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Old 8th November 2004, 07:38 PM   #1
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Default Comments on unusual bridge amp?

Hi Everyone,

I have been trying to design a symmetrical bridge amp and have come up with an (I think) unique design. I would like to discuss this design if you have anything interesting to say I would like to hear it.

I tried to make the input stage fully symmetrical but I am not sure if there are better ways to do it. Any advice? Is my input ground wrong?

I know the output transistor bias is non-optimal, I left that out for clarity. But the way the compensation cap is set (like I remember from NDFL amps) seems to help remove crossover distortion at high freq, so maybe I might not even change the bias.

What input voltage should satuare the output? I figure a bit less than 1v, right?

You can see a GIF of the circuit made with LTspice at:
http://www.deleveld.dds.nl/bridgebjt.gif

Thanks for any comments,

Doug Eleveld
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Old 9th November 2004, 12:22 AM   #2
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Hi Doug,
I would suggest that if symmetry is the goal, you should scrap the quasi-complementary output circuit. This only complicates things because the signal for driving the positive half is a follower, and the negative half is a voltage amp. Use complementary pairs. There are many devices to choose from better than 2N3055. Your VAS stages could use current sources, as does the output(& driver) bias. Optional of course, but will help with stability. Does this circuit clip at equal voltages, positive and negative? If not, bridged output will be limited by the lesser.

You could use two seperate differentials for the input, one for each channel, driven by a phase splitter circuit. LTP is not required, but a single transistor input stage has it's drawbacks. I don't see how this circuit would make two equal but opposite signals.
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Old 9th November 2004, 12:08 PM   #3
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I guess its a fine point but I want looking for symmetry in the NPN, PNP sense, I was looking for it in the bridge sense. NPN/PNP symmetry looks nice on paper but in reality, even for 'matched-paírs' they are different enough not to be truly symmetrical. But a bridge is truly symmetrical even for quasi-complementary pairs.

Another advantage of bridge amps (I think) is that they can have low suseptibility to supply line variations. This means you can use lower power supply capacitance without causing any hum. With the shematic now even 1 volt p-p at the supply causes almost nothing at the output.

You make a good point about the clipping, I am worried a bit about the clipping. The action of Q5 is to hold the average of the two outputs at some steady value. This occurs even if one side of the bridge has clipped and this means a 'fake' clip for the other side. The 'fake' clip means unwanted dissipation and lost overhead.

For some bridge amps you can make the gain of one side of the bridge to be higher than the other. So when the high gain side clips the otherside still has some headroom. The output sees gain-halfing instead of hard clipping. This might sound better than hard clipping but I dont know. I would have liked to be able to do that with this amp but the use of Q5 makes this impossible. I'm reluctant to replace Q5 with a more conventional tail beacuse 1) I have to have extra circuitry to make the outputs sit at 50% of the rail. 2) its the only think that makes my amp a bit original :-/

I wanst really planning on using 2n3055 its just the only big power transistor in LTspice. I was thinking of using the TIP series that I used to use in canada, but I'm not sure if there are readily available in europe where I live now. Any suggestions for a good high-current darlington power transistor?

Thanks very much for your comments!

Doug Eleveld
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Old 9th November 2004, 10:04 PM   #4
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I see where your coming from now. But I do have a question...should the input be a balanced input?(+ 0 -). If the input was (+Vin---GND--- -Vin) each input is referenced to ground instead of each other. Then the inputs could be made fully symetrical.
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Old 10th November 2004, 06:42 AM   #5
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Yes it would if I thought I could get balanced inputs to work with my CD player. It's just standard unbalanced outputs. If I had balanced output from the CD I would have put the ground at a more usual place (-ve supply).

Bu I havent built it yet so there could well be problems with how the input grounding is.

Thanks,

Doug
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Old 10th November 2004, 08:43 AM   #6
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well interesting amp

i have few questions though
1. i donot understand how you want to avoid crossover distortion- at first look it seems pure class b, it us tricky somehow i guess
...confusion...
2. at suppy you intend to use one huge electrlitic cap from + to - rails, right ??? classic use of two caps and ground would fail
here- you would amplify 0 potential.
3. aside from symetry, how you intend to keep low dc through speaker?? as the result of using 1% components-worst case may produce huge dc

sorry if my questions are stupid- i may not understand some ideas

cheers
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Old 10th November 2004, 09:31 AM   #7
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Well there certianly might be crossover distortion. But in simulations at least C2 and C5 really lower crossover distortion, especially at HF. Notice how the compensation caps C2 and C5 go to the base and the output. Normally they go between the base and collector. At high frequency they are local feedback loops linearizing base-output transfer function. So it tends to lower crossover distortion. I got this idea from an old series of articles called NDFL (nested differential feedback loop) by a name I dont remember (Cherry?). Some people claim that it can cause oscillations but if I remeber correctly the zobel networks at the output are really necessary to reduce this.

Yes the supply is a single cap. Rememebr that caps store energy compared to the square of the voltage. So if you have +40 volts and say 1000uf you have more energy stored than 2 1000uf caps at +-20 volts. And the single cap is going to be cheaper too.

DC at the output depends pretty much only on R4 and R10 and the Q6,Q8 matching. I'm not really sure the best polace for a DC offset trimmer, maybe ontop of R23.

Doug
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Old 10th November 2004, 12:02 PM   #8
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Hi Doug,

Nice to hear about bridge amps in professional manner from u.


Your Point regarding QUASI-COMPLIMENTARY is absolutely complainces with professional standards.

Their is far better symmetry obtained using NPN/NPN devices rather than using NPN/PNP devices.
Complementary doesnt stands tall in pro environment very well.

We also use bridge topology exclusively in our amps with only N-channel Mosfets at output.

ur comments regarding advantages of Bridging are quite right.

I have studied ur circuit in detail,
Acc to me some issues must be dealt with,
Biasing Network
Balanced input
Use higher bandwidth devices at output such as MJL21196 or use Mosfets
Feedback network
Replace Bootstrapping with Active Current source.

Hope that it helps
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Old 10th November 2004, 12:34 PM   #9
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Pure " class B " - is it joke ?
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Old 10th November 2004, 03:43 PM   #10
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Quote:
Originally posted by Upupa Epops
Pure " class B " - is it joke ?

UPUPA friend I have pointed out the Class-B stage as biasing network statement , because no professional amp is pure class-B , but Class AB .

hope this helps
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