Download JFET Biasing

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.