... The output voltage of the amplifier will drive a sensing PMOS and a charging PMOS (in the VCO)of 6*12u/1 and 3*15/1.5 each. To compute the load capacitance on the amplifier, I ran a dc on the oscillator to find out the Cgs+Cgd+Cgb of the PMOS's, (input capacitance), but I am not sure I had the right answer because of the sign conventions.
The simplest way to find the
total capacitance c at an individual node is to inject a
unit ac current into this node and measure the resulting voltage. Connect an
idc current source with
AC Magnitude=1 via a huge cap (e.g. 1F) between GND and the node to be measured. Then run either an
ac or an
xf analysis over an appropriate frequency sweep range. Both analyses use a linearized model, hence the value of the current doesn't matter; a unit current value (1A) results in a voltage value which corresponds directly to an impedance (Ω) value at this node; kV -> kΩ.
For the
ac analysis, plot the node's voltage vs. frequency. Note the low frequency value of this voltage (resp. impedance)
Z0, where the capacitive effect isn't yet involved. Then note the frequency
f, where the voltage (resp. impedance) has decreased to the 1/√2 - fold (or -3dB) of the low frequency value. At this frequency
f, the capacitive node reactance (the imaginary part of the impedance) has the same value as the low frequency resistance (the real part of the impedance)
Z0 (remember: real & imaginary values are added quadratically). Hence
c = 1/sf*Z0 =
1/(2πf*Z0) .
After an
xf analysis, from the ADE menu select
Results ->
Direct Plot ->
Main Form ... In the
Direct Plot Form menu, choose the Function
Transimpedance, then either
Magnitude,
Phase or
dB20 (with
Imaginary I had no success). You'll be asked to
Select Current Source on schematic... Click the current source and you'll get the appropriate curve. The 1/√2 , the -45° or the -3dB points should give you the same frequency
f as from the
ac analysis.
This is a standard method to analyse the total cap from an individual node to GND (also possible between 2 different nodes).