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Re: [Amps] Linearization problem

To: <>
Subject: Re: [Amps] Linearization problem
From: "Doug Ronald" <>
Date: Fri, 5 Dec 2014 09:58:39 -0800
List-post: <">>
Chuck Wenzel came up with a couple HF isolators that might be of use. Here
are the links:

This one is an RF Design award winner, and can achieve 60 dB of reverse
isolation at shortwave frequencies.

-Doug Ronald

-----Original Message-----
From: Amps [] On Behalf Of Manfred
Sent: Friday, December 05, 2014 6:18 AM
Subject: Re: [Amps] Linearization problem


> I use a bird sampler
> 3-Seri es_15-35-MHz-Variable-RF-Signal-Samplers.aspx

That one is non-directional. It's a plain simple sampler, that will produce
an output attenuated a certain amount from whatever is on the line. It will
give absolutely no rejection of the distortion introduced by the amplifier
stage into the drive signal!

You can do exactly as well by connecting a resistive or capacitive divider
from the drive line.


> Maybe a band specific, power compliant isolator? 

That would be great, but the only idea I have about how to build isolators
for HF results in something very impractical!

> A directional coupler could be used to detect the output of the 
> transceiver before the isolator.

If an isolator is used, no directional coupler is needed. A simple divider
connected to the line, between the driving transceiver and the isolator,
will do fine.

The problem is just how to make that isolator!


> Simple; Dougherty amplifier.  Two tubes one class "C" (positively 
> modulated) the other class "A" (negatively modulated) many variations

That has nothing to do at all with what I'm trying to do! A Dougherty
amplifier is relatively complex, and it's hard to splice the two amplifiers
together in a way that will result in low distortion. And the resulting
efficiency isn't so great as to warrant all that complication. The approach
I'm following is far simpler!

About amplifier classes, okay, let's then agree that some people call an
amplifier "class B" when it is in fact class AB, but very close to B. In any
case, when I talk about a solid state class AB amplifier, I mean one having
just a small idling current. Just enough to get reasonable linearity in the
cross-over region. Unlike tetrodes, which are often run with a huge idling
current to linearize the mid-level amplitude region too, in solid state amps
the mid region is more linear to start with, and the final linearization is
done with negative feedback.


 > In the microwave world one would follow the coupler with a circulator and
> the energy reflected from the amplifier would dump into a dummy load.

Yes, that would be the right thing to do at HF too, if there just was a
circulator for HF!

 > Is there a circulator design made for HF?

  At HF it's surely simpler and cheaper to use several active buffer stages,
than a circulator.I have never seen one, at least not one having a size and
cost that makes sense. But I would love to get by without those buffer


 > Instead of driving the amp with RF, I would suggest doing it digitally.

In my opinion, that's certainly the way to go, for any RF power amplifier
built into a modern, DSP-based radio. I think that the HF radios we should
be using are basically a DSP box, with a full-spectrum, wide dynamic range
A/D converter at the antenna input, and a 1.5kW, switching RF output stage,
directly driven by properly predistorted digital phase and amplitude
signals. The conventional concept of a big, bulky, heavy add-on amplifier,
to provide the final 12dB of gain, looks so lame in comparison!

But the fact is that there are millions of conventional 100W HF transceivers
out there, and most hams want to keep using them. So we still need those
ugly, big, bulky, heavy, expensive add-on legal limit boxes. And what I'm
trying to do is coming up with a way to make them significantly smaller,
lighter, cheaper, more efficient, while at least maintaining decent IMD
performance, or ideally improving it.

And for an add-on amplifier, digital driving just isn't very practical,
because it means burning up the original analog 100W drive signal in a dummy
load, dissecting a sample of it into its phase and amplitude components,
then turn them into digital signals, put them into a DSP, and then built the
whole digitally driven amplifier! It's hugely more complex than most
homebrewers would want.


 > > And a bonus question: Do you think that the phase distortion in  > >
such a class-A-AB-C-E amplifier will be bad enough to still  > > cause poor
IMD performance, even while the envelope linearity  > > is excellent?
 > >

 > Yes. Around 1980 I prototyped an amplifier aimed at improving  >
efficiency in a military hf manpack. The output stage was a  > wideband push
pull affair which behaved like class B at low drive  > levels, moving
towards class D at full output. A 'polar loop'
 > (Petrovic and Gosling, Bath University) applied amplitude feedback  > via
the PA bias supply and phase feedback via the synth PLL. With  > either not
connected, the output spectrum was very wide -

Thanks for that info, even if it's bad news to me! But I suspected it.

Several years ago, while experimenting with pure EER and with combined class
AB/F EER amplifiers, I ran into severe phase distortion problems. I finally
gave up that line of development.

My present hope was that by keeping the supply voltage constant, and
modulating the gain just through the bias, it might be possible to get a
phase distortion low enough to live with it. But your experience tells

So, it looks like I should better forget this whole stuff.

 > as best
 > I remember, both loop bandwidths needed to be some 10's kHz to get  >
acceptable results.

Correction loops will always correct the distortion products that fall
within the bandwidth of the correction loop, and they will _worsen_ and
__create_ distortion products outside their bandwidth! So, the correct way
to use correction loops is to make the correction loop's bandwidth larger
than that of the amplifier's output bandpass filter. In ham practice, this
forces the use of tuned amplifiers, and with pretty high Q networks.

Now I have to think if I can come up with a reasonably simply phase
modulator in the drive signal path, that would allow implementing a phase
correction loop in addition to the bias-controlling amplitude correction

It all is starting to look unreasonably complex. I mean, just to get 12dB
more TX signal...

Last item in this post, for all those who suggested directional couplers to
my envelope signal: I did the theoretical analysis this morning. The result
that a directional coupler could indeed be used to provide a correct, 
undistorted envelope signal from the drive signal, even while the amplifier 
stage introduces a varying load and thus some distortion, but _only_ if the 
driving radio behaves like a perfect voltage source with a fixed series 
resistance! This is how signal sources are normally modeled, but
HF radios don't behave like that, at all! In practice, when the amplifier
puts a 
  non-constant load on the drive signal, the radio's output will vary in a
that is not how a perfect signal source with fixed internal resistance would
As a result, the drive signal would get distorted, at the output, input, and

side ports of the directional coupler!

Sure, an attenuator would reduce the distortion, at the cost of some drive 
power, but only by a few dB.

Do you want to see the effect yourself? Take any good SWR meter. They are 
basically directional couplers with built-in detectors. Connect the SWR
between your radio and an antenna tuner. Load the tuner with a dummy load. 
Adjust for 1:1 SWR. Then set the radio for a safe output power, say 10W, and
detune the tuner while watching the forward power indication. It will vary 
strongly! This indicates that the radio is not performing as a true 50 ohm 
signal source.

Putting any kind of directional coupler in place of that SWR meter, and my 
non-linear amp stage in place of that tuner and dummy load, would be the
situation. As my amp varies its input impedance depending on drive and bias,
sampled envelope would become untrue.

Okay. Project shelved. It was a nice dream.


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