I don't think it's your bias that results in the linear curve. You need to check the value AND the ratio of the two resistors. Pay attention to the fact that the value of the two resistors define both where your lowest TC point(the slope=0) is and how much the reference voltage or current at the zero slope point is.
The most possible explanation of your problem, I guess, is that the delta Vbe term or Vbe term is too much larger than the other. So, reratio and adjust your resistors. There is another hint that you can try to use DC simulation and Sweep the resistor to get a group of curves. And it's easy to find the trend. Then you will get your desired parabolic curve like the text book or paper gives.