Method of realizing constant and accurate current source by cascading JFETs

　　Many process control sensors, such as thermistors and strain bridges, require precise bias currents. By adding a current-setting resistor, R1, the voltage reference circuit, IC1, can form a constant and precise current source (Figure 1). However, the error of this current source is related to the accuracy of R1 and IC1, and affects the measurement accuracy and resolution. Although we can use high-precision resistors with higher accuracy than most commonly used voltage references, IC1, the error of the voltage reference affects the accuracy of the current source. Manufacturers try to minimize the temperature sensitivity and output voltage error of the voltage reference, but sensitivity to power supply variations can still affect its accuracy, especially for process control applications that must operate over a wide range of supply voltages.

　　Cascaded JFETs reduce the effect of supply voltage fluctuations on current source accuracy

　　The constant-current source formed by a cascaded JFET pair, Q1 and Q2, reduces the reference circuit's sensitivity to supply-voltage fluctuations and extends IC1's operating voltage to its maximum rating of 5.5V. In addition, Q1 and Q2 effectively increase the current source's equivalent resistance from several megohms to almost the gigaohm range. In the Norton model of the circuit, the equivalent resistance represents the parallel resistance on an ideal current source.

　　When the gate-source bias of an N-channel JFET is 0V, it is a depletion-mode device operating at the maximum saturated drain current. Compared with a depletion-mode MOSFET that requires a gate bias to turn on, a JFET operates in the default on state and requires a gate bias.

　　The JFET is turned off by applying a gate bias voltage. When the gate-source voltage is more negative than the source voltage, the drain current of the JFET tends to zero at the turn-off voltage. The drain current of a JFET varies roughly around its gate bias current: ID≈IDSS×(1+VGS/VP)2, where ID is the drain current, IDSS is the drain saturation current, VGS is the gate-source voltage, and VP is the turn-off voltage.

　　Assume that IC1's output voltage VREF is held constant at 1.8V. Since the output voltage drives Q2's gate, IC1's input voltage VIN equals VREF - VGS(Q2), or 1.8V - (-1.2V) = 3V. Therefore, Q2's gate-source voltage remains at its nominal off-state voltage of 1.2V and changes in accordance with small changes in the current source. As the supply voltage changes from 3V to over 30V, the input voltage remains essentially constant because VREF also remains constant. The cascaded FET structure increases the Norton equivalent resistance of the current source beyond that of just the voltage reference and R1. Using a single JFET is possible, but stacking two JFETs further increases the equivalent impedance of the circuit. Note that IC1 does not degrade in accuracy because the JFETs hold IC1's input voltage nearly constant, and IC1 effectively cancels out the initial gate-source voltage changes and the temperature effects caused by Q1 and Q2.

　　The Kirchhoff voltage loop consists of VIN, VREF, and VGS (Q2), and its negative feedback allows the drain current to reach an equilibrium bias point that meets Q2's transfer equation. Q2's drain current is the sum of (VREF/R1) and IC1's internal "housekeeping" current IGND, which is constant. Adding Q1 reduces the effect of Q2's output impedance to an insignificant level. Adjusting the value of R1 varies the circuit's output current over a useful range (200mA to 5 mA), with Q2's saturated drain current being the upper limit. If you choose a JFET with a higher saturation drain current, be sure not to exceed Q1's maximum power dissipation.

　　Note that the circuit's lower supply voltage limit must be higher than the sum of the circuit's 3V voltage and the voltage drop produced by the sensor: ISOURCE × R2. The circuit's upper supply voltage limit must never exceed ISOURCE × R2 + 30V. For example, when a 1kΩ pressure sensor bridge R2 is fed with 2.5 mA, the supply voltage range is limited to 5.5V ~ 32.5V. Over a wide range of supply voltages, the circuit's output current varies by less than 1mA (Figure 2).