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Mon May 26 10:37:54 EDT 2014

Headroom for LM324N / LM13700 / 2N7000

LM324N needs about 2V at V+.  This space can be used to also place
some other control hardware for e.g. linearized FET resistors
referenced to ground.

For the LM324N with 5V supply we have about 3V headroom, placing the
midpoint at 1.5V.  Many USB usb don't make it to 5V really.  What
about setting midpoint at 1V, which gives a +-1V swing.  Should be
enough.

Is there any limitation on output voltage for the LM13700?  Output
stage are a diode drop + a saturation limit on both sides, say 3 diode
drops, 4 in total is (* 0.63 4) 2.5V.  This could work but really
needs the signal ground to be in the middle of the larger supply node.
Probably will end up with multiple virtual grounds.

The 2N7000 MOSFET needs about 3V.  Designing the circuit around large
R_DS makes it possible to stick close to that threshold.




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