Gain Saturation of CMOS Common Source Amplifier

[rage_speed]

Hi,

I was designing a CMOS common source amplifier using tsmc018(level = 49) model MOS.

I was trying to increase the DC gain by reducing the overdrive of M1 and correspondingly increasing its W/L of M1 keeping the current constant at 20u.

W/L of M2 was constant throughout.

But the gain became constant (~68.8) below 140mV Overdrive. Can somebody explain why the gain saturated?

I have attached the schematic.

Thanks in advance

 

[rage_speed]

The Schematic of the common source Amp.

And the MOS model

* T58F SPICE BSIM3 VERSION 3.1 PARAMETERS
*
* SPICE 3f5 Level 8, Star-HSPICE Level 49, UTMOST Level 8
*
* DATE: Oct 31/05
* LOT: T58F WAF: 9005
* Temperature_parameters=Default

.MODEL CMOSN NMOS ( LEVEL = 49
+VERSION = 3.1 TNOM = 27 TOX = 4.1E-9
+XJ = 1E-7 NCH = 2.3549E17 VTH0 = 0.3662473
+K1 = 0.5864999 K2 = 1.127266E-3 K3 = 1E-3
+K3B = 0.0294061 W0 = 1E-7 NLX = 1.630684E-7
+DVT0W = 0 DVT1W = 0 DVT2W = 0
+DVT0 = 1.2064649 DVT1 = 0.4215486 DVT2 = 0.0197749
+U0 = 273.8094484 UA = -1.40499E-9 UB = 2.408323E-18
+UC = 6.504826E-11 VSAT = 1.355009E5 A0 = 2
+AGS = 0.4449958 B0 = 1.901075E-7 B1 = 4.99995E-6
+KETA = -0.0164863 A1 = 3.868769E-4 A2 = 0.4640272
+RDSW = 123.3376355 PRWG = 0.5 PRWB = -0.197728
+WR = 1 WINT = 0 LINT = 1.690044E-8
+XL = 0 XW = -1E-8 DWG = -4.728719E-9
+DWB = -2.452411E-9 VOFF = -0.0948017 NFACTOR = 2.1860065
+CIT = 0 CDSC = 2.4E-4 CDSCD = 0
+CDSCB = 0 ETA0 = 2.230928E-3 ETAB = 6.028975E-5
+DSUB = 0.0145467 PCLM = 1.3822069 PDIBLC1 = 0.1762787
+PDIBLC2 = 1.66653E-3 PDIBLCB = -0.1 DROUT = 0.7694691
+PSCBE1 = 8.91287E9 PSCBE2 = 7.349607E-9 PVAG = 1.685917E-3
+DELTA = 0.01 RSH = 6.7 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 = 8.23E-10 CGSO = 8.23E-10 CGBO = 1E-12
+CJ = 9.466429E-4 PB = 0.8 MJ = 0.3820266
+CJSW = 2.608154E-10 PBSW = 0.8 MJSW = 0.102322
+CJSWG = 3.3E-10 PBSWG = 0.8 MJSWG = 0.102322
+CF = 0 PVTH0 = -2.199373E-3 PRDSW = -0.9368961
+PK2 = 1.593254E-3 WKETA = -2.880976E-3 LKETA = 7.165078E-3
+PU0 = 6.777519 PUA = 5.505418E-12 PUB = 8.84133E-25
+PVSAT = 2.006286E3 PETA0 = 1.003159E-4 PKETA = -6.759277E-3
+NOIMOD=2.0E+00 NOIA=1.3182567385564E+19
+NOIB=144543.977074592 NOIC=-1.24515784572817E-12 EF=0.92 EM=41000000 )
*
*
* flicker noise parameters above added manually from some other process
*
.MODEL CMOSP PMOS ( LEVEL = 49
+VERSION = 3.1 TNOM = 27 TOX = 4.1E-9
+XJ = 1E-7 NCH = 4.1589E17 VTH0 = -0.3906012
+K1 = 0.5341312 K2 = 0.0395326 K3 = 0
+K3B = 7.4916211 W0 = 1E-6 NLX = 1.194072E-7
+DVT0W = 0 DVT1W = 0 DVT2W = 0
+DVT0 = 0.5060555 DVT1 = 0.2423835 DVT2 = 0.1
+U0 = 115.6894042 UA = 1.573746E-9 UB = 1.874308E-21
+UC = -1E-10 VSAT = 1.130982E5 A0 = 1.9976555
+AGS = 0.4186945 B0 = 1.949178E-7 B1 = 6.422908E-7
+KETA = 0.0166345 A1 = 0.4749146 A2 = 0.300003
+RDSW = 198.321294 PRWG = 0.5 PRWB = -0.4986647
+WR = 1 WINT = 0 LINT = 2.94454E-8
+XL = 0 XW = -1E-8 DWG = -2.798724E-8
+DWB = -4.83797E-10 VOFF = -0.095236 NFACTOR = 2
+CIT = 0 CDSC = 2.4E-4 CDSCD = 0
+CDSCB = 0 ETA0 = 1.035504E-3 ETAB = -4.358398E-4
+DSUB = 1.816555E-3 PCLM = 1.3299898 PDIBLC1 = 1.766563E-3
+PDIBLC2 = 7.728395E-7 PDIBLCB = -1E-3 DROUT = 1.011891E-3
+PSCBE1 = 4.872184E10 PSCBE2 = 5E-10 PVAG = 0.0209921
+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 = 6.35E-10 CGSO = 6.35E-10 CGBO = 1E-12
+CJ = 1.144521E-3 PB = 0.8468686 MJ = 0.4099522
+CJSW = 2.490749E-10 PBSW = 0.8769118 MJSW = 0.3478565
+CJSWG = 4.22E-10 PBSWG = 0.8769118 MJSWG = 0.3478565
+CF = 0 PVTH0 = 2.302018E-3 PRDSW = 9.0575312
+PK2 = 1.821914E-3 WKETA = 0.0222457 LKETA = -1.495872E-3
+PU0 = -1.5580645 PUA = -6.36889E-11 PUB = 1E-21
+PVSAT = 49.8420442 PETA0 = 2.827793E-5 PKETA = -2.536564E-3
+ NOIMOD=2.0E+00 NOIA=3.57456993317604E+18 NOIB=2500
+ NOIC=2.61260020285845E-11 EF=1.1388 EM=41000000 )
*
*
* flicker noise parameters above added manually from some other process
*

 

[dick_freebird]

Single stage gain is gm*rout.

gm has a peak, beyond which you can do no better for that.

Rout is the parallel combination of load and active drain
impedances, perhaps the load sets a ceiling there.

Maybe if you decompose the problem into these elements
it will become more clear, and show you where the improvement
might come easiest.

 

[rage_speed]

Single stage gain is gm*rout.

gm has a peak, beyond which you can do no better for that.

Rout is the parallel combination of load and active drain
impedances, perhaps the load sets a ceiling there.

Maybe if you decompose the problem into these elements
it will become more clear, and show you where the improvement
might come easiest.

Thanks for replying.

I am keeping the current constant so that Rout remains constant there by the output pole remains constant.

And to see the maximum gain I can get with that Rout I am trying to increase the gm by changing Vov and W/L of M1.

Is there a way to plot the gm for a constant current because we need to change both Vov & W/L.

Thanks

 

[Braski]

This is the same topic posted in IC section...the reason holds...weak inversion->exp equations->gain saturation to ID/nVt with n slope factor.

 

[rage_speed]

@ Braski

Can you please elaborate or direct me to some material on the topic.

Thanks