JFET Applications

JFET has a many applications in electronics and communication field .Some applications of JFET described as following:

JFET buffer amplifier:

It is a one type of amplifier.This JFET amplifier isolates preceding amplification stage from the following amplification stage.A JFET acts as an well buffer amplifier because of the high impedance on output (i.e loading of the preceding stage is high ).This allows the almost whole output from first amplification stage to enter at the buffer input stage.The heavy loads drive by low out put impedance of JFET.This indicates that entire output from the buffer stage reaches the input circuits of the next amplification stage in a JFET buffer amplifier.

JFET phase-shift oscillators:

In the BJT phase-shift oscillators there is appears loading effect because of low input impedance of BJT .To minimise the loading effect in phase-shift oscillators ,the high input impedance of JFET is used in this phase-shift oscillators.

JFET used as RF amplifier:

JFET RF amplifier is used in many electronics and communication field in receiver instead of BJT RF amplifier because of following reasons;

• JFET has very low noise level.In JFET RF amplifier in receiver JFET will not produces significant value of noise in JFET RF amplifier.
• JFET is good respond to low current signal generated by the antenna because of JFET is voltage controlled semiconductor devices .SO JFET used as RF amplifier in receiver section in communication or broadcasting field.

JFET

JEFT refer to junction field effect transistor .A bipolar junction has major two disadvantage ,one is low input impedance because of forward biased of transistor and high noise level.To over come those two disadvantage JFET is used in many application instead of BJT.Bipolar junction transistor is a current controlled electronics devices that means the output characteristics of BJT are controlled by input base current where as FET is a electronics devices whose output characteristics of its is controlled by input voltage (i.e electric field).JFET is a one type of FET (i.e field effect transistor).It is used in IC (i.e integrate-circuits) application so it is very very important semiconductor devices in this field. The JFET have three terminal and conduction is done by either electrons or holes charge carriers where as in BJT(i.e bipolar junction transistor ) the current conduction is done by both charge carriers (i.e holes and electrons) .

Symbol of JFET:

Construction of JFET:

This figure shows the construction of N-channel JFET .Here is only one type (i.e n-type ) semiconductor material is used as bar with two PN junction layer at the side of the this bar as shown in figure below.Conducting channel for the charge carriers is formed by the bar material . At the side of the bar material P-type material is diffused in proper amount if the bar is N-type material .If this conducting channel is N-type s/c material and diffused material is P-type then this type of JFET is called N-channel JFET where as If the conducting channel is P-type material and diffused material is N-type then this type of JFET is called P-channel JFET.

This above figure is N-channel JFET. In this construction two PN junction forms such as two diodes connection internally at the side of the bar.The terminal which is taken out from diffused material is called Gate terminal (G). And other two terminal which are taken out from Bar material is called Drain terminal (D) and source terminal (S).So JFET has three terminals such as gate ,drain and source terminals.

Polarities of JFET:

This above figure shows the general polaritie

s of N-channel JFET where as the polarities of P-channel JFET is just opposite is that of N-channel JFET.
JFET is three terminal electronics devices. It consists of Gate terminal ,Drain terminal and Source terminal .In put gate voltage or potential between the gate terminal and the source terminal is always reverse biased in the both type of the JFET and source terminal is such that way internal PN junction diode becomes reverse biased connection .That why JFET has high input impedance .The drain terminal and source terminal can interchange But one things can be always take care that the drain terminal is so c

onnected w.r.t source that the drain current is always flows from source terminal to the drain terminal and the amount of source current is equal to the drain current.

Principle of JFET:

This JFET circuit shows N-channel JFET with general polarities where gate input voltage is reverse bias with source.In this JFET circuit the two depletion layers are formed at the side of two PN junction in its internal body .This is N-channel JFET circuits so current conduction is

done by free electrons where as in the case of P-channel JFET circuits polarities is just opposite that of N-channel JFET circuits and current conduction is done by hole charge carriers . In the case of fixed width of conduction channel its resistance can be controlled by varying input gate voltage or potential. If the reverse input gate voltage is high the area of depletion layers is increase and conducting channel is decrease .Hence drain current is decrease .So conducting channel is varied by changing the reverse input gate voltage (i.e amount of drain current can be change according to input gate voltage .

Working of JFET:It has two condition as below:

• When Gate voltage is zero :

When the input of the biased is open (i.e potential deference between the drain and source terminal is zero or gate voltage is zero as shown in figure below .The depletion layers is form at the side of the bar by the two PN junction .Under this condition current conduction from source to drain via conduction channel between two depletion layers determine the width of the conducting channel and amount of current flow.

• When gate voltage is applied:

Connection of the gate terminal and source terminal is reverse biased .If the input voltage is applied or closed to gate and source terminal then the width of the depletion layers is increased .Hence the width of the conducting channel is decreased and resistance of N-type conducting channel bar is increased .As a result flow of current from source terminal to the drain terminal is decrease because of its resistance .Just opposite action will happen if input gate or reverse voltage is decrease .Hence the drain current is increased because of its resistance decrease.Hence it is note that flow of current from source to drain can be controlled by reveres gate input voltage .This is similar way , in the case of P-channel JFET but current conduction is done by holes instead of electrons in N-channel JFET and polarities is just opposite that of N-channel JFET.

Features of JFET: JFET has following features;

• In JFET gate-source PN junction of JFET is always worked in reverse biased connection.
• JFET is voltage controlled semiconductor devices with three terminals (i.e gate ,drain and source termianl)
• In JFET the output drin current is controlled by varying the width of the conducting channel.
• Gate current in JFET is zero.
• Since in JFET drain current is equal to the source current at that condition.
• JFET is not temperature sensitive devices.
• In JFET depletion layers of Pn junction increase or decrease by an equal amount because of same potential in two input gate terminals.
• JFET is working in two conditions that is when gate termianl voltage is zero and when gate terminal voltage is applied.

Advantages of JFET:

BJT is current controlled devices where as JFET is not current controlled devices but JFET is voltage-controlled semiconductor devices (i.e change in input voltage controlled the output drain current) .JFET is similar to a vacumm pentode but it has more advantages;

• In JFET input impedance of its is very high.
• It has high isolation between input circuits and output circuits.
• JFET is negative temperature co-efficient of resistance devices.
• JFET has no risk of thermal runway.
• JFET has long life.
• JFET has high efficiency .
• JFET is smaller in size.
• Power gain of JFET is very high ,Hence no needed of other amplifier stage.

JFET Amplifier

Here given figure shows JFET amplifier circuits of one stage .It consist of biasing voltage JFET ,coupling devices such as capacitor in out put circuits . The weak input signal which can be going to be amplified is given between gate terminal and sources terminal with the help of the reverse biasing voltage between these two gate terminal and source terminal .Amplified output signal is obtained in the output drain-source terminal circuits .For proper function of JFET we must take care in polarities of JFET and biasing voltage .Here input gate terminal must be negative w.r.t source terminal in every JFET amplifier (i.e input circuits always be reverse biased in every JFET amplifier ). It consist of extra biasing circuits So for this biasing voltage battery is connected in this JFET amplifier.
In this case small change in the reverse input biasing voltage produces a large change in drain current in output circuits of JFET amplifier.Hence JFET act as amplifier which amplified a weak input signal.

At the time of positive half-cycle of input weak signal the reverse biasing voltage on the gate terminal decreased .hence the width of the conducting channel is increased and also increased drain current in the JFET amplifier.

At the time of negative half-cycle of input weak signal the reverse biasing voltage on the gate terminal is increased .Hence the width of the conducting channel is decreased or become narrow and decreased drain current in the out put circuits of JFET amplifier.It is noted that a small change in input gate voltage at the gate terminal produces a large change in drain current at the output of JFET amplifier .So JFET act as an amplifier .

Connection of JFET amplifier:

JFET has a three terminal such as gate ,drain and source terminals But when JFET are going to be connected in JFET amplifier circuits it has needed four terminals.Two terminal for input circuits and two terminal for output circuits connection.But here one terminal is common both input circuits and output circuits .So following are the JFET amplifier connections :

• Common drain connection JFET amplifier circuits
• Common source connection JFET amplifier circuits
• Common gate connection JFET amplifier circuits

But from above connection JFET amplifier circuits the common source connection JFET amplifier circuits is must extremely used connection as JFET amplifier because of its high input impedance ,moderate output impedance and well voltage gain .But this JFET amplifier connection provides phase reversal (i.e the angle of output signal is 180 degree difference compare with the angle of input signal).JFET amplifier in common source connection JFET amplifier circuits is similar with the common emitter transistor amplifier.

JFET amplifier(practical):

JFET amplifier accomplished faithfull amplification when associated circuits used or connects properly with JFET.Above JFET amplifier shows the practical JFET amplifier which consists of following components properly as shown in above figure.

• Gate resistor(R2) : In JFET amplifier gate resistor is used to maintain gate terminal is nearly about 0 V d.c (i.e gate terminal current is approximately zero).Gate resistor is in several mega ohms value which prevents loading of the a.c signal source.


• Coupling capacitor(C1) : In JFET amplifier coupling capacitor used as coupling devices to one stage to next stage and in input gate terminal in order to pass only input a.c signal and blocked d.c biasing signal .


• Bypass capacitor(Cs) : In JFET amplifier it is used with source resistor in parallel as shown in above figure.Which provides easy path for a.c signal to grounded which prevent voltage drop in source resistor and increase voltage gain.