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.