Chapter 6 - Using JFETs and CRDs to Create Current Sources

Abstract

Junction field-effect transistors (JFETs) can be used to create ultra-low-leakage diodes, which can also be of use when creating current sources or voltage references. The JFET has many diverse applications, which exploit its various characteristics, when compared with the bipolar junction transistor (BJT). Besides voltage-controlled resistors, they include low-noise, differential, and high input-impedance (Zin) amplifiers, audio and AGC amplifiers, video and RF amplifiers, analog switches, RF mixers, H-F oscillators, and current sources. In current analog integrated circuits (ICs) designs, JFETs are commonly used for circuit start-up, biasing, and front-ends in bipolar-FET op amps. This chapter discusses the characteristics of the JFET and CRD. The silicon junction FET is made similarly to a diode or bipolar transistor (by the planar, epitaxial process), it has some unique characteristics of its own, not shared by the others. However, because the JFET is also a junction device, it inherently has some of the same characteristics as both the diode and BJT. The current regulator diode (CRD) is equivalent to the single JFET current source. JFET CRDs do not generally reflect as high a degree of accuracy or regulation as the JFET cascode circuit. However, CRDs do remain attractive solutions to many current source applications, but it is fully monolithic, with its geometry specially designed and enhanced for precise current regulation. Besides being a low-cost item, a CRD has a fixed current level, a high output impedance, and low noise. Two other important benefits include a small two-lead package and a specified low temperature coefficient.