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Yep, noted, I understnad the basic principle with the 90 degree phase shift and carrier/peaking etc
... I am seeing a nice solid 15A total now, about 11A on the carrier side, 4A on the peaking side, I think this is about flat out for this amp. ... I found a resistor fault on the biasing of the drive amp and corrected that, it is now stable where as it was all over the place before.
The SMA lead connecting the amp to the dummy load (a 250W slab load out of an old band IV transmitter) gets quite warm after 30 seconds of carrier so something is coming out :)
Going to go and have a listen on 13cm for beacons later.
On Mon, 14 Sep 2020 at 18:46, David GM6BIG <gm6big@...
Doherty amps work in a less obvious way.
The Carrier device could easily be running 10 times the current of the
The two sides may be biased differently.
The are moderately narrowband, there is much 1/4 wave phasing in there.
The method has been around a long time, so lots on the web to read up on
Essentially, instead of increasing the PA volts (Carrier device) to get
more power, decrease the Carrier load impedance (Using the Peaking
This will get you started...
An early use of the method was in big AM transmitters, an efficient way
to make AM RF without a big oily and expensive audio rated modulation
Not so easy to move in freqy, but ideal for, say, an AM broadcast
Now applied to microwave stuff...!
On 12/09/2020 22:52, Robin Szemeti - G1YFG wrote:
> In theory, it is supposed to run in Doherty mode .. the transistor on
> the right biased into Class A/B and pulling about 1.8A quiescent ... the
> one on the left turned hard off and only coming on with peaks.
> I'm finding I can pull about 10A through the right hand device, but
> barely anything through the left hand device, no matter how hard I hit
> the input. I've increased the drive by 6dB and its not really pushing it
> any harder ... something isn't quite right.
> On Sat, 12 Sep 2020 at 20:53, militaryoperator via groups.io
> <http://groups.io> <Military1944firstname.lastname@example.org
> <mailto:email@example.com>> wrote:
> The Nokia pa for 13cm. I know about the circulator, removing etc,
> but what about the bias. Has anyone sorted out better biasing for
> this unit?
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Robin Szemeti - G1YFG