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Old 11th August 2015, 06:50 PM   #1
ctrlx is offline ctrlx  United Kingdom
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Default matching n and p fets capacitances

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.

cct 1.JPG

cct 2.JPG
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Old 11th August 2015, 06:59 PM   #2
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#2 is simpler and easily adjusted.

Also loads the front end less.

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Old 11th August 2015, 10:47 PM   #3
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Cct 1 could be prone to local oscillations in the upper MOSFET.
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Old 12th August 2015, 03:52 AM   #4
anatech is offline anatech  Canada
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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.

-Chris
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Old 13th August 2015, 01:30 PM   #5
ctrlx is offline ctrlx  United Kingdom
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ok, thanks for the replys, will go with cct 2.
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Old 13th August 2015, 05:19 PM   #6
anatech is offline anatech  Canada
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Let us know how you made out, or if you run into any problems we can help with.

-Chris
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Old 4th September 2015, 07:34 PM   #7
ctrlx is offline ctrlx  United Kingdom
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Default 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.
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Old 4th September 2015, 08:15 PM   #8
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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:
http://www.ti.com/lit/an/snaa045a/snaa045a.pdf

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

Jack
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