Electronic – Slope of VTC for FET

First, \$v_{in}\$ is an AC signal voltage while \$V_{GS}\$ is a DC bias voltage. These are treated separately.

From a small-signal perspective, the current source (lower FET) is effectively an open circuit so there is effectively no voltage division for the AC signal. This is why \$v_{in}\$ appears at the output node unattenuated (assuming the output node is connected to an effectively open circuit).

From a DC bias perspective, the gate of the top FET is at (presumably) zero volts so the source, which must be more positive than the gate, must be at 2.8V (using the values in the book and assuming the FETs are identical).

However, note that this is not \$V_{GS}\$ since, in fact, \$V_{GS}\$ for both FETs is -2.8V. In other words, the voltage at the source of the top FET is $$v_{in} - V_{GS}$$

If you wish to avoid holy wars you'll need to avoid making simplistic and incomplete statements :-).

Bipolar transistors are current driven.

MOSFETs are voltage driven.

In both cases the spread of parameters during manufacturing is such that a circuit will almost always rely on feedback to produce a given voltage or current gain.

MOSFETs tend to be slightly more costly at the very bottom end for "jelly bean" applications. But, for switching more than a few ~100mA, MOSFETs are usually as cheap or cheaper than functionally equivalent transistors, are easier to drive from a uC (microcontroller) as a digital switch than bipolar transistors and tend to have very significantly superior on characteristics.

An "on" bipolar transistor exhibits a saturation voltage. This can be several tenths of a volt and to get it much under 0.1V usually requires a high base to collector current ratio that is undesirably high. At 1 A a 0.1 \$V_{sat}\$ (saturation voltage) dissipates 0.1 W and is the equivalent of a R = V/I = 0.1/1 = 100 \$m\Omega\$ transistor. But at 10A the figures are 1 Watt dissipation and 10 \$m\Omega\$. The 0.1V is very difficult to achieve at higher current levels.

The \$R_{DSon}\$ (Drain-Source on resistance) of MOSFETs is typically under 0.1 \$\Omega\$ and you can get devices with 10 \$m\Omega\$ or even sub 1 \$m\Omega\$.

As switching speeds rise MOSFETs need a gate driver to charge and discharge the gate capacitance. These can be relatively cheap.

More soon ....