my head.

However, I remember some of what I did.

Each MOSFET had a .5 ohm 5 watt (metal film) resistor from the source to ground.  This was to balance the gain of the MOSFETs (6 in parallel).  Also provides degenerative feedback, which linearizes the devices.

The gates and drains were in parallel.

Drive power to the gates was fed through an RF transformer made from a single FB-43-1020 ferrite bead.  The gates required forward bias to bring the MOSFETs into conduction (barely).

The input was more or less broadband.

The output had a resonant circuit for tuning and loading.  I don't remember the particulars - it was nothing special.

I do remember it didn't take too much to put thing thing together.

The drain voltage was around 70VDC.

If I remember more I'll post it here... or if I ever build another one.

Thanks again Steve.
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