`define dB2dec(x) pow(10,x/20)

module top;
 electrical vinp,vinm,vdd,vss,voutp,voutm;

 opamp i1(vinp,vinm,vdd,vss,voutp,voutm);
endmodule

module opamp(vinp,vinm,vdd,vss,voutp,voutm);
 inout vinp,vinm,vdd,vss;
 inout voutp,voutm;
 electrical vinp,vinm,vdd,vss,voutp,voutm;

 parameter real gain = 100 from (0:inf),          // open loop gain in dB
                three_dB_freq  = 1M from (0:inf), // 3dB frequency
                rin  = 1M from (0:inf),           // input resistance
                cin  = 1n from [0:inf),           // input capacitance
                iout_max = 100n from (0:inf),     // max. output current
                vout_offset = 0,                  // output dc offset, must be between
                                                  // Vdd and Vss, but is not checked in
                                                  // model
                rout = 80 from (0:inf),           // output resistance
                volc = 1;                         // coefficient, influences ho w hard
                                                  // output voltage is switching into
                                                  // saturation

 real vin,vout,voutmax,voutmin,vout0,iout;

 analog
 begin
   vin  = V(vinp,vinm);
   vout = V(voutm,voutm);
   voutmax = V(vdd);
   voutmin = V(vss);

   // input stage
   I(vinp,vinm) <+ vin / rin + cin * ddt(vin);

   // dominant pole
   vout0 = laplace_nd(vin*`dB2dec(gain),{1},{1,1/(`M_TWOPI*three_dB_freq)})
           + vout_offset;

   // output current
   iout = (vout0 - vout) / rout;

   // output current limitation
   case (1)
     iout >  iout_max : iout =  iout_max;
     iout < -iout_max : iout = -iout_max;
   endcase

   /* output voltage limitation
      it may have a discontinuity at vout=vout_offset
      therefor we assign iout via slew function
   */
   if (vout >= vout_offset)
     iout = iout*tanh(volc*(voutmax-vout));
   else
     iout = iout*tanh(volc*abs(voutmin-vout));

   /* slewrate applied is only an estimation to smooth out
      the discontinuity introduced by the output voltage limitation
      it is not the slew rate of a real opamp !!!
   */
   I(voutp,voutm) <+ slew(iout,iout_max*three_dB_freq);
 end
endmodule