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Old 5th November 2009, 06:42 PM   #1
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Default A question for Mr Pass on mosfet distortion

In the article of Papa "Leaving Class A" in fig. 3 is there the figure of the distorsion for 7 bias current applied to a generic push-pull mosfet output stage. The question is: the measure are made on a simple push-pull follower like the one in fig. 1 or somethin different? at what frequency? what type of mosfet? Matched mosfet or a generic couple?
Can that measure be applied for reference to an output stage like the one in the picture below? The bias in this schematic is 200 mA per mosfet.

Click the image to open in full size.

Thank you.
Michele
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Old 5th November 2009, 10:04 PM   #2
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As I recall, it was simply an F4 output stage. The principles
apply pretty much to any Mosfet complementary followers.

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Old 5th November 2009, 10:36 PM   #3
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Thank you Mr Pass.
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Old 8th November 2009, 06:41 PM   #4
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Another question.
On the simulator the use of 3 couple of fets instead of just one seems to be worse for distortion, i think because of the lower bias for each fet (0.5A instead of 1.5A).
Is the situation different in the real world?
I mean, why the use of 3 couple instead of one?
How can this amp clip at more then 20V with just 22V on the rail?
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Old 8th November 2009, 07:25 PM   #5
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There are two effects which tend to cancel. The transconductance
of each Mosfet increases with current and the transconductance
of parallel devices is that of each divided by the number of devices.

The decision to use multiple devices usually has more to
do with maximum dissipation and current.

The amp output can be driven to within a couple of volts of
the rails unless the drive is limited, which it was not.

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