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Old 16th July 2007, 02:15 AM   #1
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Default A high voltage drive fully differential driver/op amp

Hello Folks,

Yes this post does have a Pass tie in, but you may have to look for it.

A year or so ago I took a good look at the THS41xx series of fully differential op amps from TI and though that It would be great if I could scale it up so that it could handle more voltage swing. That way you could put it in front of something like a F4 or any other low/unity gain power amp, or even simply use it as a driver/front end for any number of amplifier topologies.

My design is a clone of sorts, but many of the details are left out of the THS41xx data sheets, so I had to still do a lot of work myself.

I think this design has a lot of potential as a driver/preamp, so I thought it would be good to share it and get some feedback from the gurus.

Let the fun(or flames possibly) begin.

I have tested the circuit as shown, it does work. Some resistors were replaced with pots and adjusted for desired bias. Otherwise, it was built almost exactly as shown.

One key to this amp is the common mode error amplifier which brings the output of each side to 0V (within < 1mv on the prototype) relative to GND. You don't need an output cap.

I used 20V rails, but it would not at all be difficult to go higher, say 35V-50V.

Also significant is that the design can swing very close to the rails (easily within 1V) which means with just 25V rails you could drive an F4 pretty well. Also note that you can attain extremely low offset without the need for any caps in the signal path.

One more cool factoid. The driver easily tolerates the 3V common mode DC at the inputs I through at it. This means that I can DC couple this driver from my DAC with no cap on input as the 2.5V common mode offset is completely nulled and not just relatively(output to output), but at each output to GND.

I can't think of a good name for it yet... I will entertain ideas.

Cheers!
Russ
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Old 16th July 2007, 09:34 AM   #2
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Hi Russ,

Following some discussion with CBS240 here :
http://www.diyaudio.com/forums/showt...20#post1224020
I became more and more interested by Common Mode Feedback Loops. Post #40 showed a proposal by Win de Jager for an integrated implementation of CMFB.

Previously, same Wim de Jager who is consistent in his ideas had published an "hybrid power amplifier" (bipolar differential input directly coupled to the valve push-pull output stage) where the valve grid DC voltage is maintained fixed by a CMFB using a simple bipolar transistor :
Electronics World, November 1996, p897-900.
I was thinking to open a new thread about it with something like "A new input stage ?" as title.
I will show Jager's input stage in a following post.

I then became aware of your Twisted Pair Audio site and the existence of IC like THS41xx and OPA1632 and its incredible performances.

By pure coincidence, I spent the three last days playing with a simulator and the whole idea. The feature of having a stable DC output without resorting to an overall DC feedback is very interesting. However, as being a feedback applied to a load, how can it be called ?

By the way, looking at your schematics, I am a bit puzzled to see capacitors C1, C2 and C3 connected to ground. I also have been thinking of a more traditionnal two stage architecture rather than a folded cascode scheme, with two CMFB loops, one at the load of the input stage like Jager's circuit and one around the output stage.

I would be glad to see some power supply rejection and distorsion figures.


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Old 16th July 2007, 10:29 AM   #3
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I have been thinking very similar thoughts.

See http://www.battletonphoenix.co.uk/linebal.pdf - the design would run happily off +/- 20V with transistors shown.

Only simulated, not built.
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Old 16th July 2007, 03:33 PM   #4
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Hi, Russ,

Nice design Is it intended for classA output stage? I'm not sure, but maybe if it is used for classAB, the folded cascode is not supplying enough gain for fixing output stage non-linearity. For classAB, common emitor for 2nd stage is more like it (but you need to interchange the collector takeoff point).
Your DC servo is nice, how about replacing R10 with current mirror like Extrema power amp (Bruno Putzey's)?
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Old 16th July 2007, 08:09 PM   #5
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Thanks for all the information and feedback so far.

My intent is to drive 4 F4 amps in 2 pairs differentially for 2 bridged channels.

One idea I have is to take feedback after the output of the F4s and thus wrap them in the feedback loop. I am wondering what sonic effects this will have. I am imagining I will be substituting even order harmonics for some odd, but hopefully at much diminished levels.

I do not have any test gear to speak of, but I will simulate CMRR and PSRR when I get a chance.

I may add a simple class A buffer stage at each output. Not sure yet.

Right now I have the prototype driving two simple LM3886 based class AB amps with the amps enfolded in the feedback as described before. This is working and sounds very good indeed. Each power amp stage is setup for a voltage gain of 6X. I would say this setup sounds no worse then the THS4131 in the same configuration. I will listen for a bit longer before I say it sounds better.

I also thought about a current mirror at the servo, but when I simulated it I could not keep it stable without compensation, and I hated the idea of adding a cap if I did not have to. I may try it again later.

Cheers!
Russ
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Old 17th July 2007, 04:33 PM   #6
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Default Power amp modification

This has only been simulated, and I would probably change some devices to beefier types (cascode and CCS transistors). I would also probably parallel output devices, and possibly use drivers for the FETs so I can lower the current for the folded cascode.

One could bias the output devices class A or class A/B.

Anyway, it simulates very well.

Cheers!
Russ
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Old 17th July 2007, 05:19 PM   #7
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Neat.

A couple of details. I would bump the gate stoppers up from 47Ohms to maybe 220 - depending on layout etc, you may be marginal for stability there.

You have a big cap (C4) limiting the BW of the symmetrising circuit, so it just does offset control. In my variant, I run that full BW; it then wipes out asymmetry dynamically. If you go this way, you can drop the values of R7 and R8, and remove R15, so bias current in that path no longer gives offsets.

If you put emitter resistors under Q5 and Q6 you should get more OL linearity and reduced capacitance (I think). R10 would have to go up a bit to compensate. S
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Old 17th July 2007, 06:21 PM   #8
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PigletsDad,

How does this look?
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Old 18th July 2007, 08:36 AM   #9
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That is the sort of thing.

I notice you have gone to a current mirror in the front of the symmetriser loop. This presumably puts the open loop gain up a lot; as this loop basically runs at unity gain, do you need/want this much gain?

I haven't played with it in the simulator so I don't have a feel for the pros and cons.

On the other side, the base of Q7 and Q8 is biased by a LED, to presumably round about 1.4V below the rail voltage. But the base voltage of the folded cascode transistors Q3 and Q4 is set by two Si diodes to be roughly the same voltage - shouldn't you swap the 1N4148 for green LEDs, so that there is a sane VCE on Q7 and Q8. Or have I misread the circuit?
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Old 18th July 2007, 12:00 PM   #10
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Quote:
Originally posted by PigletsDad
That is the sort of thing.

I notice you have gone to a current mirror in the front of the symmetriser loop. This presumably puts the open loop gain up a lot; as this loop basically runs at unity gain, do you need/want this much gain?

On the other side, the base of Q7 and Q8 is biased by a LED, to presumably round about 1.4V below the rail voltage. But the base voltage of the folded cascode transistors Q3 and Q4 is set by two Si diodes to be roughly the same voltage - shouldn't you swap the 1N4148 for green LEDs, so that there is a sane VCE on Q7 and Q8. Or have I misread the circuit?
Yes on both fronts.

I have since changed back from the current mirror on VCM error amp. I also found that the amp was not stable unless I added some resistance on the grounded side of that diff pair. I am not sure why.

I will change the bias scheme a bit, but interestingly it does work as shown.

Cheers!
Russ
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