What does source follower mode mean?

Question

I've been trying to build a CMOS-level XOR gate. I've been told my implementation won't work because some of the transistors are in "source follower mode".

I've tried looking this up, but I can't find a simple description of the term.

In simple terms, what does source follower mode mean?

(Here's the XOR circuit for reference)

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

This is a source follower.

^{simulate this circuit – Schematic created using CircuitLab}

The output will be equal to the input minus the MOSFET's threshold voltage; the source 'follows' the gate. It's a configuration with no voltage gain (i.e. gain ≈ 1), but high current gain. The lack of voltage gain means it's to be avoided in logic circuitry; you want outputs that snap high or low quickly, even if the input goes a bit slowly.

In your schematic, transistors M8, M7, M3, and M4 are source followers.

Answer 2

A source follower configuration is a MOSFET configuration in which the output is taken from the source side of the MOSFET. When operated as a linear amplifier, the output potential on the source of the MOSFET is approximately \$V_{th}\$ less than the potential at the gate. So, the output/source signal "follows" (at a lower level) the input/gate voltage.

There are indeed MOSFETs in your schematic configured as source followers.