what is output capcitance of CMOS?

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can any one tell me what is the meaning of output capcitance of CMOS.
i am designing CMOS multiplexer....
i want to find the Co of that cell...
is is depend upon the load capcitance CL?

or what extatly it mean?


[/i]
 

in depeneds on your design and used chips to get better idea pick a data sheet for any CMOS multiplexer like 4067 and see
 

It comes from CJ0 and CJSW(junction capacitance of transistors' drain). their value can be found in .model card of the process.
larger transistors can drive larger load capacitances but they have larger output capacitance. smaller have lower output caps but they have lower driving power. therefore thereis a optimum transistor size for the fastest response time which depends on "load capacitance", "process minimum feature size" and "VDD".


BEST!
 

It contains the gate capacitance which can equivalent to the output to ground capacitance by miller effect, the diffusion capatance,the wire capacitance ,and the load capacitance.
 

tom_hanks said:
can any one tell me what is the meaning of output capcitance of CMOS.
i am designing CMOS multiplexer....
i want to find the Co of that cell...
is is depend upon the load capcitance CL?

or what extatly it mean?


[/i]
Click to expand...

It is normally the output capacitance of the cmos device...(it can be input capacitance too)....it can affect the switching time...(and rise time of a driving signal).... and it is normally very low....or designed to be as low as practicable......it depends on the cmos construction of the device and the load it is driving. lower impedance or low resistance loads should decrease .....higher impedance or resitive load increase this capacitance.

It is important but if you are driving another device of the same cmos family it should be taken care of as far as timing is concerned......if you are driving a transistor for example then it does become importantant.

Bernard
 

Co of Standcell can be find on data sheet, it depends on the fan out capability.
 

you can find the meaninig of CMOS output cap in
analog cmos integrated circuits by razavi
 

when u design something like the multiplexer, u have an output terminal, from this terminal u find some parasit capacitances like Cgs, is the most representative. Co is the capacitance thet your transistors must to charge to get the response on the output, and those capacitances comes from parasit capacitances.
 

visit

academics.vmi.edu/.../Research/IC_Datasheets/opamps/LMC660 (4 R-R).pdf
 

CFET = CDn + CDp
- parasitic internal contributions of the diffusion layers
Cout = CFet + CL
- total output load capacitance that the gate must drive

with CL - load capacitance (i.e. wiring capacitance, other load gates, etc)
 

hi tom_hanks, you are designing with...
 

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