JFET cascade technology to improve current source performance

　　Many process control sensors, such as thermistors and strain bridges, require accurate bias current. By adding a current setting resistor R ₁ , the voltage reference circuit IC ₁ can form a constant and accurate current source (Figure 1). However, the error of this current source is related to the accuracy of R ₁ and IC ₁ and affects the measurement accuracy and resolution. Although we can use high-precision resistors that are more accurate than most commonly used voltage reference ICs, the accuracy of the current source is limited by the error in the voltage reference _. Manufacturers will try their best to reduce the temperature sensitivity and output voltage error of a voltage reference, but sensitivity to power supply changes can still affect its accuracy, especially for process control applications that must operate over a wide supply voltage range.

　　Using a cascaded JFET pair—Q ₁ and Q _2— to form a constant current source reduces the sensitivity of the reference circuit to supply voltage fluctuations and extends IC ₁ 's operating voltage to its 5.5V maximum rating. Additionally, Q ₁ and Q ₂ effectively increase the equivalent resistance of the current source from the multi-megaohm range to almost the giga-ohm range. In the Norton model of a circuit, the equivalent resistance represents the parallel resistance across an ideal current source.

　　When the gate-source bias voltage of the N-channel JFET is 0V, it is a depletion-mode device operating at the maximum saturated drain current. Compared with depletion-mode MOSFETs that require gate bias to turn on, JFETs operate in the default on state and require gate bias to turn off. When the gate-to-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: I _D ≈I _DSS ×(1+V _GS /V _P ) ² , where I _D is the drain current, I _DSS is the drain saturation current, V _GS is the gate-source voltage, and VP _is the turn-off voltage. Assume that the output voltage V _REF

of 　　IC ₁ remains constant at 1.8V. Since the output voltage drives Q ₂ 's gate, IC ₁ 's input voltage V _IN is equal to V _REF -V _GS(Q2) , or 1.8V-(-1.2V)=3V. Therefore, Q2 _'s gate-source voltage remains at its nominal turn-off voltage of 1.2V and changes consistently with small changes in the current source. When the supply voltage changes from 3V to over 30V, the input voltage remains essentially constant because V _REF also remains unchanged. The cascaded FET structure increases the Norton equivalent resistance of the current source beyond the case of just the voltage reference and _R1 . It is also possible to use a single JFET, but stacking two JFETs can further increase the equivalent impedance of the circuit. Note that IC ₁ does not lose accuracy because the JFET maintains IC ₁ 's input voltage at a nearly constant state, while IC ₁ also effectively eliminates the initial gate-source voltage variation, as well as the temperature effects caused by Q ₁ and Q ₂ . 　　The Kirchhoff voltage loop is composed of V _IN , V _REF and V _{GS (Q2)} , and its negative feedback can make the drain current reach a balanced bias point, consistent with the transmission formula of Q ₂ . The drain current of Q ₂ is the sum of (V _REF /R ₁ ) and IC _1's internal "housekeeping" current I _GND , which is constant. Adding Q _reduces the effect of Q _'s output impedance to the point where it is inconsequential. Adjusting the value of R1 _can vary the output current of the circuit within a useful range (200mA~5 mA), while the saturated drain current of Q2 is the upper limit _. If you choose a JFET with a higher saturation drain current, be sure to not exceed the maximum power dissipation value of _Q1 . 　　Note that the lower limit of the circuit's power supply voltage must be higher than the sum of the circuit's 3V voltage and the voltage drop produced by the sensor: I _SOURCE ×R ₂ . The upper limit of the circuit's supply voltage must never exceed I _SOURCE ×R ₂ +30V. For example, when adding 2.5 mA current to a 1kΩ pressure sensor bridge R2 _, the supply voltage range is limited to 5.5V ~ 32.5V. The circuit's output current varies less than 1mA over a wide range of supply voltages (Figure 2).