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CHAPTER 7 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 = 0 ID = IDSS [1 – VGS / VGS(off)]2 • VGS(off) is the gate-source voltage for ID = 0 • gm0 is the max value of gm; occurs at VGS = 0 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 1. 2. 3. 4. 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 1 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. 1. VGS = V across Rg2 – Vs, where Vs is the drop across Rs. So VS = RS ID = VG – VGS (remember: ID = IS) 3. 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 ) = 1 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 = 1 / 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 = 1. 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 + 1 / gm) • An example: For gm = 2 mS , 1 / 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 = 0. 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.