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Old 18th November 2007, 05:41 AM   #1
CBS240 is offline CBS240  United States
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Smile My Bridge Amp

Hi

Lately I've been working on a new circuit topology that I haven't played with before, and have obtained some initial waveforms. Capacitive loading seems thus far a little bit troublesome, but I think the problem lies somewhere within the output section. Just a quick description of the project, this is a bridge amp, with +/- 25V(minus some sag) supply for the output mosfet's. Output goal is 100WRMS into 8Ohm. I have not built an amp before that is half worth a hill of beans with hexfet's, but this one may be promising. The front end, VAS, and drivers use +/- 40V @~35mA regulated. The output transistors used here are 2 pair of these, not quite so complementary, El-Cheap-O Q-fet or hexfet's from Fairchild: http://www.fairchildsemi.com/ds/FQ%2FFQP16N15.pdf, and http://www.fairchildsemi.com/ds/SF/SFP9540.pdf
The N-ch I have is the 14A version that has apparently been discontinued It has a Rds-on of 0.21 Ohms and the input capacitance is slightly lower. The like polarities don't match well either, apparently, but that doesn't matter in this circuit because they are not parallel, but bridged.
Just for the fun of it, this circuit is constructed completely from discrete components, including the regulator circuit. I am ashamed to admit, but there is one capacitor in the signal path, the 1uF polypropylene input cap. There is a lot of feedback but NONE of it is global. DC is set via a DC servo circuit that does have to sense the output, but that doesn't count. The VAS is a cascode differential balanced bridge, with a closed loop gain of 6.2. The +/- inputs are driven by two "gain cell" circuits, that have a CL gain of 6, so the circuit has an overall Av of about 37 and does not require a balanced line input but rather asymetric. The waveforms shown is the output of both phases which are bridged across the load, both outputs maintain less than 25mVDC from GND, and 10mVDC from each other. This has been an interesting journey so far into new frontiers for me, but it still requires a bit of tweaking, mostly with the output stage circuit....there is still a tiny oscillation of like 5mV at 30 some odd MHz...
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Old 18th November 2007, 05:47 AM   #2
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Where is so-called schematic of your bridge upon the river amp??
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Old 18th November 2007, 05:53 AM   #3
CBS240 is offline CBS240  United States
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Here is some more, there may be a little phase shift at 15Hz, but seems to be a little better at 20Hz. Maybe something else to tweak. It's not too bad though. I don't know what can be tested with a triangle wave, but it sure looks pretty. BTW, this is all with the output fet's bias at around 75mA, I think maybe it should be higher. The temp-co seems to be a little be overcompensated too because it starts out at over 100mA until it is warm, then decreases.
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Old 18th November 2007, 06:06 AM   #4
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Quote:
Originally posted by Workhorse
Where is so-called schematic of your bridge upon the river amp??

Well, there is two places that it exists at this moment. One is between my ears, and the other is sketched out on a poster paper stuck on the wall because it contains 146 transistors and it is the only way I can draw it all out and still see and read the whole thing. Not sure if I can take a photo of it, maybe a few photos and photshop it. he he, I doubt if anyone here would want to copy it anyway. To take a general quote from the forum, "that's just too damn many transistors."


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Old 18th November 2007, 07:07 AM   #5
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I think you should take some rest before anything else
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Old 18th November 2007, 07:30 AM   #6
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Yeah WHHeww!

I missed a couple when I was counting, there is actually 148 transistors. All of them are SOT-23 except 14, 4 outputs (TO-220), 4 drivers (TO-126), 4 Vbe X'ers (TO-126), and 2 TO-92 devices as the pass transistors on the regulator circuit. Otherwise a mix of J-fet and BJT. Many of the BJT's could be condensed into available SOT-563 pakages if a proper PCB were made.
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Old 18th November 2007, 04:47 PM   #7
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After some sleep, I got around to sketching a block diagram of the amp. I took a more 'modular' approach to the design, each circuit performing a general function. With no GNF, this made it easier to construct because I could focus on one circuit network at a time until I get it working properly, then move to the next one, ect. Eventually I was able to exterminate most of the bugs and get all the circuits to work together. Most of my surprise comes as I haven’t yet blown any of the output fets building this circuit. I'm usually par for roasting at least a couple of output pairs experimenting with new circuitry.

To input a disproportional signal to the +in of the first gain cell circuit and the -in of the second, a balanced drive can be created for the VAS circuit using two equal gain cell circuits.

Now if I can only get rid of this damned MHz oscillation. I'll try larger gate resistors on the output fets and see if that stops it. Trial and error I suppose....
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Old 20th November 2007, 08:30 AM   #8
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Here is the 'gain cell' circuit I used. It is self bias using common mode feedback. The transistors labeled 1/2 & 2/2 could be arrays or any general device, I guess. The transistors labeled 1/4 - 4/4 is a THAT 340 matched array. http://www.thatcorp.com/datashts/300data.pdf
I like these, IMO they work quite well, despite only an Hfe of around 100 or so. But high gain is not their intent here. The 5460 has a Vgs of about 2.2V @100uA. 5089 is a low noise, high gain transistor. http://www.fairchildsemi.com/ds/MM%2FMMBT5089.pdf I have found that, at least for the KSK595's I have, they work great in the 7-10 Vds and 25-50 uA range. I measured Vgs @ 25uA of about -0.35V and Rds of 1500 Ohms. It appears Fairchild MAY have discontinued these, they were 4 cents a piece when I bought them, now they are more than 50 cents and Fairchild appears to be out of stock. Seems to be a common trend with them. That's OK, I have about 300 of them. I would be willing to bet the circuit could work with just about whatever devices are out there, just adjust the resistor values appropriately. The 4091 is like a J111 and has a Vgs @ 25-50uA of around 9V. The ones I have measure Rds of 22 Ohms, and Vp around -11-12V. The Vgs of the J201 I have @ 50uA is about -0.8V. The output from this circuit drives one side of the complementary input J-fet diff of the VAS. My experience with this method of CMRR has been good, the VAS circuit uses this concept as well. I maybe drifting out in the audiophile high seas of absurdness, but I have had interesting and good enough results to not abandon the concept yet. Much to the contrary. Another positive result of this circuit is it limits current which is always a good thing when building solid state circuits, especially since those THAT chips cost like $8 a piece.
Don't suppose anyone else has tried anything like this, eh?
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Old 20th November 2007, 08:51 AM   #9
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Don't you have some white paper and black pen to draw something more brighter.......

You are using balanced drive for your floating bridge amp...
BTW: I have made balanced grounded bridge single supply amp with all N-fets.....
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Old 20th November 2007, 09:21 PM   #10
CBS240 is offline CBS240  United States
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Hi Kanwar

I will work it onto a PC drawing eventually.
Quote:
You are using balanced drive for your floating bridge amp...
There are two of these circuits, the other has the input fed to it's -in node via a voltage divider. The result is an equal balanced signal. These two signals drive the complementary input J-fet's for the VAS section. A floating bridge amp...hope it doesn't float away! The CM feedback loop appears to anchor it well. I have never seen the bias drift. The only trim pots are in the output stages to set their bias. Also the active CMFB bias is not affected by operating temperature. Since the AC doesn't appear to be affected by this bias scheme, I haven't found a reason yet to abandon this idea. Is there a clear reason why this shouldn’t work?

Quote:
BTW: I have made balanced grounded bridge single supply amp with all N-fets.....
I bet it has less transistors than mine. When you say grounded, you mean the bridge input is referenced to one side common since it is single supply or is it floating?
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