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Author Topic: Doherty amplifiers for HF?  (Read 2392 times)
M0HCN
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Posts: 473




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« on: November 18, 2012, 08:46:51 AM »

Just wondering if anyone out there has practical experience with trying to implement the Doherty PA for HF as opposed to microwave.

Basically this consists of two amplifier devices set up so that the 'peaking' amplifier can effectively load pull the load seen by the main amplifier to a lower impedance on modulation peaks.For modes featuring very large power back off ratios (Like say SSB), the efficiency improvements look to be worth having.

Now the matching would need to be per band (It needs a 90 degree phase shift), so probably best integrated with the LPF bank,  and the control of the peaking amplifier is probably best done by a fast DAC controlled by calculated modulation envelope, but with modern practice being heavily DSP based it looks to me to be possible.

Something like a DUC transmitter with two DACs would make this sort of thing straightforward as the phasing networks could amount to as little as a single bifilar inductor and high voltage cap per band.

Anyone out there tried it, or am I going to have to find out why it does not work?

73 Dan.
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KA5N
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« Reply #1 on: November 18, 2012, 11:10:57 AM »

Doherty high efficiency Linear Amps were widely used in lower power AM broadcast
transmitters back in the 40's and 50's.  Theory and circuits are described in the
literature of that era.  It has been too long since I have been exposed to them that
I just don't remember the particulars.  Perhaps Terman's Book has info.
Good Luck

Allen KA5N
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M0HCN
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« Reply #2 on: November 18, 2012, 11:37:17 AM »

Yea, much easier to do for narrow operating bandwidths however....

The mobile phone guys are starting to look at this stuff in the microwave bands as a means to reduce base station power consumption with the modern modes that have very high peak to average ratios, and I got to wondering if it could not be applied to a ham SSPA given that we have per band output filters anyway (A reasonable place to put the phase shift network).

Regards, Dan.
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W9PMZ
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« Reply #3 on: November 18, 2012, 03:38:06 PM »

My company has beenn making 40W Dougherty PAs now since 2006.

Another issue is you have to match the peaking amp.  Its output impedance is less than the main amp.

If you are going to have two transistors anyway stick to AB, unless you are into doomsday and have to conserve power and want to be loud and targeted...

73,

Carl - W9PMZ
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N3OX
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« Reply #4 on: November 18, 2012, 04:08:57 PM »

Quote
the control of the peaking amplifier is probably best done by a fast DAC controlled by calculated modulation envelope, but with modern practice being heavily DSP based it looks to me to be possible.

I think this probably sinks a lot of ham experimentation...

You're one of the few people I've ever seen in a web forum who has access to the signals needed control what you're suggesting.

A lot of the high efficiency amplifier ideas out there aren't really suitable for add on amplification of SSB or sometimes even CW. If you build the whole transmitter, like I know you have and like the mobile phone people are doing, you can do something reasonable.

I don't have any good direct leads for you.  You probably already know XQ6FOD's page but if you don't, you might want to dig around there for a while...

http://ludens.cl/Electron/mosfetamps/amps.html
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
M0HCN
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Posts: 473




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« Reply #5 on: November 19, 2012, 11:28:41 AM »

Yea, add on amps have issues all of their own, and will always be a bit of a compromise.

Still it seems to me that we may well be at the point where putting the exciter in a separate box to the amp actually makes little sense as the additional complexity cost of an exciter and (say) a ethernet port becomes comparable to the RF switching, frequency detection and protection/ALC pain that you need if you take RF as the input.
It made sense back when the shared IF, mixers and a VFO were a big deal, but now all of that can happen in an FPGA for transmit without breaking the bank.

You can buy a LOT of small signal electronics for the cost of even something like a decent tube socket.....

Getting access to whatever signals I need for whatever I want to play with is just a case of firing up the tools and assigning a pin on the gate array (Ideally one I have brought out to external IO), VHDL rocks for hacking about with circuits, but it is a bit of a new world.

Incidentally, while it is a bit of a pain, Doherty will work with just an RF link, playing bias games based on the envelope makes things easier, but as the peaking amp does NOT need to be particularly linear, a simple level detector at the input will pretty much get the job done, some other things need more, but Doherty is relatively benign in this respect.

73, Dan.
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