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Offline promachTopic starter

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Issue interpreting AC phase plot
« on: January 29, 2019, 03:42:33 am »
I am having some issue interpreting the following AC phase plot.
The AC gain plot is ok, rolling off at high frequencies, but what about the
which increases from 0 towards 180 degrees at higher frequencies ?

Note: The current input stimulus and feedback resistor is configuring the inverter as a transimpedance amplifier. It’s to avoid disturbing the dc bias.



test_cmos_inverter.asc

Code: [Select]
Version 4
SHEET 1 2264 2956
WIRE 256 -48 240 -48
WIRE 528 -48 256 -48
WIRE 240 0 240 -48
WIRE 256 48 256 -48
WIRE 256 48 240 48
WIRE 192 80 160 80
WIRE 160 160 160 80
WIRE 240 160 240 96
WIRE 240 192 240 160
WIRE 272 192 240 192
WIRE 384 192 272 192
WIRE 112 224 0 224
WIRE 160 224 160 160
WIRE 160 224 112 224
WIRE 240 224 240 192
WIRE 0 272 0 224
WIRE 256 272 240 272
WIRE 528 288 528 -48
WIRE 160 304 160 224
WIRE 192 304 160 304
WIRE 240 352 240 320
WIRE 256 352 256 272
WIRE 256 352 240 352
WIRE 0 416 0 352
WIRE 256 416 256 352
WIRE 256 416 0 416
WIRE 352 416 256 416
WIRE 384 416 384 272
WIRE 384 416 352 416
WIRE 528 416 528 368
WIRE 528 416 384 416
WIRE 352 448 352 416
FLAG 352 448 0
FLAG 112 224 in
FLAG 272 192 out
SYMBOL nmos4 192 224 R0
SYMATTR InstName M1
SYMATTR Value TSMC180nmN
SYMATTR Value2 l=3u w=1.2u
SYMBOL voltage 528 272 R0
SYMATTR InstName V1
SYMATTR Value 1.0
SYMBOL pmos4 192 0 R0
SYMATTR InstName M2
SYMATTR Value TSMC180nmP
SYMATTR Value2 l=3u w=0.4u
SYMBOL current 0 272 R0
WINDOW 123 24 102 Left 2
SYMATTR Value2 AC 1
SYMATTR InstName I1
SYMATTR Value 0
SYMBOL res 256 144 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 2 135 VTop 2
SYMATTR InstName R1
SYMATTR Value 1Meg
SYMBOL current 384 192 R0
WINDOW 123 39 50 Left 2
WINDOW 3 68 23 Left 2
SYMATTR Value2 AC 0
SYMATTR InstName I2
SYMATTR Value 0
TEXT 864 176 Left 2 !.op
TEXT 872 200 Left 2 !.ac dec 100 1 10g
TEXT 816 328 Left 2 !.include tsmc180nmcmos.lib
TEXT 808 256 Left 2 !;.dc I1 -165n 165n 1n


tsmc180nmcmos.lib

Code: [Select]
* PSPICE TSMC180nm.lib file  RWN  04/18/2010
* library  file for transistor parameters for TMSC 0.18 micron process
* uses BIM parameters added 01/15/98
* can configure and attach to Nbreak and Pbreak transistors in PSpice
****
******************  180nm TSMC parameters  *************
*T14B SPICE BSIM3 VERSION 3.1 PARAMETERS
* downloaded from MOSIS 04/18/10
*http://www.mosis.com/cgi-bin/cgiwrap/umosis/swp/params/
* tsmc-018/t92y_mm_non_epi_thk_mtl_params.txt
*SPICE 3f5 Level 8, Star-HSPICE Level 49, UTMOST Level 8
* DATE: Jun  8/01
* LOT: T14B                  WAF: 06
* Temperature_parameters=Default
*$
.MODEL TSMC180nmN NMOS (                                LEVEL   = 7
+VERSION = 3.1            TNOM    = 27             TOX     = 4.1E-9
+XJ      = 1E-7           NCH     = 2.3549E17      VTH0    = 0.354505
+K1      = 0.5733393      K2      = 3.177172E-3    K3      = 27.3563303
+K3B     = -10            W0      = 2.341477E-5    NLX     = 1.906617E-7
+DVT0W   = 0              DVT1W   = 0              DVT2W   = 0
+DVT0    = 1.6751718      DVT1    = 0.4282625      DVT2    = 0.036004
+U0      = 327.3736992    UA      = -4.52726E-11   UB      = 4.46532E-19
+UC      = -4.74051E-11   VSAT    = 8.785346E4     A0      = 1.6897405
+AGS     = 0.2908676      B0      = -8.224961E-9   B1      = -1E-7
+KETA    = 0.021238       A1      = 8.00349E-4     A2      = 1
+RDSW    = 105            PRWG    = 0.5            PRWB    = -0.2
+WR      = 1              WINT    = 0              LINT    = 1.351737E-8
*+XL      = -2E-8          XW      = -1E-8         
+ DWG     = 1.610448E-9
+DWB     = -5.108595E-9   VOFF    = -0.0652968     NFACTOR = 2.4901845
+CIT     = 0              CDSC    = 2.4E-4         CDSCD   = 0
+CDSCB   = 0              ETA0    = 0.0231564      ETAB    = -0.058499
+DSUB    = 0.9467118      PCLM    = 0.8512348      PDIBLC1 = 0.0929526
+PDIBLC2 = 0.01           PDIBLCB = -0.1           DROUT   = 0.5224026
+PSCBE1  = 7.979323E10    PSCBE2  = 1.522921E-9    PVAG    = 0.01
+DELTA   = 0.01           RSH     = 6.8            MOBMOD  = 1
+PRT     = 0              UTE     = -1.5           KT1     = -0.11
+KT1L    = 0              KT2     = 0.022          UA1     = 4.31E-9
+UB1     = -7.61E-18      UC1     = -5.6E-11       AT      = 3.3E4
+WL      = 0              WLN     = 1              WW      = 0
+WWN     = 1              WWL     = 0              LL      = 0
+LLN     = 1              LW      = 0              LWN     = 1
+LWL     = 0              CAPMOD  = 2              XPART   = 0.5
+CGDO    = 7.7E-10        CGSO    = 7.7E-10        CGBO    = 1E-12
+CJ      = 1.010083E-3    PB      = 0.7344298      MJ      = 0.3565066
+CJSW    = 2.441707E-10   PBSW    = 0.8005503      MJSW    = 0.1327842
+CJSWG   = 3.3E-10        PBSWG   = 0.8005503      MJSWG   = 0.1327842
+CF      = 0              PVTH0   = 1.307195E-3    PRDSW   = -5
+PK2     = -1.022757E-3   WKETA   = -4.466285E-4   LKETA   = -9.715157E-3
+PU0     = 12.2704847     PUA     = 4.421816E-11   PUB     = 0
+PVSAT   = 1.707461E3     PETA0   = 1E-4           PKETA   = 2.348777E-3     )
*
*$
.MODEL TSMC180nmP PMOS (                                LEVEL   = 7
+VERSION = 3.1            TNOM    = 27             TOX     = 4.1E-9
+XJ      = 1E-7           NCH     = 4.1589E17      VTH0    = -0.4120614
+K1      = 0.5590154      K2      = 0.0353896      K3      = 0
+K3B     = 7.3774572      W0      = 1E-6           NLX     = 1.103367E-7
+DVT0W   = 0              DVT1W   = 0              DVT2W   = 0
+DVT0    = 0.4301522      DVT1    = 0.2156888      DVT2    = 0.1
+U0      = 128.7704538    UA      = 1.908676E-9    UB      = 1.686179E-21
+UC      = -9.31329E-11   VSAT    = 1.658944E5     A0      = 1.6076505
+AGS     = 0.3740519      B0      = 1.711294E-6    B1      = 4.946873E-6
+KETA    = 0.0210951      A1      = 0.0244939      A2      = 1
+RDSW    = 127.0442882    PRWG    = 0.5            PRWB    = -0.5
+WR      = 1              WINT    = 5.428484E-10   LINT    = 2.468805E-8
*+XL      = -2E-8          XW      = -1E-8         
+DWG     = -2.453074E-8
+DWB     = 6.408778E-9    VOFF    = -0.0974174     NFACTOR = 1.9740447
+CIT     = 0              CDSC    = 2.4E-4         CDSCD   = 0
+CDSCB   = 0              ETA0    = 0.1847491      ETAB    = -0.2531172
+DSUB    = 1.5            PCLM    = 4.8842961      PDIBLC1 = 0.0156227
+PDIBLC2 = 0.1            PDIBLCB = -1E-3          DROUT   = 0
+PSCBE1  = 1.733878E9     PSCBE2  = 5.002842E-10   PVAG    = 15
+DELTA   = 0.01           RSH     = 7.7            MOBMOD  = 1
+PRT     = 0              UTE     = -1.5           KT1     = -0.11
+KT1L    = 0              KT2     = 0.022          UA1     = 4.31E-9
+UB1     = -7.61E-18      UC1     = -5.6E-11       AT      = 3.3E4
+WL      = 0              WLN     = 1              WW      = 0
+WWN     = 1              WWL     = 0              LL      = 0
+LLN     = 1              LW      = 0              LWN     = 1
+LWL     = 0              CAPMOD  = 2              XPART   = 0.5
+CGDO    = 7.11E-10       CGSO    = 7.11E-10       CGBO    = 1E-12
+CJ      = 1.179334E-3    PB      = 0.8545261      MJ      = 0.4117753
+CJSW    = 2.215877E-10   PBSW    = 0.6162997      MJSW    = 0.2678074
+CJSWG   = 4.22E-10       PBSWG   = 0.6162997      MJSWG   = 0.2678074
+CF      = 0              PVTH0   = 2.283319E-3    PRDSW   = 5.6431992
+PK2     = 2.813503E-3    WKETA   = 2.438158E-3    LKETA   = -0.0116078
+PU0     = -2.2514581     PUA     = -7.62392E-11   PUB     = 4.502298E-24
+PVSAT   = -50            PETA0   = 1E-4           PKETA   = -1.047892E-4    )
*
*.ENDS   
*$
 

Online Ian.M

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  • Posts: 13216
Re: Issue interpreting AC phase plot
« Reply #1 on: January 29, 2019, 05:52:09 am »
You've got the two traces mixed up. Add a cursor to the plot of V(out)/V(in) and you'll see that the phase starts at 180 deg at low frequencies (as it should be for an inverter) and starts decreasing at about 1MHz, crossing 90 deg at 64MHz and becoming close to 0 deg from about 1GHz upwards.   Meanwhile the gain is about 7.5dB at low frequencies, increasing above 1MHz to 13.8dB at 64MHz and 20dB above 1GHz. 

I am perplexed by the increasing gain . . .
 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #2 on: January 29, 2019, 03:32:07 pm »
The increasing V(out)/V(in) gain magnitude is surprising. I have no idea why. Any idea why ?

the V(out)/V(in) phase is due to V(in) and V(out) acted together.

I have the following an updated AC plot :

« Last Edit: January 29, 2019, 03:38:05 pm by promach »
 

Offline iMo

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Re: Issue interpreting AC phase plot
« Reply #3 on: January 29, 2019, 06:47:38 pm »
I can get some amplification out of it with different L and W and Vcc.
The ac analysis shows crap. The dc sweep looks ok.
It could be the spice models you have imported do not match the model levels in the LTspice.
Readers discretion is advised..
 

Offline MrAl

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Re: Issue interpreting AC phase plot
« Reply #4 on: January 29, 2019, 06:49:11 pm »
I am having some issue interpreting the following AC phase plot.
The AC gain plot is ok, rolling off at high frequencies, but what about the
which increases from 0 towards 180 degrees at higher frequencies ?

Note: The current input stimulus and feedback resistor is configuring the inverter as a transimpedance amplifier. It’s to avoid disturbing the dc bias.



test_cmos_inverter.asc

Code: [Select]
Version 4
SHEET 1 2264 2956
WIRE 256 -48 240 -48
WIRE 528 -48 256 -48
WIRE 240 0 240 -48
WIRE 256 48 256 -48
WIRE 256 48 240 48
WIRE 192 80 160 80
WIRE 160 160 160 80
WIRE 240 160 240 96
WIRE 240 192 240 160
WIRE 272 192 240 192
WIRE 384 192 272 192
WIRE 112 224 0 224
WIRE 160 224 160 160
WIRE 160 224 112 224
WIRE 240 224 240 192
WIRE 0 272 0 224
WIRE 256 272 240 272
WIRE 528 288 528 -48
WIRE 160 304 160 224
WIRE 192 304 160 304
WIRE 240 352 240 320
WIRE 256 352 256 272
WIRE 256 352 240 352
WIRE 0 416 0 352
WIRE 256 416 256 352
WIRE 256 416 0 416
WIRE 352 416 256 416
WIRE 384 416 384 272
WIRE 384 416 352 416
WIRE 528 416 528 368
WIRE 528 416 384 416
WIRE 352 448 352 416
FLAG 352 448 0
FLAG 112 224 in
FLAG 272 192 out
SYMBOL nmos4 192 224 R0
SYMATTR InstName M1
SYMATTR Value TSMC180nmN
SYMATTR Value2 l=3u w=1.2u
SYMBOL voltage 528 272 R0
SYMATTR InstName V1
SYMATTR Value 1.0
SYMBOL pmos4 192 0 R0
SYMATTR InstName M2
SYMATTR Value TSMC180nmP
SYMATTR Value2 l=3u w=0.4u
SYMBOL current 0 272 R0
WINDOW 123 24 102 Left 2
SYMATTR Value2 AC 1
SYMATTR InstName I1
SYMATTR Value 0
SYMBOL res 256 144 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 2 135 VTop 2
SYMATTR InstName R1
SYMATTR Value 1Meg
SYMBOL current 384 192 R0
WINDOW 123 39 50 Left 2
WINDOW 3 68 23 Left 2
SYMATTR Value2 AC 0
SYMATTR InstName I2
SYMATTR Value 0
TEXT 864 176 Left 2 !.op
TEXT 872 200 Left 2 !.ac dec 100 1 10g
TEXT 816 328 Left 2 !.include tsmc180nmcmos.lib
TEXT 808 256 Left 2 !;.dc I1 -165n 165n 1n


tsmc180nmcmos.lib

Code: [Select]
* PSPICE TSMC180nm.lib file  RWN  04/18/2010
* library  file for transistor parameters for TMSC 0.18 micron process
* uses BIM parameters added 01/15/98
* can configure and attach to Nbreak and Pbreak transistors in PSpice
****
******************  180nm TSMC parameters  *************
*T14B SPICE BSIM3 VERSION 3.1 PARAMETERS
* downloaded from MOSIS 04/18/10
*http://www.mosis.com/cgi-bin/cgiwrap/umosis/swp/params/
* tsmc-018/t92y_mm_non_epi_thk_mtl_params.txt
*SPICE 3f5 Level 8, Star-HSPICE Level 49, UTMOST Level 8
* DATE: Jun  8/01
* LOT: T14B                  WAF: 06
* Temperature_parameters=Default
*$
.MODEL TSMC180nmN NMOS (                                LEVEL   = 7
+VERSION = 3.1            TNOM    = 27             TOX     = 4.1E-9
+XJ      = 1E-7           NCH     = 2.3549E17      VTH0    = 0.354505
+K1      = 0.5733393      K2      = 3.177172E-3    K3      = 27.3563303
+K3B     = -10            W0      = 2.341477E-5    NLX     = 1.906617E-7
+DVT0W   = 0              DVT1W   = 0              DVT2W   = 0
+DVT0    = 1.6751718      DVT1    = 0.4282625      DVT2    = 0.036004
+U0      = 327.3736992    UA      = -4.52726E-11   UB      = 4.46532E-19
+UC      = -4.74051E-11   VSAT    = 8.785346E4     A0      = 1.6897405
+AGS     = 0.2908676      B0      = -8.224961E-9   B1      = -1E-7
+KETA    = 0.021238       A1      = 8.00349E-4     A2      = 1
+RDSW    = 105            PRWG    = 0.5            PRWB    = -0.2
+WR      = 1              WINT    = 0              LINT    = 1.351737E-8
*+XL      = -2E-8          XW      = -1E-8         
+ DWG     = 1.610448E-9
+DWB     = -5.108595E-9   VOFF    = -0.0652968     NFACTOR = 2.4901845
+CIT     = 0              CDSC    = 2.4E-4         CDSCD   = 0
+CDSCB   = 0              ETA0    = 0.0231564      ETAB    = -0.058499
+DSUB    = 0.9467118      PCLM    = 0.8512348      PDIBLC1 = 0.0929526
+PDIBLC2 = 0.01           PDIBLCB = -0.1           DROUT   = 0.5224026
+PSCBE1  = 7.979323E10    PSCBE2  = 1.522921E-9    PVAG    = 0.01
+DELTA   = 0.01           RSH     = 6.8            MOBMOD  = 1
+PRT     = 0              UTE     = -1.5           KT1     = -0.11
+KT1L    = 0              KT2     = 0.022          UA1     = 4.31E-9
+UB1     = -7.61E-18      UC1     = -5.6E-11       AT      = 3.3E4
+WL      = 0              WLN     = 1              WW      = 0
+WWN     = 1              WWL     = 0              LL      = 0
+LLN     = 1              LW      = 0              LWN     = 1
+LWL     = 0              CAPMOD  = 2              XPART   = 0.5
+CGDO    = 7.7E-10        CGSO    = 7.7E-10        CGBO    = 1E-12
+CJ      = 1.010083E-3    PB      = 0.7344298      MJ      = 0.3565066
+CJSW    = 2.441707E-10   PBSW    = 0.8005503      MJSW    = 0.1327842
+CJSWG   = 3.3E-10        PBSWG   = 0.8005503      MJSWG   = 0.1327842
+CF      = 0              PVTH0   = 1.307195E-3    PRDSW   = -5
+PK2     = -1.022757E-3   WKETA   = -4.466285E-4   LKETA   = -9.715157E-3
+PU0     = 12.2704847     PUA     = 4.421816E-11   PUB     = 0
+PVSAT   = 1.707461E3     PETA0   = 1E-4           PKETA   = 2.348777E-3     )
*
*$
.MODEL TSMC180nmP PMOS (                                LEVEL   = 7
+VERSION = 3.1            TNOM    = 27             TOX     = 4.1E-9
+XJ      = 1E-7           NCH     = 4.1589E17      VTH0    = -0.4120614
+K1      = 0.5590154      K2      = 0.0353896      K3      = 0
+K3B     = 7.3774572      W0      = 1E-6           NLX     = 1.103367E-7
+DVT0W   = 0              DVT1W   = 0              DVT2W   = 0
+DVT0    = 0.4301522      DVT1    = 0.2156888      DVT2    = 0.1
+U0      = 128.7704538    UA      = 1.908676E-9    UB      = 1.686179E-21
+UC      = -9.31329E-11   VSAT    = 1.658944E5     A0      = 1.6076505
+AGS     = 0.3740519      B0      = 1.711294E-6    B1      = 4.946873E-6
+KETA    = 0.0210951      A1      = 0.0244939      A2      = 1
+RDSW    = 127.0442882    PRWG    = 0.5            PRWB    = -0.5
+WR      = 1              WINT    = 5.428484E-10   LINT    = 2.468805E-8
*+XL      = -2E-8          XW      = -1E-8         
+DWG     = -2.453074E-8
+DWB     = 6.408778E-9    VOFF    = -0.0974174     NFACTOR = 1.9740447
+CIT     = 0              CDSC    = 2.4E-4         CDSCD   = 0
+CDSCB   = 0              ETA0    = 0.1847491      ETAB    = -0.2531172
+DSUB    = 1.5            PCLM    = 4.8842961      PDIBLC1 = 0.0156227
+PDIBLC2 = 0.1            PDIBLCB = -1E-3          DROUT   = 0
+PSCBE1  = 1.733878E9     PSCBE2  = 5.002842E-10   PVAG    = 15
+DELTA   = 0.01           RSH     = 7.7            MOBMOD  = 1
+PRT     = 0              UTE     = -1.5           KT1     = -0.11
+KT1L    = 0              KT2     = 0.022          UA1     = 4.31E-9
+UB1     = -7.61E-18      UC1     = -5.6E-11       AT      = 3.3E4
+WL      = 0              WLN     = 1              WW      = 0
+WWN     = 1              WWL     = 0              LL      = 0
+LLN     = 1              LW      = 0              LWN     = 1
+LWL     = 0              CAPMOD  = 2              XPART   = 0.5
+CGDO    = 7.11E-10       CGSO    = 7.11E-10       CGBO    = 1E-12
+CJ      = 1.179334E-3    PB      = 0.8545261      MJ      = 0.4117753
+CJSW    = 2.215877E-10   PBSW    = 0.6162997      MJSW    = 0.2678074
+CJSWG   = 4.22E-10       PBSWG   = 0.6162997      MJSWG   = 0.2678074
+CF      = 0              PVTH0   = 2.283319E-3    PRDSW   = 5.6431992
+PK2     = 2.813503E-3    WKETA   = 2.438158E-3    LKETA   = -0.0116078
+PU0     = -2.2514581     PUA     = -7.62392E-11   PUB     = 4.502298E-24
+PVSAT   = -50            PETA0   = 1E-4           PKETA   = -1.047892E-4    )
*
*.ENDS   
*$


Hi,

Why dont you have any load connected?  You should probably connect some resistive load like maybe 1k or something.
With no load you might be seeing the effects of parasitics.
 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #5 on: January 30, 2019, 01:03:01 am »
The AC gain issue is solved by using supply voltage of 2.0V instead of just 1.0V.

Could you guys comment about it ?



 

Online Ian.M

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Re: Issue interpreting AC phase plot
« Reply #6 on: January 30, 2019, 01:58:28 am »
Its a lot saner with some parasitics added.  Even 50 femtofarads across the feedback resistor makes a major difference.

Here's a single file version (MOSFET .lib embedded) of the sim that steps the supply voltage over the range 1.0V to 2.0V.    Its much better behaved with 50fF across the feedback resistor.  I am still suspicious of it having 1.7dB gain at 10GHz, but maybe calculating appropriate parasitics for the physical layout and adding them would help with that.
 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #7 on: January 30, 2019, 01:31:12 pm »
See the following modification done by others



 

Online Ian.M

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Re: Issue interpreting AC phase plot
« Reply #8 on: January 30, 2019, 02:42:16 pm »
So, that's with more parameters for the MOSFETs, which hopefully are the result of measuring off masks or die photos, not random guesswork.  I assume the default parameters in the models weren't well matched to the channel dimensions you initially entered.

However its still got some issues - increase the supply to 1.2V, and its still got 0.3dB gain right up at 1 Terahertz which is patently ridiculous for real silicon MOSFETs.  Maybe filling in realistic values for the source area and perimeter parameters would help?
« Last Edit: February 04, 2019, 06:16:46 am by Ian.M »
 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #9 on: February 04, 2019, 06:13:47 am »
The middle plot is the input impedance, why does it start low ?

 

Online Ian.M

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Re: Issue interpreting AC phase plot
« Reply #10 on: February 04, 2019, 10:01:11 am »
Scaling off the low frequency gain from the plot I get about 31dB, which is a voltage gain of about x 36.  The feedback resistor is effectively divided by (1+gain),   giving an effective input resistance of about 37K.   That's not quite consistent with your input impedance plot, but is no more than one pixel out, which could be rounding errors in the plot.  Use a cursor and check both the gain and the input impedance properly!

You do make this hard on us by just posting screenshots, without even using measurement cursors on the waveforms.  Next time if you want a quicker better answer  attach the sim .asc file.  If it needs any 3rd party symbols or libraries, zip them up with the asc file, so its ready to run.  If it needs 3rd party models you have added to the LTspice standard model files, you need too identify them and place them as dot commands on the schematic.

 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #11 on: February 04, 2019, 12:46:25 pm »
See the attached asc file and mosfet library
 

Online Ian.M

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Re: Issue interpreting AC phase plot
« Reply #12 on: February 04, 2019, 01:21:15 pm »
Yep.  Changing the gain plot y axis to linear for easy measurement and using cursors, I get gain=21.36103 and input impedance=44.720659Kohm, both at 1Hz, which are consistant with the formula I posted above.

If you are having difficulty understanding it, divide the feedback resistor into two series parts, one of 44720.659 ohms connected to the input and the other of 955279.341 ohms connected to the output, and consider the voltage on the node in the middle!

Hint: you can share your waveform view by saving the plot settings and including the .plt file in the zip with the sim.  Don't include .raw files as they get very big and are regenerated on every run.
 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #13 on: February 06, 2019, 11:17:38 am »
Rin = R1 / (| Gain | +1). At higher frequencies, the output capacitor reduces the gain and Rin -> R1.

but why Rin drops again after 2MHz ?
 

Online Ian.M

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Re: Issue interpreting AC phase plot
« Reply #14 on: February 06, 2019, 11:41:46 am »
You'd have to do a detailed mathematical analysis including modelling the MOSFETS to be certain, but  I'd certainly expect a dropoff with increasing frequency due to the gate capacitance. 
 

Offline promachTopic starter

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Re: Issue interpreting AC phase plot
« Reply #15 on: February 06, 2019, 12:41:48 pm »
true, the small device parasitic capacitance make a pole near 2 MHz with R1=1Meg

By the way, why all three waveform plots changes so much by just increasing supply voltage from 1.0V to 1.8V ?

 

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Re: Issue interpreting AC phase plot
« Reply #16 on: February 09, 2019, 08:26:17 am »
Removing the large output capacitance C1 (1n) makes more sense.

1) But why input impedance starts with 44kOhm at low frequencies range ? Any way to calculate this value ?

2) Besides, why the phase of the input impedance starts rising again after 200MHz to 150 degrees ?

3) How would I measure overall transconductance (Gm) of this CMOS inverter using input AC current source stimulus (I1) and feedback resistor (R1) ?

 


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