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Old 10th March 2010, 07:00 AM   #51
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Quote:
Originally Posted by gaetan8888 View Post
Hello apexaudio

Any special reasons to use a design with BJT transistors and not Mosfet for the output transistors of your APEX H900 amp ?

Thank

Bye

Gaetan
Reason to use BJT to get more power, and there was many limitation, see thread APEX MOSFET.
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Old 10th March 2010, 01:06 PM   #52
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FETs just don't seem to work well above +/-100V rails for a number of reasons. If you want "complementary", and need something bigger and badder than the IRFP240/9240, you're SOL. The driver circuits for all-N-channel outputs are more suited to switching than linear amplifiers. Paralleling more and more devices results in stability issues that just don't happen with bipolar emitter followers. Even with class D, there is a significant penalty in device performance going from 200V FETs to 250's, and above 250V it gets even worse.
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Old 10th March 2010, 03:12 PM   #53
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Originally Posted by wg_ski View Post
That one does have too many parts. A spearate commutating diode for each output????? That seems silly.
At first it does, but done that way there is no need for emitter resistors for the upper rail transistors. So you get to choose - one diode per transistor pair or one emitter resistor per pair plus a big common diode that needs to be heatsinked. If each pair gets its own diode you should be able to get away with large axial types without extra heatsinking, like what is used in that circuit. Increase airflow until cold enough
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Old 10th March 2010, 06:33 PM   #54
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No need to use BJT for rail transistor.
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Old 10th March 2010, 08:24 PM   #55
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It's a matter of "class G" vs "class H" rather than BJT vs MOSFET.

A high power "class G"/linear lifter will need more than one high rail transistor and emitter/source resistors (or separate diodes) irrespective of their type as the rail lifter needs to dissipate a lot of power (but the stress on the inner devices is lightened).

"Class H"/rail switcher circuits like yours get away with fewer high-dissipation transistors and can often use only a single switching FET in the rail switch so they are a bit more economical, especially for high powers. I like your use of IR2117 in the rail switches: simple and low component count!

Last edited by megajocke; 10th March 2010 at 08:27 PM.
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Old 10th March 2010, 09:37 PM   #56
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I like to say that I have best relation power/square inch of PCB There is more space for PSU
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Old 10th March 2010, 11:12 PM   #57
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Yes, quite compact even though you neither use a double-sided board nor SMD components!
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Old 11th March 2010, 02:46 AM   #58
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by apexaudio View Post
I like to say that I have best relation power/square inch of PCB There is more space for PSU
You cheat. You used a jumper wire between the NPN and PNP banks. Just make sure you don't run it parallel to or close to your feedback resistors or input trace. And I wonder if the power traces need to be wider.... Remember how many amps are being pushed through them. Even if the average heating isn't an issue with 1/8 power pink noise, it's just evil to lose voltage swing in a PC trace.
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Old 11th March 2010, 03:31 AM   #59
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hi wgski
greetings i did not get your point can you explain please as i am trying to make
pcb hand etched so lots of hard work would go down have managed to get all parts
except commutator diodes can 2 byg 80 be used in parallel
thanking you
andrew lebon
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Old 11th March 2010, 03:58 AM   #60
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I will post schematics. I dont espect any interested about that kind of amplifier in DIY.
too much of interest
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