.PS
cct_init

# Usual defs...
qrt=dimen_/4;
hlf=dimen_/2;
dim=dimen_;

Op1:opamp(right_,,,,P)
line right_ hlf
" $v_o$" ljust_
line from Op1.In1 left_ hlf
R1:dot; resistor(left_); rlabel(,R_2,); "$v_2$ " ljust_
line from R1 up_ hlf
resistor(left_); rlabel(,R_1,); "$v_1$ " ljust_
line from R1 down_ hlf
resistor(left_); rlabel(,R_3,); "$v_3$ " ljust_
line from Op1.In2 left_ qrt then down_ hlf
ground(,T)

move to R1 then right_ qrt; dot
line up_ hlf then right_ qrt
resistor(right_); llabel(,R_f,)
line right_ qrt
line to (Here,Op1.Out); dot

"\tiny Weighted" at (1.5*dim, -0.5*dim) rjust_ 
.PE