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Author Topic: 10W class driver, what fet?  (Read 5765 times)
M0HCN
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Posts: 473




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« Reply #15 on: September 15, 2012, 03:55:34 AM »

Good call,
I didn't think they were still available.

This will be an AB stage, I am not running the thick end of 50W of heat for a few watts of RF.

Regards, Dan.
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KE5JPP
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Posts: 0




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« Reply #16 on: September 15, 2012, 06:50:09 AM »

Good call,
I didn't think they were still available.

This will be an AB stage, I am not running the thick end of 50W of heat for a few watts of RF.

Regards, Dan.

So you think VRF148s are overkill but you will consider a BLF175?  Hmm.

Gene
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KE5JPP
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« Reply #17 on: September 15, 2012, 06:54:26 AM »

Quote
Do you know what an ERF2030 is?  It is a remarked IRF510 FET.
Quote
He is not building a 10-4 Good Buddy CB amp!  Those "ERF2030" FETs are run in class C EVEN in SSB on those CBs - that is why they splatter like Hell!  No one who wants to build a quality linear amplifier would use those switching type FETs!  Good Grief!

IRF-510's are used as PA devices on %80
of the QRP kits on the market. Usually in push-pull,
sometimes individually. Not suitable for this op's
project, but "splatter like hell"?
I've never had a problem.
Depends on the circuit and how they're biased.
Just a thought.....
73, Ken  AD6KA

IRF510s are just OK for cheap QRP rigs, but you certainly would not want to use them as a driver for a larger amp.  No matter how clean the larger amp is, the IRF510s would set the limit.  The only reason anyone uses them are because they are cheap.

Gene
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TANAKASAN
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Posts: 933




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« Reply #18 on: September 15, 2012, 09:56:37 AM »

Application note # NCO8705 shows a nice 10W power amplifier that has IMD at about -42 dB with 28 dB of gain using the BLF175:

www.nxp.com/documents/application_note/NCO8705.pdf

How would I alter the feedback components to give 10 dB of gain rather than 28 dB?

Tanakasan
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AD6KA
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Posts: 2237




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« Reply #19 on: September 15, 2012, 11:19:10 AM »

Quote
IRF510s are just OK for cheap QRP rigs, but you certainly would not want to use them as a driver for a larger amp.  No matter how clean the larger amp is, the IRF510s would set the limit.  The only reason anyone uses them are because they are cheap.

Which is precisely what I said:
"Not suitable for this op's project"
73 de AD6KA
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KE5JPP
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Posts: 0




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« Reply #20 on: September 15, 2012, 01:24:20 PM »

Quote
IRF510s are just OK for cheap QRP rigs, but you certainly would not want to use them as a driver for a larger amp.  No matter how clean the larger amp is, the IRF510s would set the limit.  The only reason anyone uses them are because they are cheap.

Which is precisely what I said:
"Not suitable for this op's project"
73 de AD6KA

You need to read more  before you jump in and get defensive.  I said that the Erf2030 replacements for cb use run the fet in class c mode even on ssb.  That is why they splatter like Hell.  The erf2030 is basically a remarked irf510.  You inorrectly assumed i was talking about their use in qrp rigs.

Gene
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KE5JPP
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« Reply #21 on: September 15, 2012, 01:26:58 PM »

Application note # NCO8705 shows a nice 10W power amplifier that has IMD at about -42 dB with 28 dB of gain using the BLF175:

www.nxp.com/documents/application_note/NCO8705.pdf

How would I alter the feedback components to give 10 dB of gain rather than 28 dB?

Tanakasan

Since the source is isolated from the flange, you can use source degeneration to decrease the gain.  This will increase linearity too.

Gene
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M0HCN
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Posts: 473




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« Reply #22 on: September 16, 2012, 03:00:33 AM »

So you think VRF148s are overkill but you will consider a BLF175?  Hmm.
They are both total overkill, at least for the architecture I originally had planned, but there seems to be little that is smaller and otherwise acceptable.
Moving the power splitter down the chain to just before the finals (My original design had the power splitter before the drivers) would mean that I would need maybe 15W of drive output, so while a pair is still massive overkill, it is at least a little less massive.
 
The two parts actually appear to be very similar, and would probably both work in the same circuit.

And, yea source degeneration is being used in both stages (It also helps stabilise the bias point), probably with some feedback from the DC injection bifilar as well in the usual manner.

Interesting: Just compared prices and digikey actually want more money for the VRF148 then they do for the VRF151.

Regards, Dan.
« Last Edit: September 16, 2012, 03:26:58 AM by M0HCN » Logged
TANAKASAN
Member

Posts: 933




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« Reply #23 on: September 16, 2012, 04:37:44 AM »

Dan, do you know of any European distributors of RF power transistors?

Tanakasan
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M0HCN
Member

Posts: 473




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« Reply #24 on: September 16, 2012, 05:12:16 AM »

Farnell have a few, as do RS & Barend Hendrikson (who has now mostly retired) still has some stock.

www.rfmicrowve.it have a selection (And also do ATC caps), and there are one or two other specialist places, but other then that you are looking at full up 'distributors' rather then small quantity retail, and my budget does not stretch to ordering full reels!

For many things, mouser/digikey (Or even Richardson!) can actually be cheaper then using the locals.....

Regards, Dan.
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N3QE
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Posts: 2163




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« Reply #25 on: September 16, 2012, 06:48:52 AM »

Ok, bit of a headscratcher here, given a power stage that needs about 5W or so of drive, HF + 6M, available supplies are 12 & 50V and I would like a push pull driver stage, what would you use?

RD15HVF1 is readily available and obvious, but ideally there would be something that could run off the main 50V rail rather then needing the 12V rail as an additional supply.

50V rail and 5W out implies a collector or drain impedance of 500 ohms or so. Not unachievable, but running against the modern grain of the low impedances that work nicely over HF range with broadband transformers. What is the next stage's input impedance? Is it the grid of a kW class tetrode? 500 ohm impedance might be about right for that!!!

A small 50V to 12V step down at several amps can be done with a "simple switcher" e.g. LM5576 if you need to.
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M0HCN
Member

Posts: 473




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« Reply #26 on: September 16, 2012, 07:15:21 AM »

I figured that 9:1 would get me to somewhere in the 50 Ohm ballpark, then 4:1 or so for the gates of the next stage.

Yea, maybe a 12V part is a better plan, just trying to avoid the extra rail, and the RD15s are cheap enough, although I like the source degeneration thing that you cannot easily do with those parts, ho hum decisions decisions....

Possibly something like a pair of RD15 as drivers on each output board, which should get me ~30dB gain overall for 250W output from a quarter watt in. Assume 15dB per stage makes first stage output a nads under 10W PEP, which should be clean enough with a pair of RD15 @12V into somewhere around 6 ohms, not a million miles from the sort of impedance presented by the power stage gates.

This is  for an experimental DUC/DDC rig using the usual FPGA and fast AD/DA converter for the signal generation and detection, with the AD being used to close the loop on transmit.

The bench lashup works, I am trying to turn it into a publishable project with parts that are actually available (As opposed to junkbox specials).

Regards, Dan.
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