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Old 13th May 2009, 01:23 AM   #1
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Default Can someone tell me how this works?

Hi all.

I was experimenting (yet again) with the Allison pair. I started this design not focusing on the allison, but on a simple way to cascode the VAS to make it more linear.

In simulation lots of strange things have sprung up in my experiments. At the beginning I would dismiss these as "simulator trash". But with my experience of the simulator I have come to understand that there is a reason for everything, and usually the simulator is saying something important, even though it is easy to misinterpret the output.

When testing this circuit for distortion, I decided to add R10 to see what would happen. The 1KHz THD did not improve, but the 20KHz THD did by far, and is about 1/3 the 1KHz THD. Something that I don't see often is HF performance better than LF. When I do see it, I make sure to figure out exactly why it happened that way.

This is an interesting case. In this case, increasing Cdom actually decreases HF distortion. Here are the observations:

1: Decreasing C8 and C9 increases THD20.
2: Increasing Cdom will decrease THD20 only until you go above 86p.
3: Replacing D1 and D2 with a constant voltage source does not affect THD.
4: THD vs freq becomes normal with the deletion of R10. (normal constitutes no negative correlation of THD with increasing frequency.)
5: THD20 shows dominant 3rd harmonics, whereas 1KHz THD is 2nd harmonic dominant with the other harmonics following in a roughly downward line.

THD @ 1KHz = .00088%
THD @ 20KHz = .00027%

Maximum load capacitance is 2nF.

Gain is 10.

Here is the schematic:

Click the image to open in full size.

And here is the frequency response:

Click the image to open in full size.

I have the LTSpice .asc file attached. The 2Sx/MJL output device model are included in the file for your convenience, the rest should be default LTSpice models.

I am fairly sure that the answer lies in the interaction between Cdom, C8 and C9.

If the answer is that R10 makes the VAS current more linear, then why is it only affective at high frequencies?

Thank you for your time,
- keantoken
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File Type: zip allisonquestionmark.zip (1.8 KB, 25 views)
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Old 13th May 2009, 02:27 AM   #2
Mr Evil is offline Mr Evil  United Kingdom
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The effect of Cdom is expected - it's just the trade between linearity of the VAS and open-loop gain.

I wouldn't expect the bias voltage for the cascode to have much effect simply because the VAS doesn't usually benefit from cascoding much, so it doesn't matter if the cascode is perfect or not.

A resistor to ground from the output of the VAS usually increases distortion (some people do this deliberately) by needlessly throwing away gain, but I have seen cases like yours where it actually improves distortion (2nd harmonic specifically, which is what the VAS produces in the first place). Here's an experiment for you: Measure the open-loop gain at various DC output voltages. My guess is that the gain of the VAS changes with output voltage and the resistor to ground flattens that out.
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Old 13th May 2009, 05:08 AM   #3
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Quote:
Originally posted by Mr Evil
The effect of Cdom is expected - it's just the trade between linearity of the VAS and open-loop gain.

I wouldn't expect the bias voltage for the cascode to have much effect simply because the VAS doesn't usually benefit from cascoding much, so it doesn't matter if the cascode is perfect or not.

A resistor to ground from the output of the VAS usually increases distortion (some people do this deliberately) by needlessly throwing away gain, but I have seen cases like yours where it actually improves distortion (2nd harmonic specifically, which is what the VAS produces in the first place). Here's an experiment for you: Measure the open-loop gain at various DC output voltages. My guess is that the gain of the VAS changes with output voltage and the resistor to ground flattens that out.
You are right that using a better voltage source won't improve the cascode much. I was simply trying to see if the cascode was a factor in our strange phenomenon.

The gain of the VAS changes by 40 from peak to peak (is fairly constant at 1.2k) at 20KHz. Without cascoding, it changes by 3.8K at the same output power and frequency (goes from 0.9k to 4.7k). I would say that cascoding has an enormous affect on VAS performance!!! In my experience on the simulator I have had more luck producing well-behaved amplifiers using a cascoded singleton VAS than I have using a darlington or CFP. It just seems that a single transistor VAS is more stable, and there is less need for a large Cdom (which, except in special cases, increases HF distortion) to make it so.

If your amplifier has .01% THD, it probably won't matter much (unless the VAS is most of your problem). But if you are getting to levels like those of this circuit, it matters quite a bit. I think you should run the circuit in the sim and look at the affect of cascoding on the VAS and THD.

I have begun to use it regularly in my circuits, and it's fairly simple to implement. The VAS will also run cooler, improving the temperature balance of the LTP.

I think you are partially right in that the gain changes at different output powers, because the input impedance of the Allison has a strong 3rd harmonic. But I don't know if this is enough to cause the strangeness here. The open-loop gain stays at 33.6k pretty constantly.

Here is another thing to note:

The best value for Cdom, if we go by THD, is about C8+C9. This means also that the best value of Cdom will pass the same amount of current as C8 and C9 combined. Any thoughts on this? My thoughts are that the capacitor currents at all parts result in a cancellation of device currents, reducing gain. What if the current in Cdom somehow compensates for the current in C8 and C9?

I think this will be very enlightening when we find the answer.

All comments welcome,
- keantoken
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Old 13th May 2009, 05:58 AM   #4
Bonsai is offline Bonsai  Taiwan
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You are not the only person to report an improvement in THD by loading the VAS resistively (R10 in your case). I may have to be corrected, but Pavel has also made this comment. However, quite a few practitioners claim it makes no difference or is a waste of loop gain at lower frequencies. Most people who load the VAS resistively do it to get 'wider open loop bandwidth' at the expense of lower loop gain at lower frequencies.

I have simulated this, but on a fully balanced design (+-35V rails and 2 x 22k to ground from each side of the Vbe multiplier. What I found was that without the resistors, the current waveform in the VAS emitter resistors was quite distorted and after I added the resistors, this waveform cleaned up dramatically and in fact was almost a perfect sinusoid (I run the VAS at 27mA). Let me add, that the waveform on the 27mA VAS current standing current that I was looking at was about 500uA - so about 2%.

The overall distortion improvement was not that great at 20KHz - from about .06% to about .04% after I added the resistors.
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Old 13th May 2009, 06:09 AM   #5
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Could you attach your .asc file? I'm not sure what you are talking about and you refer to emitter resistor in the VAS, where there are none in my schematic.

Also, try making those 22k resistor 47k. Going lower than 22k net impedance increases distortion dramatically.

- keantoken
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Old 13th May 2009, 08:04 AM   #6
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Hi keantoken

Tested in the square wave?
That peak below 0dB, can be oscillation...
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Old 13th May 2009, 10:10 AM   #7
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I saw schematics with a capacitor of about 1u in series with the resistor to not load the VAS at lower frequencies. Maybe you also should try it to improve lower frequencies THD.
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Old 13th May 2009, 12:08 PM   #8
Bonsai is offline Bonsai  Taiwan
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Keantoken,

the circuit I use is a std fully balanced VAS topology - no rocket science there.

Since I run th e VAs at about 25-30mA, driving 10k and higher VAS resistive load to ground is no problem.
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Old 13th May 2009, 04:27 PM   #9
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I might be wrong but D1, D2 should not be in the current mirror. The base of Q4 should be a constant V, the voltage across D1,D2 varies with the mirrror current, (and it varies non linearly). They might be adding feedback?

To get a better idea what R10 is doing try a loopgain plot with and without it (and try 10k 40k) (f you dont know how check the last few pages of the Spice thread) This R will change the VAS gain but it will probably also effect the pole caused by Cdom, (and maybee the pole of the driver stage Alison new to me, Common base driver with local feedback??)
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Old 13th May 2009, 05:28 PM   #10
Mr Evil is offline Mr Evil  United Kingdom
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Ok, if cascoding works for you then I believe you! I don't usually see it making any difference because Cdom negates the improved bandwidth. Perhaps it's because of the way you have it connected, both sides of the LTP see a more equal load? Try moving the cascode bias voltage out of the LTP and see if it makes things worse.

It might also be interesting to try replacing R10 with a resistor from VAS output to the inverting input, sized to give the same open-loop gain.
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