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Class D Switching Power Amplifiers and Power D/A conversion

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Old 1st September 2004, 04:03 PM   #11
Konrad is offline Konrad  Norway
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Better, but there is notthing that supresses common mode. It may not do any harm, but the opamps driving the diff stage may go into klipping. Then its a problem.
It may bee a god idea to start with a singel amp before you go for a balanced amp.
And the diff is near to transistors, not to exspensive to fix that 'problem'.

In spice all the components are equal, this is not so in real life as you say.
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Old 1st September 2004, 11:23 PM   #12
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Quote:
Originally posted by classd4sure
Hi there,



Unless you get this "The image that you have attached is too big. Please make it no bigger than 800 x 1200." Though.. rotating it might have been good for a laugh.

Open the file in "Paint" and resize it smaller(strech;askew) then resave. File will be smaller.
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Old 2nd September 2004, 12:00 AM   #13
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Quote:
Originally posted by Konrad
Better, but there is notthing that supresses common mode. It may not do any harm, but the opamps driving the diff stage may go into klipping. Then its a problem.
It may bee a god idea to start with a singel amp before you go for a balanced amp.
And the diff is near to transistors, not to exspensive to fix that 'problem'.

In spice all the components are equal, this is not so in real life as you say.
Hi,

Nothing that suppresses common mode? Please elaborate, sounds interesting. There should be no common mode on the feedback signals. Other than that they're far from clipping. 1V differential input max, easily adjusted via the gain resistor in the input stage.

Parts were selected with +- 30 volt rails in mind, not 35 or 40. Parts selected are all that limits it from higher power.

Half bridge versions are in the other thread.

If you have any concrete improvements in mind please post them.


Regards,
Chris
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Old 2nd September 2004, 06:27 AM   #14
Konrad is offline Konrad  Norway
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The triggering point to the upper and lower halves can bee adjusted with somthing like this to improove common mode supression in the diff stage.

The emitters of Q3 and Q12 together to R3.
And emitters Q9 and Q4 connected to R4
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Old 2nd September 2004, 06:52 AM   #15
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Nicely done Konrad
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Old 2nd September 2004, 11:01 PM   #16
Konrad is offline Konrad  Norway
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This seems to work!

Basic circuit without classd4sure's input stage.

The input/controll-cuircuit shown by classd4sure would improove performance as open loop gain also in higher.

Abt the open loop gain have someone tried higher open loop gain?
I mean like replacing the resistors R121 and R122 with smal caps in Classd4sure's input stage ?
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Old 2nd September 2004, 11:17 PM   #17
Konrad is offline Konrad  Norway
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ooobs... one smal drawing error on the circuit above. Thouse analysing will see it instantly.
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Old 2nd September 2004, 11:24 PM   #18
JohnW is offline JohnW  Hong Kong
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Hi Chris & Konrad,

Nicely done to both of you - a good starting point - and clever CM balance Konrad

Output stage needs a bit more work - to make it faster & hence reduce the switching losses (how about adding a EF buffer with say FMMT617 / 717) - you could also add feedforward from the buffered driver to reduce the effects of circuit capacitances in the level shifter section.

John
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Old 2nd September 2004, 11:36 PM   #19
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Hi,


Still sleeping here must study all the new goodies but, it seems to me there is a "kind" of feedforward already, in fact I couldn't ever get it to work without it.

Chris
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Old 2nd September 2004, 11:57 PM   #20
JohnW is offline JohnW  Hong Kong
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Hi,

In fact giving just a tiny bit more thought, you would only want to speed up Toff (PNP) so something with good Gain at high current like a FMMT717 / 718.

Ton (NPN) could be handled by a “limited” Current / Gain device – like BC something….

This would result in slower Ton times (nice "Soft" deadtime), and faster Toff times to compensate for Toff Delay.

With the extra EF stage you might also want to AC couple the output devices to swing the Gate to zero – or there about, this would reduce Enhancement voltage, but I would recommend 10V at Max – I normally only Enhance my output devices to about 9V – it’s a balance between RdsOn losses, and increased switching losses due to Toff Delay.

John
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