LF: Re: [rsgb_lf_group] Linear Amp for LF

[Andy Talbot <andy.g4jnt@gmail.com>]

With the re-interest there appears to be in WOLF, and the gradual moving on with processing and data comms technology, it might be time to revisit modes that require a bit of linearity.  

Its well known that idPSK and coherent modes in general will offer an immediate 3dB advantage over non-coherent modes.  However, in many practical situatins they are let down by the non linear processing needed to recoder carrier coherency and symbol timing.   So non-coherent - FOR PRACTICAL PURPOSES wins.

Recent tests using EBnaut show just what can be done if the need for carrier / clock recovery are removed (by using ultra stable sources) , so there may be some mileage to be gained by looking again.

The magic point to reach is to make sure the advantages gained by coherency, in terms of S/N and hence transmitted power for any given path, offset the reduced efficiency of a linear PA stage.  Typically  85% (class-E) down to  to 45% (Class AB).   So we really need to make sure our new data modes give us that extra 2 to 3dB.  Which, judging by EBnaut with its simple BPSK mayty be possible.

Once linear amps are in use, then the route to faster data in a given narrow bandwidth opens up - and more scope for espreimentation.

As an aside - I am wondering if a speeded up version of WOLF might work for the higher microwavbe bands.  Say 1000B/s or even faster BPSK with all the processing overhead of WOLF.

Andy  G4JNT

 

On 9 January 2016 at 09:14, David G0MRF g0mrf@aol.com [rsgb_lf_group] <rsgb_lf_group@yahoogroups.co.uk> wrote:

 

Hi Terry.

 

Ask away...You have almost read my mind.......

 

I've been looking at the class D circuit in detail over the past few weeks....FETs obsolete....SWR detector not quite right etc.

I've even used it on its limits to make a nice QSO with SV on 630m last weekend.  - Got so hot the snubber resistor on one of the drains fell off

 

I plan on producing around 50 boards if my Chinese manufacturer hasn't gone bust in the stock market crash.... 

However, I'm not sure the linear board has much sales potential as CW / WSPR / JT9 can all be done with non linear amplifiers.

 

Also, the recently introduced power FETs are all optimised for rapid switching in power supply applications. That's very different from 15yrs ago when the Vg and Id transfer characteristics were much more 'linear' in nature.

 

What do you think??

 

David

 

 

-----Original Message-----
From: 'Terry GW0EZY' gw0ezy@sky.com [rsgb_lf_group] <rsgb_lf_group@yahoogroups.co.uk>
To: rsgb_lf_group <rsgb_lf_group@yahoogroups.co.uk>
Sent: Fri, 8 Jan 2016 23:51
Subject: RE: [rsgb_lf_group] Linear Amp for LF

 

Hi David

 

I don’t suppose you would consider making some more PCBs for your amplifier…….?

 

Sorry – I couldn’t resist asking!

 

73 Terry

 

From: rsgb_lf_group@yahoogroups.co.uk [mailto:rsgb_lf_group@yahoogroups.co.uk]
Sent: 08 January 2016 23:25
To: rsgb_lf_group@yahoogroups.co.uk
Subject: Re: [rsgb_lf_group] Linear Amp for LF

 

 

Hi Andy.

 

If it helps, I found a series combination of a silicon diode and a schottky diode has the correct characteristic to compensate for a FET in linear service. - Avoiding components in the source.

I just put them at the lower end of a potential divider chain that developed the bias voltage.

Worth a try perhaps

 

73

 

David

 

 

-----Original Message-----
From: Andy Talbot andy.g4jnt@gmail.com [rsgb_lf_group] <rsgb_lf_group@yahoogroups.co.uk>
To: rsgb_lf_group <rsgb_lf_group@blacksheep.org>; rsgb_lf_group <rsgb_lf_group@yahoogroups.co.uk>
Sent: Fri, 8 Jan 2016 21:51
Subject: [rsgb_lf_group] Linear Amp for LF

 

Are there any proven reliably designed LF amplifier designs power amplifiers  for 137 / 475?   Wideband, covering both ideally    Rated for at least 300 Watts output, and ideally a lot more.

 

I saw G3YXM's low-HF design, and it looks promising for low frequency use, but he appears to have not gone that way and went for a switchmode version at LF/MF

 

Of course, I could just redesign that, or something similar, using suitable ferrite and a bit of feedback ,but it would be nice to avoid reinventing too many wheels.

 

Using switching MOSFETs as linear amps still doesn't feel quite right, but enough people seem have managed it now (including my own design for a low power one) that it looks a sound-enough route to follow up.    The one thing that does bother me about the technique is bias compensation with temperature.   It is really asking for active bias control, but in a class-B or -AB design, where bias just affects standing current, that gets more-than a bit tricky as a concept.

 

Andy  G4JNT

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Posted by: David G0MRF <g0mrf@aol.com>

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