I am receiving PWM signal at 7.8V from a given controller, with which we need to control the speed of a DC motor at 18 V. I implemented the following circuit for this. However, the output to motor signal (blue) is not following the original PWM (yellow) and duty cycle is increased by 50%. What could be the culprit. I dont think it is the awkward taper off on the downleg of the input signal .

What could be a remedy?