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Re: [Amps] Linearization problem

To: Manfred Mornhinweg <>,
Subject: Re: [Amps] Linearization problem
From: Radio WC6W via Amps <>
Reply-to: Radio WC6W <>
Date: Thu, 4 Dec 2014 08:50:16 -0800
List-post: <">>
Hi Manfred,
    Directional coupler.

73 & Good morning
    Marv WC6W

On Thu, 12/4/14, Manfred Mornhinweg <> wrote:

 Subject: [Amps] Linearization problem
 Date: Thursday, December 4, 2014, 6:05 AM
 Attention to all technically minded
 Maybe somebody has a good idea, that could contribute to
 some work I'm doing, 
 which might end up being useful for the ham community.
 First I will explain the situation, then comes the
 Picture the following amplifier setup: The core is a simple
 class E amplifier 
 stage, using one MOSFET, or several in parallel, with a
 single-band tuned 
 matching network, operating from a fixed-voltage power
 supply. This is a "black 
 box", having three inputs and one output: Power supply
 input, RF drive input, 
 gate bias input, RF output. Let's assume this amplifier
 block to be optimized 
 for operation at high efficiency (say, 90%), at 1500W
 output, a condition at 
 which it's driven into moderate, but not very deep
 saturation, from a 50-100W 
 drive signal.
 Such an amplifier will of course be very nonlinear. But it
 is possible to vary 
 its gain over a significant range, by varying the gate bias.
 So, we will now 
 wrap a linearization circuit around this black box: A highly
 linear envelope 
 detector at the output, another such envelope detector at
 the input, and an 
 integrating comparator that drives the gate bias. Of course
 there is a clamp 
 too, that limits the highest bias to a safe value. The
 transceiver's ALC could 
 be driven from this circuit too, so that the ALC is
 activated when the bias 
 control circuit starts running out of headroom.
 When this contraption is driven by a SSB signal, the bias
 control circuit will 
 at all times apply such a bias voltage that the output
 amplitude remains very 
 highly proportional to the input amplitude. The amplifier
 will move through 
 several operation classes, from class A at extremely low
 signals, over class AB, 
 right into non-saturated class C where it will stay over
 most of its dynamic 
 range, and moving into class E at the highest amplitude
 levels.  The result will 
 be very good amplitude linearity, along with pretty good
 average efficiency over 
 the entire dynamic range. I expect something like 60%
 average efficiency in SSB, 
 compared to 20% or so for conventional linear amps.
 Now the question: How can I get an undistorted sample of the
 drive signal, for 
 my input-side envelope detector?
 The problem is that the MOSFET amplifier has strong internal
 feedback from the 
 output to the input, and as a result the drive signal
 produced by a transceiver, 
 with its non-zero output impedance, gets quite distorted.
 However good my bias 
 control circuit might be, the output envelope cannot be less
 distorted than the 
 amplifier-polluted drive signal is!
 Please don't suggest placing an attenuator between the
 transceiver and the amp. 
 There are only a few dB of excess power to burn up in an
 attenuator, and as a 
 consequence the attenuator can only improve the situation by
 a few dB too, which 
 isn't enough.
 And don't suggest getting the drive envelope from somewhere
 inside the 
 transceiver! That would work, but then the amplifier
 couldn't be simply 
 connected to the transceiver's antenna output anymore. This
 would make it 
 unacceptable for most hams, who wouldn't want to do surgery
 on their radios to 
 bring out an internal signal.
 At this time my best bet is adding a two-stage buffer/driver
 at the input of my 
 amplifier, and getting the drive sample signal from the
 input of it. That should 
 solve the problem, but adds a significant amount of
 Any ideas are welcome.
 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?
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