CHAPTER 7

Junction

Field-Effect

Transistors

OBJECTIVESDescribe 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. 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.

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. ID will be limited by the negative VGS.

4. 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 common-collector 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.