I have a 2-input NAND gate spice netlist (generated from a Tanner Ledit layout) where I have to find each input's capacitance and the output resistance. I am to use a 1nF load capacitor and a 10 Kohm for the calculations. I honestly have no idea how to do this using spice and could really use some help getting started. Here is the netlist:

*** Extration Obtained 
.include Model.txt

*       1 = Vdd (23.999,44)
*       2 = Gnd (28.999,0.5)
*       3 = Nand_out (20.999,21)
*       4 = A (46.999,19.5)
*       5 = B (49.999,24)

M1 6 4 3 2 NMOS L=2u W=10u AD=70p PD=34u AS=55p PS=31u 
* M1 DRAIN GATE SOURCE BULK (30.5 5 32.5 15) 
M2 2 5 6 2 NMOS L=2u W=10u AD=70p PD=36u AS=70p PS=34u 
* M2 DRAIN GATE SOURCE BULK (39.5 5 41.5 15) 
M3 1 4 3 1 PMOS L=2u W=10u AD=80p PD=38u AS=110p PS=62u 
* M3 DRAIN GATE SOURCE BULK (30.5 27 32.5 37) 
M4 3 5 1 1 PMOS L=2u W=10u AD=110p PD=62u AS=80p PS=38u 
* M4 DRAIN GATE SOURCE BULK (39.5 27 41.5 37) 

* Total Nodes: 6
* Total Elements: 4
* Extract Elapsed Time: 0 seconds

Any help would be appreciated!


3 Answers 3


As mentioned by @analogsystemsrf, the capacitance varies, so a .TRAN analysis will be a bit more involved, but since this is SPICE world you can measure it easily in .AC by adding a current source at the input with ac 1 as value and the impedance will be Vin/Iin. Since Iin is unity, you can read the value of the impedance in Volts as Vin:


I didn't use your models, or your configuration (you didn't provide it), or your load (same), this is just for exemplification. The analysis goes from 1 MHz to 1 THz because the capacitances are so small and there are resistances about that cause that zero to appear. The capacitance will be determined by the slope of the magnitude. Also, the output capacitance influences the input, and you can see that the phase after 1 GHz is not quite so smooth. Since the values are not fixed, it leaves room for experimentation, whether you want the input impedance analyzed in open configuration (Vdd disconnected, no load), or statically (as it is now), or dynamically (you'll need .TRAN for that, slightly more complicated).

(answer to the comment) If what you need is a dynamic analysis, here is a possible setup for finding out how the input impedance varies with input voltage:


The ramp at the input is the usual suspect in determining the transfer function. A pulsed input can also be used, but it tends to complicate things.

However, given your description in the OP, the input/output characteristics are usually found by doing an .AC analysis. The resulting values are what you see in the tables in the datasheets. Their dynamic variance is based on that initial value, and it's depicted in graphs according to whatever criteria need to be met. The ramp here was used for the lack of other descriptions.

For the sake of completion, if the ramp at the input has a unity dV/dt, then its derivative will be 1 and the current through the gate will be directly proportional to the capacitance:


If you consider this graph as an example, which value from it would you use in a datasheet as input capacitance?

Reply to the comments below.

I never said the gate capacitance is not a function of voltage, in fact, I said, several times, that it gets more complicated if you choose to analyze it in time domain vs frequency. Do note that this is a . I never said there are datasheets (a word nicely emboldened) for the transistors composing the gates, I said datasheets show Ciss, as an example, which is specific to MOS transistors. You keep saying that you need to determine

the effective capacitance seen by a driver as the gate input voltage changes from Vss to Vdd

while referring to a specific value representing a range of use cases, but that's exactly what I did: determine one value, but I never said one measurement. This is also valid for your saying that I

keep trying to treat the problem as a transfer-function, frequency response question

There's no transfer function involved as the linked answer shows: it's a simple calculation based on a simple .AC analysis. And, BTW, saying that

Matching exactly the value in the .model file should be a red flag. We know that the capacitance varies with voltage, but the values in the .model file are obviously constant.

would warrant a long stare. Yes, it's constant, but, as I said, that value serves as calculations for any dynamic regime. You seem to overlook the fact that, internally, the .model makes use of that value for its equations which vary the capacitance dynamically.

What's more, when I said that using a unity dV/dt ramp at the input will give the exact value of the capacitance as a function of the input, you said it's not true, even though not only I posted my 3rd picture in this answer, but also linked to this picture, showing why it's not so. Here is, again, what happens if you calculate the exact value of the capacitance with an .AC analysis and a sweep of input's DC:


{x} is the DC level and the capacitance is measured on the fixed slope, @1 MHz, because that point is surely dominated by the gate's capacitance. Since the ac source is unity, the reverse of the current though the gate displays the impedance (see waveform label), which can be used to calculate the capacitance (see the .meas). Because {x} is .stepped, the values for the measured c can be plotted and they are displayed in the 3rd window. What a coincidence, the graph looks very similar to the one in my 3rd picture. In fact, I'd say they are exact within float precision, for the displayed points. This not only proves that what I said about the unity dV/dt is true (as expected for a physical phenomenon), but also that your contradictions are wrong.

If you are to actually determine a value (I avoided this, reasons later on), you would use the average of two values, for ON and OFF:

Measurement: c
  step  mag(i(v2))/2/pi/1meg    at
     1  (1.42605e-10,0) 1e+06
     2  (3.92237e-10,0) 1e+06

Their average is ~267.5 pF. But the gate has a transition region, and the gate @6 V is 647.918 pF. If you make the average now, you get ~394 pF. And if you make the average of all that nonlinearity, you get what you see in the picture, ~463 pF. This would be the equivalent of what you said about integrating the values. But the highest value is not at the middle, and its value is quite large there. Also, since it's switching, the edges will not have the same weights as the ON/OFF, so how do you determine the weighting? It turns out that you can live with an unweighted average between the two maximum values. Here is a document showing that the determination for the input capacitance is about the same procedure.

You said your choice would be using a current source and measuring the time it takes to reach Vdd. I said it's not reliable, and here's why:


It takes ~5.61 s to reach ~12 V. The resulting value would be (1 nA * 5.61 s)/(12 V) = 467.5 pF. Compare this with the 463 pF above. This average means considering equal weights for ON, OFF, and transitions, which is hardly the case (also mentioned above). If it is, the ON/OFF times would be about the same, or comparable to the transitions, and the dissipated power would be prohibitive. Using this value would also mean overstating by a factor of almost 2, confusing the user.

But this should have been the conclusion OP would have reached given my words at the ending of the 1st edit. I never said "that value, right there, is the only one you'll need". I said you can use this way to measure, easily since it's SPICE, leaving room for OP to think for him/herself. This was "just for exemplification". It's also the reason I made an INV, not (N)AND, to avoid being explicit. But it's a homework, not a quest for a datasheet, and even datasheets display only one value for the input capacitance.

And, finally, when I said we were talking about different things, I meant that while I said that you can measure the input capacitance with .AC and unity dV/dt, you replied saying that the capacitance is a function of voltage.

  • \$\begingroup\$ Also see this. \$\endgroup\$ Commented May 11, 2020 at 7:22
  • \$\begingroup\$ I would be suspicious of using an .AC analysis. My understanding is that an .AC analysis assumes that the circuit is linear but we know that the input capacitance of the gate is a non-linear function of voltage. My preference is to use a constant current and measure the time it takes for the input to swing from Vss to Vdd. \$\endgroup\$ Commented May 11, 2020 at 15:01
  • \$\begingroup\$ Also, you must make sure that your ac source voltage swing is over the full power supply voltage range and does not exceed that range. Pretty difficult if your source is an ac current. \$\endgroup\$ Commented May 11, 2020 at 15:02
  • \$\begingroup\$ @ElliotAlderson Then what you want is a dynamic analysis (as mentioned in the last phrase). I'll update my answer to include a possible setup, but an.AC analysis like the one I proposed are pretty standard for finding out static values. \$\endgroup\$ Commented May 11, 2020 at 15:08
  • 2
    \$\begingroup\$ Your comments only reinforce the impression that you're not replying to my words, or that you are even reading them. I've made my last reponse in the edit. \$\endgroup\$ Commented May 12, 2020 at 9:23

We created a schematic for the posted NAND2 layout (with the electrically correct connections (treating drain and source as asymmetrical): The NAND2 circuit posted

We could not find any SPICE model cards for a channel length L=2u (that large a process has not been used in integrated circuit design for thirty years or more), so we used BSIM4 model cards for nmos and pmos (and provide a link to those files at our GitHub account) suitable for the NAND2 (and INV) 45nm PDK cell layout (and power supply voltage, operating frequency and load capacitance from the relevant standard cell library).

We write circuit files to run in Ngspice-27. Hopefully the code comments and discussion here will make adapting the methods to other SPICE variations.

MOSFETs in CMOS gates are voltage-controlled switches so it would be inappropriate to drive them with a current source if the intent is to measure gate input capacitance seen by another CMOS driving circuit (that is the goal we have set here). Accordingly, we use roughly the output \$R_{ds}\$ of a CMOS circuit for our standard library 45 nm process, \$1 \, \mathrm{k}\Omega\$, resistor between the pulse generator \$^+\$lead in series with the connection to the NAND2 X1 input (in1) input which is our test target for dynamic input capacitance.

We will use the so-called differential form of the electrostatics definition \$C =Q/V\$ of capacitance: $$ C = \frac{dQ}{dV} $$

Ngspice circuit file implementing a NAND2 (X1) standard cell circuit with BSIM4 model cards (link to those at end of article)


* a less sophisticated simple input capacitance
* driving gate with voltage pulse
* and calculating charge delivered

* meas capacitance of NAND2_X1 gate input 1

* using Ngspice-27 Creation Date: Tue Dec 26 17:10:20 UTC 2017
* using BSIM4 level=54 mosfet models from  process file
* /FreePDK45/ncsu_basekit/models/hspice
* /tran_models/models_nom/NMOS_VTL.inc
* in which we updated the version statement from 4.0 to 4.8
* and enabled RF high speed support

* the W and L dimensions of INV_X1 and NAND2_X1 
* come from NCSU FreePDK 45nm
* used by Christopher Torng 2019 for 
* a 45nm ASIC design kit for mflowgen
* we use ngspice 25 degree C default circuit temperature, 
* and vdd 1.10v per
* NangateOpenCellLibrary_typical 
* Build Date: Thursday Feb 17 15:07 2011 library

* PARAMETERS for command lines, 
* CSPARAMETERS for lines within control section or echos/prints
* refer to ngspice-27 manual for syntax

* Simulation time setup
* desired pulse train frequency in Hz
.param testfreq=1000e6
* printable as MHz
.csparam testfreqPrintable={testfreq/1e6}
* the period is then 1/frequency in seconds
.param testperiod='1/testfreq'
.csparam testperiodctl={testperiod}
.csparam testperiodctl2={testperiod*2}
*.csparam testperiodSmallerForPlotctl={testperiod*0.7}
.csparam testperiodSmallerForPlotctl={testperiod*1.1}
* how many cycles at specified frequency should be analyzed
.param testcycles='5'
* length of the simulation in seconds will be 
* number of cycles times period
.param ttime='testperiod*testcycles'
.csparam ttimectl={ttime}
* how many simulation steps desired:
.param numberOfStepsInSim=100000
* resolution is length of simulation divided by steps
.param transstepsize='ttime/numberOfStepsInSim'

* power supply DC voltage, using 
* Nangate typical corner Vdd=1.10 v, see 
* NangateOpenCellLibrary_typical Build Date: 
* Thursday Feb 17 15:07 2011
.param DCsupplyVoltage=1.10
* want to space multiple voltage plots clearly, 
* so get ceiling of DCsupplyVoltage
.param PlotMultiVoltsSpacer=(DCsupplyVoltage+.9)
* need a csparam form to use in the control section
.csparam PlotMultiVoltsSpacerForCTL={PlotMultiVoltsSpacer}
* similarly, need DCsupplyVoltage param that works in control section
.csparam DCsupplyVoltageForCTL={DCsupplyVoltage}
* for y limit if plot n traces together, say n=5
.param numOfVoltTraces=5
.csparam DCsupplyVoltageForYLIM={PlotMultiVoltsSpacer*numOfVoltTraces}

* pulse train input stimulus parameters

* note rise/fall based on 20% pulse width
.param RiseFallPercent=0.20
.param HalfPeriod='(testperiod/2)'
.param PulseWidth='(HalfPeriod)'

.param TriseOrFall='(PulseWidth*RiseFallPercent)'
.csparam TriseOrFallPrintable={TriseOrFall*1}

* load capacitance for NAND output
.param NANDLoadCapacitance=59fF
.csparam NANDLoadCapacitancePrintable={NANDLoadCapacitance*1e15}

* BSIM4 level=54 mosfet models from 
* 2006 45nm_bulk.pm process file
* which we updated the version statement from 4.0 to 4.8

.include modelcard.nmos
.include modelcard.pmos

* use NMOS_VTL for the model name nmos in instantiation
* use PMOS_VTL for the model name pmos in instantiation

* NAND gate power supply:
vdd3 vddnand 0 {DCsupplyVoltage}
* the NAND subckt simply uses global gnd node 0


* CREATE the NAND gate whose input gate A will be
* target capacitance 

* global gnd 0 not included in parameter list
.SUBCKT NAND in1 in2 out VDD
* PMOS parallel PUN pair
*  nd  ng  ns  nb  model
M3 out in1 vdd vdd PMOS_VTL W=0.630000U L=0.050000U 
M4 out in2 vdd vdd PMOS_VTL W=0.630000U L=0.050000U 

* NMOS series PDN pair
*   nd   ng  ns    nb model
M1 net.1 in2 0     0  NMOS_VTL W=0.415000U L=0.050000U 
M2 out   in1 net.1 0  NMOS_VTL W=0.415000U L=0.050000U 
* above we use the W and L dimensions of NAND2_X1
* from NCSU FreePDK 45nm
* from Christopher Torng 2019 45nm ASIC design kit for mflowgen

* INSTANTIATE a NAND gate X3 using the SUBCKT:
*  in1 in2  out  VDD
X3 in1 in2  out  vddnand NAND



* measure effective capacitance of the in1 input to NAND gate
* when driven voltage pulse train

* provide low resistance path
* to NAND input 1 in1 from pulse
* source, use approx. CMOS output R
* as voltage pulse source resistance
RTOTARGETC PulseInput in1 1000

* tie unused NAND input in2 of NAND to vddnand
RX32 in2 vddnand 10k

* LOAD the NAND X3 output:
CLOAD out 0 {NANDLoadCapacitance}



* syntax note: initial plus sign is extension of previous line
* to keep columns less than equal 66 for printing

* run transient analysis defined outside the control section
* make the plot white background instead of default black
 set color0 = white ; plot window -background color
 set color1 = black ; plot window -grid and text color
* thinner grid and plot lines?
 set xbrushwidth=0.5

* plot 2 cycles NAND output and in1 NAND input 
plot in1 out xl 0 $&testperiodctl2
+ title "NAND input (in1, in2 at VDD) and output, 2 cycles"

* measure input capacitance of the NAND:
 let rcurrent = ( (pulseinput - in1) / @rtotargetc[resistance] )
 echo "charge transferred to NAND input: (Coulombs)"
 meas tran incurlh INTEG rcurrent from=0 to=$&TriseOrFallPrintable
 echo "voltage change on NAND input: (Volts)"
 meas tran maxin1 MAX in1 from=0 to=$&TriseOrFallPrintable
 echo ""
 echo "capacitance of NAND input measured, fF:"
 let NANDincap = (incurlh / maxin1)*1e15
 print NANDincap
 echo ""

 settype current rcurrent
 plot rcurrent xl 0 $&TriseOrFallPrintable
+ title "NAND in1 current L to H"
 plot in1 xl 0 $&TriseOrFallPrintable 
+ title "NAND in1 gate voltage L to H"

 echo "(Nangate typical build library suggests"
 echo "NAND2_X1 A1 input is 1.599 fF)"

 echo ""
 echo "Load capacitance on NAND output:  " 
 echo $&NANDLoadCapacitancePrintable " fF"

 echo ""
 echo "Test frequency was " $&testfreqPrintable " MHz"
 echo "Input rise/fall time was " $&TriseOrFallPrintable " s"
 echo "pulse source output impedance: (ohms)"
 print @RTOTARGETC[resistance]
 echo "Vdd supply voltage was "
 echo $&DCsupplyVoltageForCTL " volts DC"


* PULSE DRIVE of NAND input 1

VIN1 PulseInput 0 PULSE(0 {DCsupplyVoltage} 
+ 0 {TriseOrFall} {TriseOrFall} {PulseWidth} {testperiod})

* transient analysis
.tran 'transstepsize' 'ttime'


In the Ngspice-27 code above (in the CONTROL section; note that we give Ngspice variable names from code above in parentheses) we:

  • obtain the voltage drop across the pulse source resistor (rtotargetc) by taking the difference of the voltage from the pulse generator ideal voltage source (pulseinput) and the voltage at the NAND2 input (in1). This is a vector (a matrix of data with an element for each time step of the simulation) which will be the voltage drop across the source resistance for each simulation time step (because we are running a transient analysis)
  • obtain the current flow (rcurrent) into the NAND2 input in1 by dividing the voltage drop across the pulse source resistor (the vector above) by the scalar resistance of the source (rtotargetc).
  • integrate that gate input current (the rcurrent vector) from zero to the rise time of the pulse stimulus (TriseOrFallPrintable). store this in variable (incurlh). This is the charge transferred to the gate input over the interval of integration.
  • obtain the voltage change from zero to the maximum, i.e., the maximum value from zero to the rise time of the pulse stimulus (TriseOrFallPrintable). store that in variable (maxin1).
  • divide the charge transferred (incurlh) by the voltage change (maxin1) and echo that capacitance value to the screen.

Terminal output from the Ngspice-27 code run above:

charge transferred to NAND input: (Coulombs)
incurlh             =  1.39093e-15 from=  0.00000e+00 to=  1.00000e-10
voltage change on NAND input: (Volts)
maxin1              =  1.088130e+00 at=  1.000000e-10

capacitance of NAND input measured, fF:
nandincap = 1.278276e+00

(Nangate typical build library suggests
NAND2_X1 A1 input is 1.599 fF)

Load capacitance on NAND output:  
59  fF

Test frequency was  1000  MHz
Input rise/fall time was  1E-10  s
pulse source output impedance: (ohms)
@rtotargetc[resistance] = 1.000000e+03
Vdd supply voltage was 
1.1  volts DC

We measure \$1.278276 \, \mathrm{fF}\$ above, \$20 \%\$ lower than Nangate standard library typical corner suggested value of \$1.599 \, \mathrm{fF}\$. CBCM (charge-based capacitance measurement) is more accurate (within a few percent), but the CBCM method requires more circuit code.

Why test frequency of \$1 \, \mathrm{GHz}\$? We made use of the familiar relation between rise time \$t_r\$ and signal \$-3 \, \mathrm{dB}\$ cutoff frequency, \$f_{-3 \, \mathrm{dB}}=0.35/t_r\$.

One of the Nangate stdcells-databook (for our 45nm CMOS process) applicable test input rise time specifications for our typical corner circuit NAND2_X1 loaded with \$C_L = 59.3567 \, \mathrm{fF}\$ is \$0.1985 \, \mathrm{ns}\$. \$t_r = 0.1985 \, \mathrm{ns}\$ is then roughly equivalent to a clock speed of \$1.8 \, \mathrm{GHz}\$. $$ 0.35 / 0.1985 \, \mathrm{ns} = 1.8 \, \mathrm{GHz} $$

Transit time estimates also put us in this general neighborhood, so we went with the conservative \$1\, \mathrm{GHz}\$, maximum clock estimates not necessarily being the reliably usable frequency of operation. Edge rate enters in also. You will note in the Ngspice-27 code that we automate the specification of rise/fall time of the input signal, using 20% of the pulse width, the latter being half the test frequency period. The frequency and rise time used is printed in the terminal after a run (as seen above, i.e., rise time was \$100 \, \mathrm{ps}\$ and test frequency \$1\, \mathrm{GHz}\$).

Why \$V_{dd} = 1.1 \, \mathrm{V}\$? That is the specified voltage supply for this typical corner in 45nm CMOS (VTL).

What about output resistance of the NAND2 gate? We treat the output voltage waveform of the switch as an exponential with a time constant defined by \$R_S\$ and \$C_{eq}\$.

$$ V_o = V \left[ 1 - \exp \left(\frac{-t}{R_S C_{eq}} \right) \right] $$

where \$R_S\$ is the output impedance of the gate and \$C_{eq}\$ the equivalent capacitive load on the output. For our purposes, \$C_{eg}=C_L= 59 \, \mathrm{fF}\$ (to be clear, we hang a \$59 \, \mathrm{fF}\$ capacitor on the output of the NAND gate, as we did above), which should dominate any intrinsic capacitance of the PMOS or NMOS transistors.

Considering the equivalent output resistance \$R_S=R_{DS}\$ of the PMOS transistor in the output transition from LOW to HIGH, our time interval will be the rise time \$t_r\$ which we define here as the time required to rise from \$0.1\$ to \$0.9\$ of the final output voltage at the NAND2 output. We solve for \$t_r\$ using the equation above:

$$ \begin{align} \frac{V_o}{V} &= 0.9 = \left[ \exp \left(\frac{-t_{0.9V}}{R_S C_{eq}} \right) \right] \nonumber \\[1ex] \ln(0.9) &= -0.1053 = \left(\frac{-t_{0.9V}}{R_S C_{eq}} \right) \quad \quad \Longrightarrow 0.1053R_S C_{eq} = t_{0.9V} \nonumber \\[1ex] \frac{V_o}{V} &= 0.1 = \left[ \exp \left(\frac{-t_{0.1V}}{R_S C_{eq}} \right) \right] \nonumber \\[1ex] \ln(0.1) &= -2.3026 = \left(\frac{-t_{0.1V}}{R_S C_{eq}} \right) \quad \quad \Longrightarrow 2.3026 R_S C_{eq} = t_{0.1V} \nonumber \\[1ex] t_r &= R_S C_{eq} \left( t_{0.1V} - t_{0.9V}\right) = 2.19722 (R_S C_{eq}) \approx 2.2 (R_S C_{eq}) \nonumber \\[1ex] \end{align} $$

Knowing \$C_{eq}=C_L\$, if we measure the times at which the \$0.1\$ and \$0.9\$ voltage points on the rising output transition occur to obtain \$t_r\$, we then obtain an estimate of the PMOS drain source resistance with \$R_S= R_{DS} = t_r /(2.2 C_L\$.

First we obtain the final voltage of our output rising signal, selecting an interval to measure on where NAND (out) appears to be leveling out, e.g., \$t=0.3\$ to \$t=0.4 \, \mathrm{ns}\$ using Ngspice-27 interactive commands in the terminal window after running the simulation:

ngspice 1 -> meas tran maxinvolt MAX_AT out  from=0.300n to=0.400n
maxinvolt           =  3.999750e-10 with=  1.094780e+00

It appears our final voltage on NAND (out) rising will be \$1.09478 \, \mathrm{V}\$. We then calculate the \$0.1\$ and \$0.9\$ fractions of that voltage, making use of Ngspice interactive calculator capability:

ngspice 1 -> print 1.094780e+00*0.1
1.094780e+00*0.1 = 1.094780e-01
ngspice 1 -> print 1.094780e+00*0.9
1.094780e+00*0.9 = 9.853020e-01

We now have the two output voltage trigger points, so will enter the Ngspice command to measure the time difference between the output out at those two points:

meas tran tdiff TRIG out VAL=1.094780e-01 RISE=1 TARG out VAL=9.853020e-01 RISE=1

And response in terminal:

tdiff= 1.231943e-10 targ= 2.910126e-10 trig=1.678183e-10

We obtain \$t_r = 123.1943 \, \mathrm{ps}\$ for the \$\Delta t\$ (between \$t_{0.1V}\$ and \$t_{0.9V}\$). Now we apply our equation above using our load capacitance value of \$C_L= 59 \, \mathrm{fF}\$:

ngspice 1 -> print 1.231943e-10/(2.2*59e-15)
1.231943e-10/(2.2*59e-15) = 9.491086e+02

We obtain an estimated average \$R_{ds}= 949 \, \Omega\$ for the single M3 PMOS transistor (see NAND schematic given earlier) in saturated pullup mode, i.e., charging the output capacitance \$C_L\$.

For a detailed discussion of the above circuits and measurements, including:

  • standard cell libraries parameters and the FreePDK45, including the CDL text of the NAND2 circuit and the Nangate suggested values we used
  • relevant CMOS layout making context with schematic and silicon clear
  • bandwidth estimation (multiple methods)
  • output resistance (multiple methods)
  • input capacitance measurement (multiple methods)
  • propagation delay
  • brief examination of the BSIM4 MOSFET capacitance model, including the intrinsic core model (presenting the Ward-Dutton transcapacitance matrix), the BSIM4 RF (high-speed) settings we used, and for the curious, a look under the hood as it were, graphing some of the internal BSIM4 capacitance and charge variables
  • implementation of the charge-based capacitance measurement presented by Dennis Sylvester and Chenming Hu “Analytical Modeling and Characterization of Deep-Submicrometer Interconnect,” PROCEEDINGS OF THE IEEE 89 (2001)

with all reference cites (many including url's), see our paper hosted at GitHub (along with the nmos and pmos BSIM4 model cards we used, multiple Ngspice-27 circuit files, copies of the Ngspice-27 manual to facilitate understanding the code):

the article and circuit files listed

CBCMcmosArticle.pdf, the article

We recommend that you download and view in the article in apdf viewer rather than a browser to preserve the hyperlinked document cross-references, table of contents, and reference cites.


The input capacitance will vary, perhaps greatly, as the input voltage slews between the rails.

Similarly the output resistance, though not greatly (maybe 2:1), as the input voltage slews between the rails.

By measuring the input (transient) current during slewing, you can compute the Cin.

Note the heavy Cload will slow the output and thus reduce the charge demanded across the FETs C_gate_drain, thus artificially lowering the Cin.

Foe Rout, remove the Cload and Rload, run a input transient at risetime of your chosing, while driving the output with 100 uA sinusoid current source.


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