Overview: Conventional solutions using depletion mode FETs have wide variability in current and temperature coefficients. Figure 1 shows a two-terminal current source with two transistors and two Zener diodes. It provides both currents at both ends, but only a few percent accuracy. This circuit operates in an open loop and therefore does not provide the same precision as a closed loop feedback circuit. Because the temperature coefficient of the Zener diode does not match well with the transistor, drift and errors can occur in the circuit based on changes in the Zener diode and the base emitter voltage VBE of the transistor. In addition, a minimum of approximately 3V is required across the circuit for proper operation. A Zener diode (actually an IC) such as the LT1004 reduces the minimum operating voltage. (Please read the PDF for details) CNG/LPG Differential Pressure Sensor Cng/Lpg Differential Pressure Sensor,Barometric Pressure Sensor,Digital Pressure Sensor,Strain Gauge Pressure Transducer Shenzhen Ever-smart Sensor Technology Co., LTD , https://www.fluhandy.com
Current source design is a more difficult challenge than regulator design. Current sources at both ends introduce a host of new problems, especially when high temperature and stability are still desired when temperature changes. The current source must operate over a wide voltage range and exhibit high DC impedance and AC impedance when connected in series with an unknown reactance. It also requires good regulation and temperature coefficient. For the best two-terminal solution, the power supply bypass capacitor should not be used as it will reduce the AC impedance. 