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Opamp pyramid stacking for higher voltage operation
Opamp pyramid stacking for higher voltage operation
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Old 21st October 2019, 09:15 PM   #11
Mark Tillotson is offline Mark Tillotson
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Of course an input opamp is needed if a gain more than 3 is wanted, but the output swing is about 90V p2p. Click the image to open in full size.
Note the opamp's rails swing well above and below ground as they track the output, but the input voltages should just stay within limits - a protection resistor and clamps on the non-inverting input would be a wise precaution.


This circuit could drive a similar circuit using the LTC6090 to triple the swing again (LTC6090 swing is 140V), so upto 270V swing would be possible.

Last edited by Mark Tillotson; 21st October 2019 at 09:21 PM.
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Old 21st October 2019, 09:35 PM   #12
stocktrader200 is offline stocktrader200  Canada
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hows the performance, especially when clipping
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Old 21st October 2019, 09:45 PM   #13
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Mark Tillotson View Post
I've realized the transistor bootstrapping circuit of Surjan Dogran's can go up to a tripling of output voltage swing if the gain is set to 3, non-inverting. This version uses 50V rails, runs an opamp at +/-18V (limited by a zener for protection) and has the bootstrapping split into a DC and AC path to allow the bootstrap factor to set optimally even though the rail voltage isn't 3 times 18V.
Click the image to open in full size.
Well, in the end, there seems to be a consensus on the practicality and convenience of bootstrapped rails:
EZ-Dump: dump your current without really trying
But of course, this does not detract from the merit due to the pyramidal scheme, which is compatible with the bootstrap, in case the swing available is not sufficient.

I happily welcome any new and creative approach, and the pyramid certainly counts as such
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Old 21st October 2019, 10:41 PM   #14
Mark Tillotson is offline Mark Tillotson
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Originally Posted by stocktrader200 View Post
hows the performance, especially when clipping

Click the image to open in full size.Click the image to open in full size.
Seems OK, if the emulation is good. I added 1k input resistor to the non-inverting input. In practice it won't clip much as the opamp driving it will clip anyway.
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Old 21st October 2019, 10:42 PM   #15
Mark Tillotson is offline Mark Tillotson
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Originally Posted by Elvee View Post
I happily welcome any new and creative approach, and the pyramid certainly counts as such

Thanks, it was quite pleasing to figure it out - actual breadboarding it might not happen for a while as I mainly have SMT dual opamps at hand...


[ The tripler circuit may get breadboarded soon, though, I have a +/-48V supply. ]
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Old 22nd October 2019, 04:53 AM   #16
Iain McNeill is offline Iain McNeill  United Kingdom
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Opamp pyramid stacking for higher voltage operation
I built some systems using Doug Self's Class G topology which has similarities. I never could get it to sound better than his blameless class B. So forgive me if I smell a free lunch here.

I'd check a few things before building.
PSRR - what does output look like if you put 1uV signal into either supply line.
real load - what happens if you add a bit of parallel C or series L in that 2K load resistors.
Op-amp supply voltage - in-circuit, worst case load, related to PSRR but any datasheet violations?

there's a lot of complexity with this solution so expect unforeseen interactions.
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Old 22nd October 2019, 11:54 PM   #17
Mark Tillotson is offline Mark Tillotson
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Opamp circuits aren't very sensitive to rail noise, I sometimes test simulations by injecting 1V tones into the rails and see how much they bleed into the output or intermodulate the output.

BTW are you talking about the pyramid driving an OS - I presume so due to the complex load question - I'm not advocating any particular OS, that's just the motivation for the topology.

The tripler is just part of a VAS stage so its load would be the drivers or pre-drivers.

And I've tested the tripler using +/-45V rails and an LT1113 I had knocking about. Seems to work stably and was getting 80V p2p out into 10k load. I used KSA992/KSC1845's as the bootstrap transistors so they were running a little hot, didn't want to drive a heavy load.

I realized the zener that protects the opamp from over-voltage can lead to heavy current draw through the transistors if the biasing network isn't setup correctly for the supply voltage - it might need a bit of rethinking.
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Old 23rd October 2019, 03:02 AM   #18
hitsware is offline hitsware
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Simpler approach :
https://www.analog.com/media/en/tech.../ADA4700-1.pdf
Better yet :
https://www.analog.com/media/en/tech...ets/6090fe.pdf

Last edited by hitsware; 23rd October 2019 at 03:11 AM.
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Old 23rd October 2019, 03:46 AM   #19
Iain McNeill is offline Iain McNeill  United Kingdom
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Opamp pyramid stacking for higher voltage operation
Yes, driving an OS as in your schematics above, in turn driving a real speaker with challenging crossover.
Good point about the bootstrap transistors, they're dissipating almost 0.5W each quiescent. Driving the drivers will increase this dissipation.

Is there any way the V+ supply to op-amp could be less than the in/out voltage (vice versa for V- rail)?

A square wave test would be revealing.

There's so much more to consider with a complex design like this and I can't convince myself on first view that this is unconditionally stable. Personally, I've lost many battles to a simple cascode VAS as demonstrated by Self et al. so I appreciate what you're trying to do here.

sorry to drag the meal metaphor along, but the proof of the pudding....
Look forward to hearing more.
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Old 23rd October 2019, 04:53 AM   #20
hitsware is offline hitsware
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Quote:
Originally Posted by Iain McNeill View Post
Is there any way the V+ supply to op-amp could be less than the in/out voltage (vice versa for V- rail)?
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