That's very clever.
Perhaps clever, but Hawksford error correction tends to minimize K2, unless it is deliberately maladjusted to increase K2. By connecting the cascode feedback resistors to the opposite polarities (OUT- and FE-OUT+), which I call Anti-Hawksford-Feedback, at high amounts of feedback gives the opposite phase of K2.
In the following plots, I show the harmonics of 4 different situations. Sorry, but these plots do not show the "optimal" adjustment of the HEC situation.
upper-left: output stage with ideal (undistorted) inputs
upper-right: no cascode FB, and 10dB global FB thru input stage.
lower-left: Anti-Hawksford FB and about 10dB global FB thru input stage.
lower-right: misadjusted (Pseudo) Hawksford FB and about 10dB global FB thru input stage.
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Here is another feedback scheme based on the following objectives:
- Use minimal feedback around the output stage needed to obtain the desired damping factor.
- Apply as much feedback as needed to the input and VAS stages to drive the output stage with minimal distortion.
- Use the input, cascode, VAS, and output stage topology previously shown.
- input+cascode+VAS
- cascode+VAS+output
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And yet another feedback scheme. The idea here is negative feedback using a mix out OUT and FE-OUT, not using the cascode feedback option.
Shown is the basic concept, and schematic and sweep with the best achieved parameters I have found. It might be improved with more tweaking.
The schematic and sweep was with parameter a=.35.
Shown is the basic concept, and schematic and sweep with the best achieved parameters I have found. It might be improved with more tweaking.
The schematic and sweep was with parameter a=.35.
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Here is a result from a totally different approach that I previously rejected because the local feedback approach seemed to be distasteful. After changing my opinion about the ultimate objective being to preserve the character of the output stage I can accept this approach.
Here are simulation results. The top sweeps are for the output stage driven by an ideal sine generator. The bottom plots are for the amplifier using this feedback scheme. The damping factor is around 200. The feedback ratio is about 10dB. As you can see, the result is a very good match to the ideal output stage except very high frequencies.
Here are simulation results. The top sweeps are for the output stage driven by an ideal sine generator. The bottom plots are for the amplifier using this feedback scheme. The damping factor is around 200. The feedback ratio is about 10dB. As you can see, the result is a very good match to the ideal output stage except very high frequencies.
- Harmonics vs. Watts at 1 kHz
- Harmonics vs. Frequency at 1 Watt
- Harmonics vs. Frequency at 20 Watts
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I think that will keep everyone busy for a while...
The project could expand even further by varying the number of output FETs, the rail voltages, and the bias currents. How about an XA1000.8?
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