I'm studying this kind of CMOS switch:
I have some doubts about some sentences of my book.
"For many analog applications, a switch is used merely to transfer a charge from one node to another and to set a voltage on a high impedance point."
What does it mean "high impedance point"? In our switch I just see a capacitor at the output node, but the impedance of a capacitor is not necessarily high
"With the switch in Fig. 2.17 in the on state, after a transient, we have Vout=Vin hence the drain to source voltage of M1 goes to zero. The MOS enters the linear region and its on-resistance is approximated by
[...] We assume an exponential response of the circuit (we neglect any operation in the saturation region)"
The concept that, after a transient, the drain to source voltage goes (ideally) to zero is clear: indeed, assuming the capacitor initially discharged, the current which flows through the capacitor charges the capacitor and the output voltage increases, thus reducing the drain to source voltage. Moreover, if Vds is very small, the author replaces M1 with a resistor, thus obtaining an RC circuit. What is then the meaning of the sentence "we neglect any operation in the saturation region"?
"The value of the source voltage is equal to the input signal. When the input approaches the gate control, the overdrive diminishes and, at a given level, vanishes. At this point, the on-resistance goes to infinity and the switch no longer operates properly."
Why does the author assume that the source is the left terminal of the mosfet (indeed he neglects Rin, thus the input voltage is equal to the source voltage)? Assuming M1 to be an NMOS, theoretically I don't know which terminal the source is: in an NMOS the drain is at a higher potential than the source, but who knows here which terminal is at higher potential? Indeed, if Vin suddenly increases, the output will be a little lower than Vin, thus the left terminal of M1 should be the drain.
It seems that the author of my book assumes a constant input signal. Can the input be, in general, a variable signal (e.g. a sinusoidal signal)?