NMOS Inverter vs CMOS Inverter -- transfer characteristics

[yannik33]

Why does a CMOS inverter go down to gnd voltage but a NMOS inverter doesn't?

Source: https://cnx.org/content/m38528/latest/

 

[crutschow]

Because in the NMOS inverter the top transistor is always on (rather like a resistor) so the bottom transistor has to sink that current to ground to pull the output low. That current through its ON resistance generates the voltage that is shown

 

[yannik33]

Thanks you very much. The following Circuits consists of the NMOS Transistor and a PMOS current source - why doesn't the voltage in this circuit also go down to gnd?


1. NMOS is off
PMOS is in saturation

2. NMOS enters linear region
PMOS still saturated

3. NMOS and PMOS are saturation

4. NMOS in saturation
PMOS enters linear region

5. PMOS is off
NMOS is in saturation

Shouldn't Vout be on GND level?

 

[Audioguru]

The attachment in your last post does not work.

 

[yannik33]

ok, i updatet the post.

 

[Audioguru]

I have never seen symbols for Mosfets like yours. Usually the arrow is on the middle line and is in the reverse direction to yours.

A Cmos inverter has a P-channel and an N-channel Mosfet as common source and their gates are tied together. Then when the input is high, the N-channel Mosfet is turned on and the P-channel Mosfet is turned off.
When the input is low then the P-channel Mosfet is turned on and the N-channel Mosfet is turned off. No current source.