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Old 13th February 2008, 12:38 AM   #11
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Is a DC servo (all NPN of course) allowed or does that count as AC coupling?
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Old 13th February 2008, 03:20 PM   #12
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Much talk but no schematic yet
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Cheers// Ove
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Old 13th February 2008, 04:59 PM   #13
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sorry, was thinking output stage only. My bad.

-SL
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Old 13th February 2008, 10:14 PM   #14
latala is offline latala  United Kingdom
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I see no problem with a dc servo ! Anything other than complimentary devices in this case I have said NPN only
The amplifier can have a input capacitor and a feedback to gnd capacitor
the spk is to be dc coupled to the output stage
I have as yet no schematic of my own the idea is to get away from what are esentlialy only variants on standard schematics!
level translation could be via diodes ,zeners/opto devices
capacitor or even galvanic
Just no pnp devices
regards Trev
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Old 14th February 2008, 04:18 AM   #15
Tim__x is offline Tim__x  Canada
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Whoop, this is the result of ten minutes. The output stage could be drastically improved and scaled for a typical load, but that's the easy part, this is just an example topology.

Around 40db open loop DC gain climbing to 80 some db, currently uncompensated and marginally stable with a 260mhz gbw.
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File Type: txt draft12.asc.txt (4.5 KB, 98 views)
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Old 14th February 2008, 05:04 AM   #16
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Quote:
Originally posted by latala
I see no problem with a dc servo ! Anything other than complimentary devices in this case I have said NPN only
The amplifier can have a input capacitor and a feedback to gnd capacitor
the spk is to be dc coupled to the output stage
I have as yet no schematic of my own the idea is to get away from what are esentlialy only variants on standard schematics!
level translation could be via diodes ,zeners/opto devices
capacitor or even galvanic
Just no pnp devices
By NPN only, are you being literal in that you only want NPNs as in bipolar NPNs, or does that also include N-type FET devices? If so, here's a concept for you.

Click the image to open in full size.

se
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Old 14th February 2008, 06:23 AM   #17
latala is offline latala  United Kingdom
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Tim X I can not open your attachment all I get is a wiring list ?
please give us a diagram !!!
The other one by steve eddy is interesting ! and very unexpected
lots of iron though! still its food for thought
I shall submitt my concept sketch tonight
regards Trev
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Old 14th February 2008, 11:58 AM   #18
DRC is offline DRC  United Kingdom
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This OP stage is mostly NPN !!

The inner diff pair / current source can be readily replace with NPN but this leaves a level shifting problem - maybe a current sourced NPN E-F and a resistor to shift the level ?
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Old 14th February 2008, 06:36 PM   #19
Tim__x is offline Tim__x  Canada
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It was an LT Spice simulation file, earlier I wasn't on a computer with image editing software installed.

Obviously I ignored the power goal, and only focused on making a workable topology with only NPN devices, I chose to go DC coupled throughout including the input. The two caps in the second stage serve only to enhance AC gain, there is still considerable DC gain and complete control of the output stage down to DC.
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Old 14th February 2008, 06:42 PM   #20
Tim__x is offline Tim__x  Canada
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I should note: it's VERY unpolished, it is just a work of ten minutes and ONLY an example of topology, not a fleshed out buildable schematic. DON'T build it as is..
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