opamp verilog model
An operational amplifier, which is often called an op-amp, is a DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output.[1]. Typically the op-amp’s very large gain is controlled by negative feedback, which largely determines the magnitude of its output (“closed-loop”) voltage gain in amplifier applications, or the transfer function required (in analog computers). Without negative feedback, and perhaps with positive feedback for regeneration, an op-amp essentially acts as a comparator. High input impedance at the input terminals (ideally infinite) and low output impedance (ideally zero) are important typical characteristics.
verilog model
`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