|
When the green Vds drain voltage trace is above the blue (overdrive) crossover trace, then the Mosfet is in Saturation. The wave shape is a good replica of the original signal, and only the green light is on.
When that green drain voltage trace touches the blue crossover trace, (as seen here), then the Mosfet is hopping in and out of Ohmic. Both lights are seen. The green drain wave shape is poor if you look closely.
When the green drain voltage trace stays below the blue crossover trace, then the Mosfet is in Ohmic. Only the red light is on.... then the green drain wave trace remains a poor replica of the a.c input signal of 200mV
The curvature of head and foot peaks is not the same.
Just alter the top left drain resistor Rd 750 Ω to alter the green drain volts Vds
Somewhere near 7V is a good place to put the wave here if you are thinking about a Q position, (≈half of 15v).
You might want to leave that Rd at 750 Ω and adjust the dc Vgs instead.... it is at 5v presently.
A divider of bias gate resistors is often used to attain the necessary Q adjustment instead of this 5v source.
Now you know how to alter some settings for a mosfet to get it into a Q position in the Saturation region, where the wave shape holds good. (ignore the rest of the schematic for now... it is there just for the above demo).
When the green trace Vds drops to touch the blue crossover trace of (Vgs-Vth) , or (Vgs-Vth) rises to touch Vds, then that touching is the crossover region point .... at the 'knee', on a graph of Id vs Vds.
That is when the mosfet gate bar alters between half and full, and the lights change in this schematic.... and the wave shape becomes distorted.
You can also view clipping, by altering that wave amplitude as reqd. until the wave peak hits the 15v top rail. See also the effect of altering the Vdd supply of 15v
The Mosfet Vth here is 1v in the mosfet settings, so (Vgs-Vth) is 4v +/- 200mV peak presently, and that blue trace will alter here automatically if you alter the present d.c. Vgs of 5v at the bottom far left source.
If you alter the 1v VTO Mosfet setting, then also alter the 1v source at the schematic bottom to be the same.
You could now highlight the drain current Id and see how that is affected by the above alterations, but don't be concerned about the Id wave touching the (Vgs-Vth) crossover wave.
If you wish to keep the Nmos in Ohmic ... full bar seen (for full switching) then adjust any of these:
1: Increase the Vgs (or lower Vth in EC) to increase Vov, ... ie increase the Overdrive Volts.
2: Increase the Load drain resistor Rd ... to lower the Vds.
3: Decrease Vdd ... to lower the Vds.
To keep the Nmos in Saturation ... Half bar seen (for small signal amplification): do the opposite adjustments for those three.
look also at part 6 of the 'Fun Mosfet' below....
http://everycircuit.com/circuit/4940688430202880
Keeping the mosfet in the required region can be considered here with the earlier 'Fun Mosfet' article http://everycircuit.com/circuit/5245744790634496
Your board setup for Ohmic (not Saturation) Full Switching ... see Simulation Guide here: http://everycircuit.com/circuit/6260747729633280
|
