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Old 27th December 2009, 04:02 PM   #1
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Default MOSFET Power Amplifier

I found another schematic on a Power Mosfet curcuit,and I made a few modifications,and I wanted some opinions on the design,and maybe some pointers on improving the design.

Here is the list of semiconductors,I don't have resistor values,other than the source resistors.


Q1,Q2,Q3 MPSA43
Q4,Q5 BC546
Q6 MJE340
Q7 MJE350
Q8,Q9 IRFP240
Q10,Q11 IRFP9240


Source resistors are 10ohm 3 W wirewound.

Thanks
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Old 27th December 2009, 04:24 PM   #2
jaycee is offline jaycee  United Kingdom
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Driving 2 pairs straight from the VAS will give you poor results. Add driver stages.
The bias network is not suitable for vertical MOSFETS, use a VBE multiplier.
For 40V supplies you would be fine with one pair of IRFP240/9240

Look at Quasi's NMOS200 amp for a good example. That is an N-channel only (quasi complimentary) amp but could use N and P pairs with a small change.
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Old 27th December 2009, 05:44 PM   #3
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Also, I would suggest adding a equally large resistor to the non-active leg of the input diff amp. This leg can feed another transistor connected as a part of the VAS stage to make it more linear. And yes, adding drivers would certainly improve the slewrate about 10 times or so.

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Old 27th December 2009, 06:48 PM   #4
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For 40V supplies (but not more) much better pair would be IRFP240/9140. You will need some version of "Vbe" circuit to control bias for HEXFET-s. Your version (just a trimpot without LED and diode) might be OK for laterals but not for HEXFET-s.
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Old 27th December 2009, 08:11 PM   #5
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I suspect you will get oscilation on the outputs without a Miller cap on Q7.
I made a similar design and found that problem. 220pF sorts that problem from Q7 c to b.
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Old 27th December 2009, 08:19 PM   #6
russel is offline russel  Europe
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10 Ohm source resistor seem rather high, a bit less perhaps.
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Old 27th December 2009, 08:32 PM   #7
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Quote:
Originally Posted by russel View Post
10 Ohm source resistor seem rather high, a bit less perhaps.
I use 0R22 with 3 pairs of MOSFETs.
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Old 30th December 2009, 01:43 PM   #8
Kay is offline Kay
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it's a modified circuit from redcircuits.com
?

60 Watt MosFet Audio Amplifier - RED - Page100

why do you change it?
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