I have some doubts about different configurations of differential pair I have seen in many cases:
In these schemes, biasing network for the gate is not shown for simplicity. All of them are excited by an input differential signal, but
In 1) two different opposite signal sources are applied to the transistors. The output is taken between the drain of one MOSFET and GND. It is clear for me.
In 2) the pair receives a differential signal in a different way, by using only 1 voltage source. I have the following doubt: how can that signal be amplified by the MOSFETS, if they are not referred to GND? How can that circuit work?
3) is like 1), but the output signal is taken from both drains and not from 1 drain and GND. This allows to double the gain. But, despite that difference, is there a reason to prefer 3) on 1) or viceversa? I'd say that 3) is better because noise on GND is not transmitted to the output signal. Is it true?
4) is like 2), but the output signal is taken from both drains. I have the same doubts said about 3) and 1).
I have seen that the current source is often replaced by a simple resistor. In this case, it is important for it to be very high in order not to have common mode gain. But, in general, which is the reason for choosing a current source between Source and GND, or an high resistor between Source and GND?
Sometimes this kind of circuit works with VDD and -VDD, so with dual voltage supply. Which is the reason for choosing it or a single supply?