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' [email protected] [rsgb_lf_group] <[email protected]>
To: rsgb_lf_group <[email protected]>
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: [email protected] [mailto:[email protected]]
Sent: 08 January 2016 23:25
To: [email protected]
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 [email protected] [rsgb_lf_group] <[email protected]>
To: rsgb_lf_group <[email protected]>; rsgb_lf_group <[email protected]>
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 <[email protected]>

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