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Old 27th December 2005, 08:15 AM   #1
bob123 is offline bob123  United Kingdom
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Default Mosfet output stage help (please)

Hi all
I have just built a bread board class D amplifier, It works fine with bipolars on the output (npn and pnp), When i put mosfets on there it gives a wierd wave and starts misbehaving, I know this is due to the input capacitance of the mosfet. How can i simply get round this as i dont have a clue as to driving mosfets at high frequency.

The FET are

The switching frequency is 200Hz and they are currently driving by the output of a comparitor, I have added a resisor and diode to give some dead time as they were getting hot.

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Old 27th December 2005, 09:00 AM   #2
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Post your schematic..

It is very doubtful that your comparator will drive the mosfets directly.

You'll probably want to keep your previous NPN/PNP output stage as a low impedance buffer (high current driver), and use that to drive the gates, possibly each through a level shifting capacitor.

Good luck.
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Old 3rd January 2006, 03:25 PM   #3
nitrate is offline nitrate  United Kingdom
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Hi there,

200Hz ? I do hope you meant 200Khz LOL

Post the schematic so we can help...


P.S Hope your not planning on plugging this one into the mains
The perfect amplifier is a piece of wire with gain....
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Old 3rd January 2006, 08:38 PM   #4
bob123 is offline bob123  United Kingdom
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Well i dont have time to model it on LTspice but heres a simalar crocclips model, I have changed a few things such as

Due to the lm311 being a comparitor i have put a 1k resistor from the output of the LM311 comparitor to the +15v rail.

The component values on this are quite likly to be wrong but you get the idea, It is not the exact cicuit that i have built.

As for the output stage using transistor as is on the circuit the transistor output works but does not give a square wave, It has a dip in the middle of the wave. Does anyone know what that is?
When i put mosfets on it the whole thing gives a very odd wave shape.

What can i use to drive the mosfets as i assume i need lower output inpedance for the base of the mosfets to drive the capacitance.

I am using IRF530 and its complement.

Oh and yes i did mean 200KHz
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Old 3rd January 2006, 09:18 PM   #5
bob123 is offline bob123  United Kingdom
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The circuit:
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File Type: jpg classd.jpg (29.5 KB, 248 views)
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Old 3rd January 2006, 11:48 PM   #6
kartino is offline kartino  Indonesia
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Hi Rob,

You need mosfet driver to switch the mosfet fastly.

Here is the simplest driver for P-N mosfet that may suitable to directly joint with your comparator.

Good luck!
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File Type: pdf n-p_driver.pdf (23.3 KB, 186 views)
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