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This is sentence from Design of Analog CMOS Integrated Circuit that I don't understand.

Note that the current source injects current into ground or draw current from VDD. In other words, only one terminal of each current source is "floating".

Does this mean that, for a MOSFET to be used as a current source, the source of an NMOS should be connected to ground and the source of an PMOS should be connected to VDD? No other cases are possible? I mean for example for NMOS, the source may be connected to something else than to ground. enter image description here

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  • \$\begingroup\$ FWIW, he seem to be talking about this book. \$\endgroup\$ – Fizz Nov 17 '15 at 2:14
  • \$\begingroup\$ well, sorry. I forgot to add the picture. I just modified it with picture. Thanks. \$\endgroup\$ – anhnha Nov 17 '15 at 2:24
  • \$\begingroup\$ Can you clarify more your question? Can you imagine by connecting something between the Source and GND of a NMOS you make it difficult to drive? For example, imagine if a resistor is connected between the source of PMOS and GND. The voltage over this resistor would depend on your current, then the gate voltage will have to be conditioned for the minimum Vgs to drive the MOSFET. \$\endgroup\$ – Gabriel Rezende Germanovix Nov 17 '15 at 3:22
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"... No other cases are possible?" Other cases are possible. You can design a floating current source

floating current source

Note that you really have to have a feedback mechanism that ensures a correct operating point for all the devices. Please consult this paper A low-offset low-voltage CMOS op amp with rail-to-rail input and output ranges and many more by searching "floating current source"

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The FETs don't require that the source be connected to a supply, but it's easier to create a circuit that generates a specific current that way.

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