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ctrlx 11th August 2015 05:50 PM

matching n and p fets capacitances
2 Attachment(s)
cct 1 has caps, based on the spec sheet, with equal value gate R's

cct 2 has a higher gate resistor for the n-device.

i not sure which setup to use.:confused:

Attachment 498102

Attachment 498103

Nelson Pass 11th August 2015 05:59 PM

#2 is simpler and easily adjusted.

Also loads the front end less.


franzm 11th August 2015 09:47 PM

Cct 1 could be prone to local oscillations in the upper MOSFET.

anatech 12th August 2015 02:52 AM

Hi ctrlx,
Listen to Nelson. He's been there and done that - plus a lot more. Setup #2 is very common in commercially built amplifiers. It works well.


ctrlx 13th August 2015 12:30 PM

ok, thanks for the replys, will go with cct 2.

anatech 13th August 2015 04:19 PM

Let us know how you made out, or if you run into any problems we can help with.


ctrlx 4th September 2015 06:34 PM

does C4 need a series resistor?
i've read that c4 is a "feedforward" compensation cap, and as such may need an resistor in series with it.

i dont know if it needs one, and if it does what value it should be - any advice regarding this will be greatly appreciated.

jackinnj 4th September 2015 07:15 PM

A bit taxing to the amplifier as you need to run it pretty hot -- you place a trimmer in one of the gate circuits, say 500 ohms, set to mid-value. Using a square wave source, adjust the trimmer until the amplifier "slews" identically for the N and P channels. The square wave source would be about half the input voltage you would normally use for sine-wave testing.

Explained in page 16 of this application note from Texas Instruments:

You probably don't need the protection diodes for the Lateral Mosfets --


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