Why I can't see the effect of W/L on Ids

Hi:

According to the equation of Ids=1/2K'W/L(Vgs-Vt) (in saturation), the Ids should increase with the ration of W/L. When I use Pspice to do the simulation, I can't observe the increase in Ids in the saturation region. I am using Level 1 model, and the parameter is :
VTO=1.4 KP=0.05e-3 LAMBDA=0.05

and I set the width W as the parameter to do the parameteric sweep. My intention is to observe the increase in Ids with increasing the ration of W/L. There are five different W/L are set, but they give me same curve for Ids versus Vgs.

If anybody can help me to explain it, and would you please show me how to obeserve the variation of Ids when I change the W/L.

The current can only increase if you keep Vgs-Vt constants. Moreover, how do you set up the bias current to the MOSFET? Can you provide your circuit schematic?

You can increase Ids if (Vgs-Vt) is constant!

Everytime you change the W/L, some of the node voltage is also change to balance you circuit simulation!

I think you better provide your circuit here for to make sure we know 'deeply' about your problem...

terryssw said:

The current can only increase if you keep Vgs-Vt constants. Moreover, how do you set up the bias current to the MOSFET? Can you provide your circuit schematic?

Click to expand...

Hi:

Thank you for the reply. You are right, I just want to compare the current at the same Vgs-Vt, the only variation is the W/L.
I have attached my schematic. I set VDD as 1.8V, and and sweep Vin from 0 to 1.8V. If I change W/L, let's say 2 and 4, I should see two different Ids versus Vin curves. I have observed the variation when I do the simulation separately. That means I set W/L as 2 first, and then to 4. But I can't see the variation when I do the parametric sweep for W. The final result has only one curve with the initial W value.
I tried another parametric sweep for KP, I got different curves for different KP. I don't know why I can't do the parametric sweep for W or L.
It should be appreciated if you can give me some suggestions.
Thanks

Can u remove that resistance from ur circuit..
the characteristic u r trying to plot is very elementry..I never found any problems with that..
If u r still facing problems post ur stimuli and netlist files..

opamp741 said:

Can u remove that resistance from ur circuit..
the characteristic u r trying to plot is very elementry..I never found any problems with that..
If u r still facing problems post ur stimuli and netlist files..

Click to expand...

Hi:

I tried the way your suggested, but it gave me the same result. I don't think the resistor will give trouble. You said you never meet problems with that, do you mean you do the parametric sweep for W or L. Becuase I let my friends who take the same course as me to do the same simulation, they also couldn't see the effect. We are using Orcad9.1 student version. If you have ever seen the effect in doing parametric sweep, would you please tell me how did you do it. I posted my stimuli and netlist below, you may can help me to look into it. Thanks.

**** 06/29/04 21:14:08 ********* PSpice 9.1 (Mar 1999) ******** ID# 1000000670
** circuit file for profile: common_amplifier


**** CIRCUIT DESCRIPTION


******************************************************************************




** WARNING: THIS AUTOMATICALLY GENERATED FILE MAY BE OVERWRITTEN BY SUBSEQUENT PROFILES

*Libraries:
* Local Libraries :
.LIB ".\common_source.lib"
* From [PSPICE NETLIST] section of pspice91.ini file:
.lib ".\common_source.lib"

*Analysis directives:
.DC LIN V_Vin 0 5 0.1
.STEP LIN NMOS Mbreakn(W) 2e-6 4e-6 1e-6
.PROBE
.INC "common_source-SCHEMATIC1.net"


**** INCLUDING common_source-SCHEMATIC1.net ****
* source COMMON_SOURCE
V_Vin N00031 0 0Vdc
V_VDD N00018 0 5Vdc
R_R1 N00028 N00018 10k
M_M2 N00028 N00031 0 0 Mbreakn
+ L=1e-6
+ W=2e-6

**** RESUMING common_source-schematic1-common_amplifier.sim.cir ****
.INC "common_source-SCHEMATIC1.als"



**** INCLUDING common_source-SCHEMATIC1.als ****
.ALIASES
V_Vin Vin(+=N00031 -=0 )
V_VDD VDD(+=N00018 -=0 )
R_R1 R1(1=N00028 2=N00018 )
M_M2 M2(d=N00028 g=N00031 s=0 s=0 )
.ENDALIASES

**** RESUMING common_source-schematic1-common_amplifier.sim.cir ****
.END

**** 06/29/04 21:14:08 ********* PSpice 9.1 (Mar 1999) ******** ID# 1000000670
** circuit file for profile: common_amplifier


**** MOSFET MODEL PARAMETERS


******************************************************************************




Mbreakn
NMOS
LEVEL 1
L 100.000000E-06
W 100.000000E-06
VTO .4
KP 100.000000E-06
GAMMA .01
PHI .6
LAMBDA 0
IS 10.000000E-15
JS 0
PB .8
PBSW .8
CJ 0
CJSW 0
CGSO 0
CGDO 0
CGBO 0
TOX 25
XJ 0
UCRIT 10.000000E+03
DIOMOD 1
VFB 0
LETA 0
WETA 0
U0 0
TEMP 0
VDD 0
XPART 0



JOB CONCLUDED

TOTAL JOB TIME .03