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Old 24th March 2011, 05:07 PM   #1
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Default F1 Feedback

How important, or rather, how unimportant, is the small amount of feedback on the F1? I am looking to build one, but I wanted to use input transformers rather than the input caps. In order for this to work, the gates of the two FETs must be at the same potential. This means biasing them from the same voltage, which means biasing from B+ (a resistor divider to set the bias point then connect to each gate via a 500K resistor) rather than their own drains, thus removing the feedback. I have done this on other designs (diferential followers) and it works fine, but I thought there might be something sneaky going on with the F1 that could lead to unanticipated issues. I'll still bias the tail CCS from the drains.
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Old 24th March 2011, 05:16 PM   #2
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If you match the Fets, they will balance by themselves,
but probably you should allow for some ability to trim
the values a bit. Since the F1 uses some small resistance
on the Sources of the Fets, you can trim those values
without touching the input balance.

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