How to determine load capacitance?

[Syukri]

Hi...I need help here. CAn any one please tell me how do I to know the value of load capacintance on given circuit of NAND and diff. amp?

Is the equivalent value of the whole circuit capacitance or the Cgs on pMOS and Csg on the nMOS connected to output? Or others..

 

[pengboy]

because the circuit is for digital signal, so it is not small signal operation.also remember for large signal operation the CL is nonlinear. but in digital circuit for a quick simulation, we usually define the equivalent CL.

therefore CL is not the same as differential amplifer. and also the equivalent CL changes depending on the operation of the circuit.

using the second figure as example:

assume Vin is from 0 to VDD and Vout is also from 0 to VDD
the CL of one terminal would be:

Cgs+2*Cgd

the factor of 2 is bebause that When vin from 0 to VDD, the vout is from VDD to 0.
so voltage change across gate and source is 2*VDD. so the eqivalent cap will be double the Cgd.

as you can see, it is much simplier compared with small signal amplifier where the factor is determined by the miller effect.