CHAPTER 7: Junction Field-Effect Transistors doc

Introduction • JFETs have three leads: drain, gate, and source which are similar to the collector, base, and emitter of a bipolar junction transistor BJT.. • The gate of a JFET is revers

CHAPTER 7

Junction

Field-Effect Transistors

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

 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 /  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 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

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 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

• 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