UNIT 1
Introduction to Analog Electronics
Session-2
Characteristics of JFET (Common Source, Common Drain
and Common Gate configurations) and uses
Differences between BJT and JFET
BASICS
JUNCTION FIELD EFFECT TRANSISTOR (JFET)
There are two types of JFET’s:
n-channel and p-channel.
The n-channel is widely used.
Three terminals:
Drain (D) and Source (S) are
connected to n-channel
Gate (G) is connected to the p-type
material.
Gate is always reverse biased
Gate current, ID=0
CONSTRUCTION, TYPES & SYMBOLS
(Image source: www.electronics-tutorials.ws)
BASICS
JFET AMPLIFIER-CONFIGURATIONS
1. Common Source (CS) configuration (Good voltage amplifier) 1
Mostly used, Similar to CE transistor
Generally used in audio frequency amplifiers and in high input impedance
pre-amplifier stages.
2. Common Drain (CD) configuration (Good voltage buffer)
Source follower
High input impedance and a low output impedance 2
Approx. unity voltage gain-used in buffer amplifiers.
referred to as “Common Drain” because there is no signal available at the
drain connection.
3. Common Gate (CG) configuration (Good current buffer)
Has a low input impedance, but a high output impedance.
Applied in high frequency circuits or in impedance matching circuits 3
where a low input impedance needs to be matched to a high output
impedance
Microphone amplifiers
BASICS
COMMON SOURCE AMPLIFIER
In CS configuration, the input is given
to the gate and the output is taken from
the drain.
180 degree phase shift between input
and output.
Features
Large voltage gain
Good voltage amplifier
Better transconductance amplifier
High input resistance
Medium / high output resistance
BASICS
JFET (CS) WORKING
The depletion region becomes thicker with
increased reverse bias
When VGS =0 When VGS <0
(Adapted from: Electronic devices and Circuit
theory, Robert L Boylestad and Louis Nashelsky)
CONNECTION DIAGRAM FOR
DETERMINING THE CS AMPLFIER CHARACTERISTICS
CS AMPLIFIER CHARACTERISTICS
Drain or V-I characteristics
Output characteristics ID VS VDS FOR VGS = constant
Relationship between Drain current (ID) and
Drain to source voltage (VDS) for different
values of gate to source voltage (VGS)
Operation analysis includes
Without external bias: (VGS =0)
With external bias.
VGS Depletion layer ID.
CS AMPLIFIER CHARACTERISTICS
The transfer characteristics The transfer characteristics is drawn between
input voltage and output current.
The plot of the drain current Vs gate-source
voltage for different values of drain to source
voltage (VGS)
It is observed that the value of drain current
varies inversely with respect to gate-source
voltage when the drain-source voltage is
constant.
ID VS. VGS FOR VDS = constant
CS AMPLIFIER CHARACTERISTICS
At the pinch-off point,
any further increase in VDS does not
produce any increase in ID.
VGS at pinch-off is denoted as Vp.
ID is at saturation or maximum. It is referred
to as IDSS.
(Image source: www.rezzonics.blogspot.com)
CS AMPLIFIER CHARACTERISTICS
Important terms in JFET characteristics
Knee Point: Where the variation of drain current with drain-source voltage appears
to be linear. But beyond this point, the linearity changes into a curve.
Channel Ohmic Region: The region to the left of the knee point in the
characteristics curve.
Pinch-off point: The point in the curve beyond which the drain current will not
increase further no matter how much we are increasing the drain to source voltage.
Pinch-off Voltage: The voltage at the pinch-off point is termed as pinch-off voltage
because at this voltage, the current is completely turned to be constant.
Drain-Source Saturation Current: The drain to source saturation current is the
current which becomes constant and enters into a saturation state.
COMMON DRAIN JFET AMPLIFIER (Source follower)
The output characteristics
Connect voltmeters and
ammeters to measure the
necessary voltages and
currents (as explained in CS
amplifier connections)
The transfer characteristics
COMMON GATE JFET AMPLIFIER-CHARACTERISTICS
The output characteristics
Connect voltmeters and
ammeters to measure the
necessary voltages and
currents (as explained in CS
amplifier connections)
The transfer characteristics
COMPARISON BETWEEN BJT AND JFET
BJT JFET
Bipolar device (current condition, by both types of Unipolar device (current conduction is only due to
carriers, i.e. majority and minority-electrons and one type of majority carrier either electron or hole).
hole).
The operation depends on the injection of minority The operation depends on the control of a junction
carries across a forward biased junction. depletion width under reverse bias.
Current controlled device. The base current controls Voltage controlled device. The gate voltage controls
the output current. output current.
High noise level. (current conduction through Low noise level. ( current conduction is through n-
junctions) channel or p-channel and no junction crossing)
Low input impedance (due to forward bias at input High input impedance (due to reverse bias).
side).
Gain is characterized by voltage gain. Gain is characterised by transconductance.
Low thermal stability. (positive temperature Better thermal stability.(NTC at high current levels
coefficient at high current levels lead to thermal prevent thermal breakdown)
breakdown)
Cheaper Relatively costly
REVIEW QUESTIONS
Source follower is a circuit that provides_________
In CS amplifier, the drain-source current is largest when the gate-source
voltage VGS is____
Which JFET amplifier configuration is used in microphone amplifiers?
JFET is a ___ controlled device
UNIT 1
Introduction to Analog Electronics
NEXT Session-3
Transistor Amplifier: CE amplifier
Transistor Amplifier: CC ,CB amplifier
