Temperature-stable heat pulse driver circuit for low-voltage single supply soil moisture sensors based on junction transistors

摘要

A temperature-stable heat pulse driver for low-voltage single supply soil moisture sensors based on junction transistors is presented. The temperature drift of the heat pulse, caused by the variations d/d in the transistors placed in the feedback loop, is fully compensated for. A circuit measures the variations d/d of the transistors in the feedback loop and generates a current which is proportional to the - (1/0) d/d that, when passed through another resistor inside the feedback loop ( = ), provides full temperature compensation. The topology of the circuit also provides an output voltage (the voltage used to define the heat pulse) which does not need to be calibrated, since it depends only on a reference voltage external to the feedback loop.