Voltage reference improves JFET


Posted on Feb 3, 2013

The common JFET current-source in Figure 1a has average output impedance and depends heavily on the JFET`s VP and IDSS variations. These manufacturing variations limit the initial accuracy of the current setpoint that the circuit can obtain with a fixed R1. Most users of JFET current-sources sample VP and IDSS for each lot of JFETs and then select R1 by first solving the quadratic equation ID=IDSS(1VGS/VP)2 for VGS and then solving the current-setpoint equation ISOURCE=VGS/R1.






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