The Inexpensive, simple, MOSFET A.M. linear project

[steve_qix]

This project arose from the need for a good, AM linear amplifier that doesn't require a mortgage to acquire.  Furthermore, MOSFETs do not typically wear out, so the ongoing maintenance cost of high power vacuum tubes is removed.

This amplifier is designed for the "bell curve" of AM operation.  That being said, the amplifier basic design will work on 160 thru 40 meters.  If you want anything outside of this range, do reach into your pocket and pull out your credit card  

The input circuit is, at this time, broad band.  Hopefully this will be able to remain.  The output circuit is a tuned circuit.  This allows the operator to adjust the operational parameters, and also to match the amplifier to imperfect loads (like coax fed dipoles, etc.).

The amplifier, as built, uses 12 FQA11N90 MOSFETs.  These cost $3.00 each.

The heat sink is an old Motorola heat sink that came from some sort of VHF repeater, I believe.  Other heat sinks may be used.

Operational parameters:

80V @ 7A to 8A with 5 W. (carrier) drive will produce 200W of carrier output.  Modulation capability with these operational parameters is 160% positive modulation.  Current depends on the setting of the forward bias (see schematic).  At 7A, amplifier is running in class B-Minus.

The amplifier will put out 300 watts of carrier at approximately 125% positive modulation.

The operational class is class B to class B- (B-minus).  With class B, there is some resting current.  With class B-Minus,  there is no resting current with no drive.  This class of operation (B-Minus) is NOT suitable for SSB.  

If you want to use this amplifier for SSB, the resting current must be greater than zero !!  That is simple matter of adjusting the (forward) bias voltage.  With AM, operating in this class will shift the overall modulation percentage slightly, meaning you will hit 100% negative modulation a few percent before the driver reaches true 100% negative modulation.  As a practical matter, this is not a problem, particularly if your transmitter uses a negative peak limiter.  You can, of course, increase the forward bias slightly, such that there is a zero-drive resting current.  There will be a small loss of efficiency in this case.

Harmonics are down almost 60Dbm from the fundamental frequency (a display of the output waveform and an FFT of the output waveform is shown).

Here are the schematic and pictures.  Pictures shown are with the amplifier operating in class B.  Class B-Minus, the current dropped to 7A, with the same output and modulation.

The Schematic:


Overall Picture of the amplifier:


Top View of the amplifier


Meters Closeup (operating):


The output waveform and the FFT of that waveform.  FFT:  10dbm per vertical division, 5mHz horizontal


Actual Modulation produced by the above operational conditions:

[steve_qix]

When the amplifier is set up in Class B-Minus, the drain current drops to 7A, for a carrier output of 200W, and positive modulation level of over 160%.

[W1RKW]

Steve,
Just wondering, your design has 5 paralleled MOSFETs paralleled with another 5 paralleled MOSFETs, could one build just one half of this instead of having 2 symmetrical halves?  In other words, eliminate 1 half of the 5?

[w9jsw]

I would think you would drop them in pairs...

Steve, can you add a tops down picture of the amp? Thinking about a board layout... 2.00mm 2oz kind of board. What is the peak current per drain? Guess?

On Tron's name..nah, I think we should add another pair and call it Steve's 12 pack...

John

[steve_qix]

Steve,
Just wondering, your design has 5 paralleled MOSFETs paralleled with another 5 paralleled MOSFETs, could one build just one half of this instead of having 2 symmetrical halves?  In other words, eliminate 1 half of the 5?

Also, noted what looks like a wiring error in the schematic, see attached? See circle on input.

Thanks for finding the wiring bug :-)  I will fix it !!

The groups of 5 MOSFETs are in single-ended push-pull.  That really helps in eliminating the 2nd harmonic.  That's why the split design.  Also, it is easy with that particular design to simply add more modules to get more power.  The next level up on this thing is probably to go to 20 MOSFETs in 5 modules.

Thanks and Regards,  Steve

[steve_qix]

I would think you would drop them in pairs...

Steve, can you add a tops down picture of the amp? Thinking about a board layout... 2.00mm 2oz kind of board. What is the peak current per drain? Guess?

On Tron's name..nah, I think we should add another pair and call it Steve's 12 pack...

John

Yes, I will take a top-down picture and add it to the initial post.  Stand by on the boards    The design is still in flux, but that would be VERY cool when things are stable.

[K1JJ]

Very cool Steve!

I've been wanting to try something like this for a long time. I already have a 2 MOSFET version as a class C driver for my class C big tube rigs, so this is a natural step up.

The class B- bias mode is perfect for this amp on AM.  It works very well with tubes and should for MOSFETS.  The difference in efficiency is quite big.

Next try a 2-tone test and see what it shows. If you can hit  -30 dB 3rd IMD, you are in business.     -30dB is not an easy feat.

I may make this my next project. But it would probably be a 20 MOSFET or larger version. A great addition would be a template and plan how to add on "unlimited" modules to customize it for anything you wish.  For instance, if someone wanted to build a 40 MOSFET version, here is the suggested layout.  Maybe there is a limit based on higher frequency, I dunno. For example, maybe a 75M version is the limit for 40 MOSFETS, etc.  

Also, a variable matching / tuned input circuit will eek out a few more dB of cleanliness and make a perfect 50 ohm match for an SDR, etc., and is easy to add.

** Add a jack and sample the RF output with a toroidal transformer.  Cable it back to the SDR and get pre-distortion "Pure Signal"  -55 dB 3rd order brick wall perfect bandwidth. Wow.

The 10K resistance in the adjustable bias line:  Are there times when there should be gate current or does it never draw gate current in any mode?  

I heard you on briefly today testing it but was not set up to check the sidebands. I'd be glad to assist you with that anytime.

Congrats on an innovative project that has the potential to [inexpensively] open up the AM KW world to a lot of hams.

T

[WA2SQQ]

I’ve been listening to you on 3885 testing this. It sounds very good. I think you may have just decided my next project. Got a few questions....

1 Can the design be expanded for a bit more power?
2  Parts all appear to be readily available except for the transformer. Any suggestions?

[steve_qix]

I’ve been listening to you on 3885 testing this. It sounds very good. I think you may have just decided my next project. Got a few questions....

1 Can the design be expanded for a bit more power?
2  Parts all appear to be readily available except for the transformer. Any suggestions?

The design can definitely be expanded.  My suggestion is to add more 5 MOSFET modules, 2 at a time.   Or build 4 4-FET modules, etc.  Figure about 20 watts carrier output (being conservative) per MOSFET when running on AM and able to modulate 160% in the positive direction.

Which transformer do you mean?  The power transformer?  I am using an isolation transformer with a 2:1 step down.  Ideally, you need a bit more than 60VAC on the secondary - more like 65VAC.  I "boosted" my isolation transformer to get 65VAC, which gives me about 80VDC for the power supply.  Isolation transformers that can be configured for a 2:1 turns ratio are generally quite available, and at good prices.

[N1BCG]

How clean is the keying? It would be an all-mode amp if it had C.W. capability.

[w9jsw]

Steve,

Thinking about boards even though you said not to....    

Make each module a separate board. Can then populate 4 or 5 FETs on each and chain them together as needed. Makes the overall cost a lot less.

I have a nice little linear bias board with 4 pots on it. Used it for a GFZ provided MRI board. It may be useful here in that you could independently dial in the carrier current on each module separately if they are slightly out of balance.  The board is about 1in by 4in. Dirt cheap to have more made. I can send you one if you want to play with it.

Can the input transformer be made with a BN73-202 Binocular instead of that large ferrite? Would be easier to wind and mount on a board.

My 813 crapped out last nite. Shorted out a screen bypass cap that should have been 2kv rated. I used a 1kv part. Live and learn. Parts on order but now I need a backup transmitter/amp it seems!

John

[w4bfs]

a question ....  I see FQA11N90C Chinese made fets for around a dollar each .... what is your experience with these ?

[w9jsw]

https://forum.allaboutcircuits.com/threads/chinese-chips-on-ebay.118492/

https://www.aeri.com/counterfeit-electronic-component-detection/

I prefer to buy semiconductors from a known, quality parts house. YMMV

John

[Chuck...K1KW]

Very interesting..

When one runs an AM linear, is it best to accentuate the positive peaks or raise the carrier level to achieve just 100% positive modulation?

My opinion is that anything more than 100% positive peaks is useless when it comes to using a linear amplifier on AM.

All linear amplifiers produce a certain PEP level.  Why wouldn't one opt for the biggest carrier (quieting) and just modulate up to 100% perhaps using an all pass filter to limit those useless positive peaks?

A good 6 to 8 pole all pass filter will eliminate that nasty assymetry in the human voice in your audio chain.  Plus that assymetry does NOTHING for intelligibility.


YUP....A bit of heresy but fact.

Chuck K1KW

[steve_qix]

a question ....  I see FQA11N90C Chinese made fets for around a dollar each .... what is your experience with these ?

The FQA11N90C is the current version of the FQA11N90, and that is what should be used in any new designs.  I would just ensure that they are the genuine article and not a counterfeit device.

[M0VRF]

They're obsolete and have been for ages!

JB.

[steve_qix]

They're obsolete and have been for ages!

JB.

Yes, the original FQA11N90 has been replaced by the FQA11N90C.

[steve_qix]

Very interesting..

When one runs an AM linear, is it best to accentuate the positive peaks or raise the carrier level to achieve just 100% positive modulation?

My opinion is that anything more than 100% positive peaks is useless when it comes to using a linear amplifier on AM.

All linear amplifiers produce a certain PEP level.  Why wouldn't one opt for the biggest carrier (quieting) and just modulate up to 100% perhaps using an all pass filter to limit those useless positive peaks?

A good 6 to 8 pole all pass filter will eliminate that nasty assymetry in the human voice in your audio chain.  Plus that assymetry does NOTHING for intelligibility.


YUP....A bit of heresy but fact.

Chuck K1KW

Given that, the current amplifier design and implementation meets the goals of 300 watts carrier output.  However, at 300 watts carrier out, the positive modulation is limited to about 120% positive.

I suppose to have asymmetry (or not) is a personal choice, and there are always compromises.  Personally, I prefer the sound of an unmodified waveform.  For whatever reason, it sounds better (to me).  Some people may not notice it.

So, if you don't have a lot of asymmetry (or reduce the asymmetry by artificial means such as with a phase rotator), you can run more carrier output given the same amplifier because you don't need the positive peak headroom.  And, lots of people do this to get the most carrier.

I prefer the asymmetrical waveform but of course that comes with the compromise of lower carrier output given the same linear amplifier.

The final goal of this project is an amplifier that will deliver 300 watts carrier output and at least 160% positive modulation, and then the user can actually run it any way he (or she) wants !!

Give 'em choices 

Regards,  Steve

[M0VRF]

Can you point us to a supplier of said FET?

Regards.

JB.

[KD1SH]

https://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/FQA11N90C-F109?qs=0lQeLiL1qyavxF5xp5JF5g%3D%3D

Can you point us to a supplier of said FET?

Regards.

JB.

[K9MB]

Very interesting..

When one runs an AM linear, is it best to accentuate the positive peaks or raise the carrier level to achieve just 100% positive modulation?

My opinion is that anything more than 100% positive peaks is useless when it comes to using a linear amplifier on AM.

All linear amplifiers produce a certain PEP level.  Why wouldn't one opt for the biggest carrier (quieting) and just modulate up to 100% perhaps using an all pass filter to limit those useless positive peaks?

A good 6 to 8 pole all pass filter will eliminate that nasty assymetry in the human voice in your audio chain.  Plus that assymetry does NOTHING for intelligibility.


YUP....A bit of heresy but fact.

Chuck K1KW

Given that, the current amplifier design and implementation meets the goals of 300 watts carrier output.  However, at 300 watts carrier out, the positive modulation is limited to about 120% positive.

I suppose to have asymmetry (or not) is a personal choice, and there are always compromises.  Personally, I prefer the sound of an unmodified waveform.  For whatever reason, it sounds better (to me).  Some people may not notice it.

So, if you don't have a lot of asymmetry (or reduce the asymmetry by artificial means such as with a phase rotator), you can run more carrier output given the same amplifier because you don't need the positive peak headroom.  And, lots of people do this to get the most carrier.

I prefer the asymmetrical waveform but of course that comes with the compromise of lower carrier output given the same linear amplifier.

The final goal of this project is an amplifier that will deliver 300 watts carrier output and at least 160% positive modulation, and then the user can actually run it any way he (or she) wants !!

Give 'em choices 

Regards,  Steve

This is an interesting thread.
Steve, I just have a couple of questions:
1. You spec 80-85 vdc as the voltage used. Is there any reason why one could not go to 100vdc in this circuit and reduce current to keep dissipation down?
I have some nice 70vac @14amp transformers. Could put a variac in, if needed, but they will give about 100vdc output with full input AC voltage.

2. The 1.5 ohm resistors on the source leads- are they a special low inductance thick film or just 5 watt thick film resistors with spiral windings?
The implied question is whether inductance matters at this low resistance and if so- would 3-4.7 ohm 2 watt resistors be better?
Same question on the 4.7 ohm input resistors.
3. If one went to a 4x4 setup (16 FETs) how will that affect the shunt caps?
4. Can this be bandswitched by tapping a larger output inductor and using-say- a 1000pF at 10KV vacuum variable and using vacuum relays?

[WD5JKO]

Hi All,

   I'm following this thread closely. For kicks, I added two datasheets. One is for the FQA11N90C and another for the IXFH26N100X. Mouser carries both, and the IXYS version is more expensive ($13.78 ea, or $11.16 qty=10). Might be able to cut down the number of fets though by a factor of 2...maybe by a factor of 3.

   Anyway, these IXYS devices might be a good candidate for a class AB or B linear amplifier. The extra dissipation rating (840 watts) is impressive!

   One of my day jobs is to repair commercial class E amps that run 3KW out at 13.56 Mhz. I use two IXYS devices with similar ratings (more Pd), but a different package. The design runs two in parallel with a high Q tuned gate circuit. Runs 3KW 24/7. Not sure of the Class E efficiency, but the box has three energy conversions, and has ~ 70% overall efficiency from 3 phase AC in to RF out. Devices run at about 200v Drain voltage at maximum output, and peak drain voltage is around 850v.

Link to Mouser IXFH26N100X:
https://tinyurl.com/ycfwa8mh

73,
Jim
Wd5JKO

[M0VRF]

MOSFET 900V N-Ch Q-FET advance C-Series
FQA11N90C-F109
ON Semiconductor / Fairchild
1: £2.96
297In Stock

Ok, this is the correct part no.

Nerd alert over and out....

JB.

[K4QE]

Looking for an asymmetry filter?

W3AM Presents:
The Phase Rotating Asymmetry Eliminator!

http://w3am.com/8poleapf.html

[WA2SQQ]

My interest in this amp would be using it with my Flex 6500. I have two related questions. Has anyone though of some sort of TR switching? More important, how do you safely mute the 6500 during transmit?