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| SEC. 13-1 BASIC IDEAS The junction FET, abbreviated JFET, has a source, gate, and drain. The JFET has two diodes: the gate-source diode and the gate-drain diode. For normal operation, the gate-source diode is reverse biased. Then, the gate voltage controls the drain current. SEC. 13-2 DRAIN CURVES Maximum drain current occurs when the gate-source voltage is zero. The pinchoff voltage separates the ohmic and active regions for VGS = 0. The gate-source cutoff voltage has the same magnitude as the pinchoff voltage. VGS(off) turns the JFET off. SEC. 13-3 THE TRANSCONDUCTANCE CURVE This is a graph of drain current versus gate-source voltage. The drain current increases more rapidly as VGS approaches zero. Because the equation for drain current contains a squared quantity, JFETs are referred to as square-law devices. The normalized transconductance curve shows that ID equals one-quarter of maximum when VGS equals half of cutoff. SEC. 13-4 BIASING IN THE OHMIC REGION Gate bias is used to bias a JFET in the ohmic region. When it operates in the ohmic region, a JFET is equivalent to a small resistance of RDS. To ensure operation in the ohmic region, the JFET is driven into hard saturation by using VGS = 0 and ID(sat) « IDSS. SEC. 13-5 BIASING IN THE ACTIVE REGION When the gate voltage is much larger than VGS, voltage-divider bias can set up a stable Q point in the active region. When positive and negative supply voltages are available, two-supply source bias can be used to swamp out the variations in VGS and set up a stable Q point. When supply voltages are not large, current-source bias can be used to get a stable Q point. Self-bias is used only with small-signal amplifiers because the Q point is less stable than with the other biasing methods. SEC. 13-6 TRANSCONDUCTANCE Transconductance gm tells us how effective the gate voltage is in controlling the drain current. The quantity gm is the slope of the transconductance curve, which increases as VGS approaches zero. Data sheets may list gfs and siemens, which are equivalent to gm and mhos. SEC. 13-7 JFET AMPLIFIERS A CS amplifier has a voltage gain of gmrd and produces an inverted output signal. One of the most important uses of a JFET amplifier is the source follower, which is often used at the front end of systems because of its high input resistance. SEC. 13-8 THE JFET ANALOG SWITCH In this application, the JFET acts like a switch that either transmits or blocks a small ac signal. To get this type of action, the JFET is biased into hard saturation or cutoff, depending on whether VGSis high or low. JFET shunt and series switches are used. The series type has a higher on-off ratio. SEC. 13-9 OTHER JFET APPLICATIONS JFETs are used in multiplexers (ohmic), chopper amplifiers (ohmic), buffer amplifiers (active), voltage-controlled resistors (ohmic), AGC circuits (ohmic), cascode amplifiers (active), current sources (active), and current limiters (ohmic and active). SEC. 13-10 READING DATA SHEETS JFETs are mainly small-signal devices because most JFETs have a power rating of less than 1 W. When reading data sheets, start with the maximum ratings. Sometimes data sheets omit the minimum VGS(off ) or other parameters. The large spread in JFET parameters justifies using ideal approximations for preliminary analysis and troubleshooting. SEC. 13-11 JFET TESTING JFETs can be tested using an ohmmeter or DMM on the diode test range. Care must be taken not to exceed the JFET’s current limits. Curve tracers and circuits can be used to display a JFET’s dynamic characteristics. |