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 g m ; BJT collector current is related to base current by .  I D = g m   Vgs where g m 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 g m =  Vgs /  I D is, obviously, not a constant

I D & I DSS, V GS & V GS(off ), g m & g m0 I DSS is the drain current when V GS = 0 I D = I DSS  [1 – V GS / V GS(off) ] 2 V GS(off) is the gate-source voltage for I D = 0 g m0 is the max value of g m ; occurs at V GS = 0 g m0 = (2  I DSS ) / V GS(off) g m = g m0  (1 - V GS / V GS(off) ) g m = g m0  sqrt [ I D / I DSS ] g m =  I D /  V GS

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.Since I D flows when V GS = 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. 2.The gate is grounded through a high valued resistor, and the gate current is zero. So the gate is at ground potential. 3.Based on 1 and 2, the gate becomes negative with respect to the source. I D will be limited by the negative V GS. 4.The JFET is biased.

Self-Biasing Since JFET parameters (g m0, I DSS, V GS(off) ) vary widely from device to device, self-biasing does not provide a predictable value for I D. Self-biasing holds g m reasonably constant from device to device since I D is more or less a constant percentage of I DSS (refer back to the equations). Constant g m is more important than constant I D in most applications. Voltage (Av) gain depends on g m.

Resistor-Divider Biasing If constant I D 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.V GS = V across R g2 – Vs, where Vs is the drop across Rs. So V S = R S  I D = V G – V GS (remember: I D = I S ) 3.The drop across Rs is large compared to V GS, & V G is fixed at a relatively high level, so I D = V S / R S is almost constant. Variations in V GS from device to device (or in the same device as the temperature changes) can have only a small effect on I D.

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  10 6  ) = 1 Volt!

Output Impedance: Zout For common-source amplifiers (equivalent to the common-emitter BJT) Zout = Rd where Rd is the resistor from V DD to the drain. (Note: V CC is for BJTs, V DD is for FETs.) For common-drain (equivalent to the common- collector BJT) Zout = (1 / g m ) || Rs which, in many cases, is more or less Zout = 1 / g m

Voltage Gain: Av For a common-source amplifier, Av = g m  Rd assuming Rs is bypassed with a capacitor. If not, then Av = Rd / (Rs + 1/g m ) 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 g m, and the value of the source resistor Rs. The equation is: Av = Rs / (Rs + 1 / g m ) An example: For g m = 2 mS, 1 / g m = 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

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 R DS(on), the resistance of the channel when V GS = 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.