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Old 9th June 2004, 09:44 PM   #1
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Default class ab questions

so i have a class ab setup with one pair of output devieces
and they run really hot with no input
i am using to diodes with a 5k resistor from the +50 voltrail to the two diodes
am i over biasiing the diodes and this is why my output pair is running really hot?
do i need and extra set of output pairs
i am using +- 50 volt rails as you can see
also my output devices are
mjw21193 and mjw21194

why are the devices runnign so hot and by hot i mean i cant touch the heatsink because it will burn my bad
i have the diodes on the heatsink also to prevent thermal runaway

and info insight wioould be greatly appreciated
also one last question
i am wondering where everyone one gets there rca and binding post terminals for the final projects
?
thanks
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Old 9th June 2004, 10:51 PM   #2
Steven is offline Steven  Netherlands
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Do you have a schematic diagram?

Steven
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Old 10th June 2004, 02:37 AM   #3
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here is my schematic
the output devices are mjw21193/94 from on semiconductor
the diodes are n4001 eventhoug the picture states different
the emmitter resistors are .33 5 watt
and the other resistor is currently 6k
now what do you think?
need more info?
my rails are +- 50
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Old 10th June 2004, 08:00 AM   #4
djk is online now djk
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Power rectifier diodes have too high a forward voltage to work in that kind of a circuit.

Use a diode connected transistor mounted to the heatsink.
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Old 10th June 2004, 09:55 AM   #5
AKSA is offline AKSA  Australia
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Use a couple of 1N4148 if you want to stick with diodes.

Alternatively, use one diode only and a variable resistor in series. Then you can set the quiescent to whatever you require. Make damn sure that the diode is sitting atop one of the power transistors to prevent thermal runaway.

Even then I'd suggest a Vbe multiplier using a BD139 might be more effective.

Cheers,

Hugh
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Old 10th June 2004, 05:58 PM   #6
sreten is offline sreten  United Kingdom
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Why has no-one pointed out its just an awful output stage ?

Drivers need adding and R3 should be bootstrapped or
replaced with a constant current source, and then the
2 diode biasing is irrelevant, unless its a CFP output.

sreten.
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Old 10th June 2004, 06:52 PM   #7
Steven is offline Steven  Netherlands
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Hi roofingboom,

From your first post it was not clear to me what you were trying to achieve. But sreten is right, the whole circuit will not do what I think you want.
Looking at the +/-50V supplies you want to make an output amplifier able to deliver some 100W in 8Ohm. That means you have to be able to source and sink 4..5A to and from the output. Your output transistors, not being darlingtons, have a current gain around 50. That means that the base current can be as big as 100mA. If your load is resistive that output current must be delivered by the NPN transistor when the amplifier is delivering a high positive voltage (or sinked by the PNP for a negative output voltage). At that moment the voltage across your R3 of 6k will not be much and R3 will only be able to supply a few mA of base current. Certainly not the 100mA that was required.
So, as sreten said, you need drivers in front of the output transistors. You could make a darlington or a compound transistor (CFP). This will increase the current gain and maybe R3 will be able to deliver the required base current. Only for positive output signals this will still be difficult for R3, because the current R3 can deliver will be smaller for higher output voltages. So just when you need the most base current R3 cannot deliver. For that reason you could use a bootstrap circuit or a constant current source instead of R3.
If you choose for an NPN and a PNP darlington output, your bias voltage generator should have a voltage of around 4Vbe or 2.8V. Furthermore this voltage should be temperature dependent and less for increasing temperature. Otherwise the idle current in the output transistors will rise with temperature and eventually get overheated (thermal runaway). The easiest way to make such a bias voltage is to use a Vbe-multiplier, being just a transistor and 2 resistors. The transistor should be thermally coupled to the output transistors.
Have a look at all the circuits that have been posted on this forum and try to understand the functions of all components. Read a book about power amplifiers, e.g. Self on Audio (Self) or Audio Power Amplifiers (Duncan).

Steven
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Old 10th June 2004, 10:09 PM   #8
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i posted a new schmenatic
is this called bootstrapping?
what is the principle behind it and what does it do?
how does it work mathematically and in theory?

can i replace the 2 resistors and capactiro with a current source?
what is better using a current source or bootstrapping?
i am going to eventually going to place a Vbe multipier in the circuit also
i changed the diodes to a different kind and my output transistors are still heating up really bad
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Old 10th June 2004, 10:34 PM   #9
sreten is offline sreten  United Kingdom
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I don't want to appear rude but I've no choice :

Your overheating could have nothing to do with bias, the fact
you've only posted the output stage rather than the complete
circuit means you don't understand why this is possible.

sreten.
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Old 10th June 2004, 11:00 PM   #10
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here is full schmetiac i pieced togehter from my readings
the input is via a cap and resistor to ground making a high pass at 6-7 Hz
i am wokring on making the output stage a darlington
my out[put stage is ghastly overheating
i am still wondering what is worng
emiiter resistor for LTP is 22k
emtter resistor for the driver is 33
the resistors for otuput stage are .33
any help would be great
thanks
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