The drawback of the circuit is that the self-heating of the sensor can be too high. We may connect an NTC and PTC in one circuit in order to have better sensitivity of any one simple circuit. Figure 8 ...

1. This circuit, based on a PIC microcontroller with a few added components, converts a sensor-based voltage into a 4- to 20-mA current loop. Shown is the circuit as originally drawn on a whiteboard.