CHAPTER 7: Junction Field-Effect Transistors doc

CHAPTER Junction Field-Effect Transistors OBJECTIVES Describe and Analyze: • JFET theory • JFETS vs Bipolars • JFET Characteristics • JFET Biasing • JFET Circuits & Applications • Troubleshooting Introduction • JFETs have three leads: drain, gate, and source which are similar to the collector, base, and emitter of a bipolar junction transistor (BJT) • JFETs come in N-channel and P-channel types similar to NPN and PNP for BJTs • JFETs conduct majority carriers while BJTs conduct minority carriers • The gate of a JFET is reverse biased; the base of a BJT is forward biased • JFETs have high Zin; BJTs have low Zin • JFETs are more non-linear than BJTs Introduction • JFETs are on until you apply a gate voltage to turn them off; BJTs are off until you apply base current • JFET drain current is related to gate voltage by gm; BJT collector current is related to base current by   ID = gm   Vgs where gm is the mutual conductance or transconductance, and Vgs is the gate-source voltage JFET Construction Increasing Vgs causes the depletion region to grow Transconductance Curve gm = Vgs / ID is, obviously, not a constant ID & IDSS, VGS & VGS(off), gm & gm0 • IDSS is the drain current when VGS = ID = IDSS  [1 – VGS / VGS(off)]2 • VGS(off) is the gate-source voltage for ID = • gm0 is the max value of gm; occurs at VGS = gm0 = (2  IDSS) / VGS(off) gm = gm0  (1 - VGS / VGS(off)) gm = gm0  sqrt [ ID / IDSS ] gm = ID /  VGS JFET Biasing There are several ways to set the Q-point of a JFET Self-Biasing The easiest way to bias a JFET is self-biasing Self-Biasing Since ID flows when VGS = 0, putting a resistor in the source leg makes the source pin positive with respect to ground, or ground negative with respect to the source pin The gate is grounded through a high valued resistor, and the gate current is zero So the gate is at ground potential Based on and 2, the gate becomes negative with respect to the source ID will be limited by the negative VGS The JFET is biased Self-Biasing • Since JFET parameters (gm0, IDSS, VGS(off)) vary widely from device to device, self-biasing does not provide a predictable value for ID • Self-biasing holds gm reasonably constant from device to device since ID is more or less a constant percentage of IDSS (refer back to the equations) • Constant gm is more important than constant ID in most applications • Voltage (Av) gain depends on gm Resistor-Divider Biasing If constant ID is important, this is how you get it R-Divider Biasing The gate is held at a fixed voltage (with respect to ground) by a resistor divider VGS = V across Rg2 – Vs, where Vs is the drop across Rs So VS = RS  ID = VG – VGS (remember: ID = IS) The drop across Rs is large compared to VGS, & VG is fixed at a relatively high level, so ID = VS / RS is almost constant Variations in VGS from device to device (or in the same device as the temperature changes) can have only a small effect on ID Source Biasing Can be done, but not commonly used Input Impedance: Zin • Since the gate is reverse-biased, the input impedance of a JFET is, for all practical purposes, equal to the external resistance between gate and ground • For a self-biased JFET, Zin = Rg where Rg is the resistor from gate to ground • The only limit on Rg is the reverse leakage current of the gate So Rg = 1000 Meg-Ohms is not a good idea since (1 nA)  (1000  106 ) = Volt! Output Impedance: Zout • For common-source amplifiers (equivalent to the common-emitter BJT) Zout = Rd where Rd is the resistor from VDD to the drain (Note: VCC is for BJTs, VDD is for FETs.) • For common-drain (equivalent to the commoncollector BJT) Zout = (1 / gm) || Rs which, in many cases, is more or less Zout = / gm Voltage Gain: Av • For a common-source amplifier, Av = gm  Rd assuming Rs is bypassed with a capacitor If not, then Av = Rd / (Rs + 1/gm) • For a common-drain amplifier, equivalent to an emitter follower, you would expect the gain to be Av = But it’s not; it’s less How much less depends on the JFET’s gm, and the value of the source resistor Rs The equation is: Av = Rs / (Rs + / gm) • An example: For gm = mS , / gm = 500 Ohms If Rs = 500 Ohms, then Av = 500 / 100 = 0.5 JFET Applications • A common application of JFETs is in the “front-end” of a radio receiver JFETS are inherently quieter than BJTs, meaning that the internal noise they generate is less than in a BJT Since the first amplifier is crucial in terms of noise in a receiver, it’s a good place to use a JFET Self-biasing is fine since the signal levels are typically microVolts • Another place to use a JFET amplifier is for any signal source that has a high internal resistance JFET as a Switch JFET as a Switch JFET as a Switch • JFETs can be used as voltage controlled switches for switching low-level analog signals • As seen in the previous slide, the control signal is digital: on or off • JFETs can be used as series switches or as shunt switches • When used as a switch, the key JFET parameter is RDS(on), the resistance of the channel when VGS = Troubleshooting • Unlike BJTs, JFETs can’t be checked easily with an Ohm-meter • As usual, check the DC bias levels • Check the input and output levels of signals to see if they are approximately what you expected • If it’s necessary to replace a JFET, use the same part number If that’s not an option, pick a device suitable for the application: switch, RF amplifier, etc  ... leads: drain, gate, and source which are similar to the collector, base, and emitter of a bipolar junction transistor (BJT) • JFETs come in N-channel and P-channel types similar to NPN and PNP for