Current source Loads.

Question

Hi, 
 Recently I came across a topic"Common source stage with current source as load" while going through single stage amplifiers design using MOS. I came across a statement that in Common Source stage, "PMOS is more preferred to used as current source load than NMOS". I would be grateful, if someone please explain me the actual reason of that statement. Kindly mention the reference link if any. 
 Thanks in advance!!!

MarkThoren · Accepted Answer

Sorry looks like this got overlooked - in a VERY small nutshell, from the perspective of a non-I.C. designer: If you use a PMOS at the "high side" of a circuit node, you'll be "looking into" the drain, which has a higher impedance than the source. That is, the opposite of a source follower, which has a low impedance. 
 
 The complement of this is that an NMOS is preferred as a current SINK. 
 If you want to get a better feel for this idea, make a test circuit in LTspice. Or, build this circuit: 
 https://wiki.analog.com/university/courses/electronics/electronics-lab-6m 
 This is the opposite case, using an NMOS as a current sink. And note that LTspice files are provided. 
 -Mark)