Re: [Help-gnucap] Bug? convergence problem?

[gserdyuk]

Hello Orestes,

likely that is relaxed convergence criterion. Opamp is very sensitive
to that stuff.

add

.options abstol=1.e-15

and it will fix this issue.


if you will make DC step finer - result will be the same (no peak).

regards
Gena

Thursday, February 10, 2011, 4:46:31 PM, you wrote:

OM> Hello,

OM> I'm facing a weird result when I simulate a simple inverter amplifier built
OM> around an operational amplifier. Basically I'm trying to obtain the input-
OM> output transfer characteristic (for educational purposes), but if the number
OM> of points in the DC command is low, I obtain an spurious peak at the output.

OM> The circuit is:

OM> -------
OM> .INCLUDE uA741.model
OM> Vcc 4 0 DC 15
OM> Vdd 0 5 DC 15
OM> vg 1 0 DC 2.5
OM> R1 1 2 2.2k
OM> R2 2 3 15k
OM> X1 0 2 4 5 3 uA741
OM> .PRINT DC v(3)
OM> .DC vg -5 5 0.5
OM> .END
OM> ---------

OM> If you simulate that you'll see an output peak of -17V for the Vg=2.5V 
input.

**********>>>> NOTE: Using the "OP" command yelds the correct solution (or a 
OM> more plausible one) of -14.613V ************

OM> I need some help to determine if this strange behaviour is originated by a 
bug
OM> in gnucap, an expectable convergence issue, or a wrong macromodel adaption
OM> (see below)

OM> Can anyone help me? Thanks in advance.


OM> (((More info: The AO macromodel is the pspice one slightly modified to make 
it
OM> work on gnucap. Here is the file:

OM> 
*-------------------------------------------------------------------------------
OM> * Library of linear IC definitions

OM> * This is a reduced version of MicroSim's linear subcircuit library.
OM> * You are welcome to make as many copies of it as you find convenient.
OM> *
OM> * The parameters in the opamp library were derived from the data sheets for
OM> * each part.  The macromodel used is similar to the one described in:
OM> *
OM> *       Macromodeling of Integrated Circuit Operational Amplifiers
OM> *         by Graeme Boyle, Barry Cohn, Donald Pederson, and James Solomon
OM> *       IEEE Journal of SoliE-State Circuits, Vol. SC-9, no. 6, Dec. 1974
OM> *
OM> * Differences from the reference (above) occur in the output limiting stage
OM> * which was modified to reduce internally generated currents associated with
OM> * output voltage limiting, as well as short-circuit current limiting.
OM> *
OM> * The opamps are modelled at room temperature and do not track changes with
OM> * temperature.  This library file contains models for nominal, not worst 
case,
OM> * devices.
OM> *
OM> *$
OM> 
*-----------------------------------------------------------------------------
OM> * connections:   non-inverting input
OM> *                |  inverting input
OM> *                |  |  positive power supply
OM> *                |  |  |  negative power supply
OM> *                |  |  |  |  output
OM> *                |  |  |  |  |
OM> .subckt uA741    1  2  3  4  5
OM> *
OM>   c1   11 12 8.661E-12
OM>   c2    6  7 30.00E-12
OM>   dc    5 53 dx
OM>   de   54  5 dx
OM>   dlp  90 91 dx
OM>   dln  92 90 dx
OM>   dp    4  3 dx
OM> * POLY font rewritten to make it work under gnucap
OM> * egnd 99  0 poly(2) (3,0) (4,0) 0 .5 .5
OM> egnd1 99 98 3 0 .5
OM> egnd2 98 0 4 0 .5
OM> * POLY font rewritten to make it work under gnucap
OM> * fb    7 99 poly(5) vb vc ve vlp vln 0 10.61E6 -10E6 10E6 10E6 -10E6
OM>   fb1   7 99 vb 10.61E6
OM>   fb2   7 99 vc -10E6
OM>   fb3   7 99 ve 10E6
OM>   fb4   7 99 vlp 10E6
OM>   fb5   7 99 vln -10E6
OM>   ga    6  0 11 12 188.5E-6
OM>   gcm   0  6 10 99 5.961E-9
OM>   iee  10  4 dc 15.16E-6
OM>   hlim 90  0 vlim 1K
OM>   q1   11  2 13 qx
OM>   q2   12  1 14 qx
OM>   r2    6  9 100.0E3
OM>   rc1   3 11 5.305E3
OM>   rc2   3 12 5.305E3
OM>   re1  13 10 1.836E3
OM>   re2  14 10 1.836E3
OM>   ree  10 99 13.19E6
OM>   ro1   8  5 50
OM>   ro2   7 99 100
OM>   rp    3  4 18.16E3
OM>   vb    9  0 dc 0
OM>   vc    3 53 dc 1
OM>   ve   54  4 dc 1
OM>   vlim  7  8 dc 0
OM>   vlp  91  0 dc 40
OM>   vln   0 92 dc 40
OM> .model dx D(Is=800.0E-18 Rs=1)
OM> .model qx NPN(Is=800.0E-18 Bf=93.75)
OM> .ends

OM> _______________________________________________
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-- 
Best regards,
 Gserdyuk                            mailto:address@hidden