[Ngspice-users] DC interactive not working

[steven.borley <ste...@vi...>]

I am confused over why the following simple circuit, that aims to show 
a MOSFET's characteristics, is failing to work as I expected.

I hope someone can either point out what I'm doing wrong, or confirm it 
as a bug in ngspice.

MOS OUTPUT CHARACTERISTICS - Example circuit 1
Vds  1  0 dc 10
Vgs  2  0 dc 5
M1   1  2  0  0 MOD1 L=4U W=6U AD=10P AS=10P
.MODEL MOD1 NMOS level=1 VTO=-2 NSUB=1.0E15 UO=550
.end

I run this as follows...

[mirror-doors:~/spice] steven% ngspice circuit1.cir
******
** ngspice-15 : Circuit level simulation program
** The U. C. Berkeley CAD Group
** Copyright 1985-1994, Regents of the University of California.
** Please submit bug-reports to: ngs...@li...
** Creation Date: Sat Mar 12 01:57:16 GMT 2005
******

Circuit: MOS OUTPUT CHARACTERISTICS - Example circuit 1

ngspice 119 -> listing
         MOS OUTPUT CHARACTERISTICS - Example circuit 1

      1 : mos output characteristics - example circuit 1
      2 : vds  1  0 dc 10
      3 : vgs  2  0 dc 5
      4 : m1   1  2  0  0 mod1 l=4u w=6u ad=10p as=10p
      5 : .model mod1 nmos level=1 vto=-2 nsub=1.0e15 uo=550
      7 : .end
ngspice 120 -> dc vds 0 10 0.5 vgs 0 5 1
Doing analysis at TEMP = 300.150000 and TNOM = 300.150000

No. of Data Rows : 126
ngspice 121 -> plot -i(vds)
ngspice 122 -> show
  Mos1: Level 1 MOSfet model with Meyer capacitance model
      device                    m1
       model                  mod1
          id                 1e-11
          is                     0
          ig                     0
          ib                -1e-11
         vgs                     0
         vds                     0
         vbs                     0
         von                     0
       vdsat                     0
          rs                     0
          rd                     0
          gm                     0
         gds                     0
         gmb                     0
         cbd                     0
         cbs                     0
         cgs                     0
         cgd                     0
         cgb                     0
        cbd0                     0
      cbdsw0                     0
        cbs0                     0
      cbssw0                     0

  Vsource: Independent voltage source
      device                   vgs                   vds
          dc                     5                    10
       acmag                     0                     0
           i                     0                     0
           p                    -0                    -0

ngspice 123 ->

I expect to get the classic MOSFET characteristics, but all I get is a 
straight line, and gm is zero.


Now, if instead I place a .DC line the circuit file and repeat then I 
get this...

[mirror-doors:~/spice/tested] steven% ngspice circuit2.cir
******
** ngspice-15 : Circuit level simulation program
** The U. C. Berkeley CAD Group
** Copyright 1985-1994, Regents of the University of California.
** Please submit bug-reports to: ngs...@li...
** Creation Date: Sat Mar 12 01:57:16 GMT 2005
******

Circuit: MOS OUTPUT CHARACTERISTICS - Example circuit 2

ngspice 132 -> listing
         MOS OUTPUT CHARACTERISTICS - Example circuit 2

      1 : mos output characteristics - example circuit 2
      2 : vds  1  0 dc 10
      3 : vgs  2  0 dc 5
      4 : m1   1  2  0  0 mod1 l=4u w=6u ad=10p as=10p
      5 : .model mod1 nmos level=1 vto=-2 nsub=1.0e15 uo=550
      6 : .dc vds 0 10 0.5 vgs 0 5 1
      8 : .end
ngspice 133 -> run
Doing analysis at TEMP = 300.150000 and TNOM = 300.150000

No. of Data Rows : 126
ngspice 134 -> plot -i(vds)
ngspice 135 -> show
  Mos1: Level 1 MOSfet model with Meyer capacitance model
      device                    m1
       model                  mod1
          id              0.000735
          is             -0.000735
          ig                     0
          ib            -1.001e-11
         vgs                     5
         vds                    10
         vbs                     0
         von                    -2
       vdsat                     7
          rs                     0
          rd                     0
          gm               0.00021
         gds                     0
         gmb                     0
         cbd                     0
         cbs                     0
         cgs                     0
         cgd                     0
         cgb                     0
        cbd0                     0
      cbdsw0                     0
        cbs0                     0
      cbssw0                     0

  Vsource: Independent voltage source
      device                   vgs                   vds
          dc                     5                    10
       acmag                     0                     0
           i                     0             -0.000735
           p                    -0               0.00735

ngspice 136 ->

And now I get the expected characteristic, and gm  is non zero

Why is .DC different to DC interactively?

I am using ngspice from CVS (taken yesterday).  It's on MacOSX, but I 
don't see why this should make a difference.

Regards,
Steven