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> _______________________________________________ OM> Help-gnucap mailing list OM> [email protected] OM> http://lists.gnu.org/mailman/listinfo/help-gnucap -- Best regards, Gserdyuk mailto:[email protected] _______________________________________________ Help-gnucap mailing list [email protected] http://lists.gnu.org/mailman/listinfo/help-gnucap
