JFET and its configuration

-JFET (junction gate field effect transistor) that kind of transistor that’s main difference from the BJT (bi-polar transistor) is that BJT can be considered as current control devices (dependent current source) while the JFET can be considered as voltage control device.
There are two types of JFET, the p-type JFET that consists of a middle channel of a semiconductor material having large number of positively charges carriers, while the N-type JFET that consists of a middle channel of a semiconductor material of negatively charged carriers.
Through this transistor the Electric charge passes through the channel from the source pole to the drain pole along the channel.

As shown here there are three terminals for the JFET, the Gate which controls the Electric current passes through the channel, the source which is the Source of electric charge and the Drain where the electric charge is collected.

So in this transistor as we apply some voltages at its terminals, V_GS = 0 v, V_DS = +ve voltage.

So at the channel which is N-type the electrons will be attracted to the drain which we apply a positive voltage at it, at the gate terminal which is P-type, by the applied voltages here we noticed that the p-type forms with the n-type a reverse bias which makes the depletion region larger and by increasing the drain source voltage V_DS the depletion region larges until it reaches a critical point where approximately no electric field can pass from the source to drain and that makes the resistivity of the device increase, this voltage is called the pinch-off voltage V_P, a small current can pass through the channel that us highly attracted to the drain terminal which called the saturation current I_DSS that passes through the channel when applying V_DS  ≥ V_P after exciding the pinch off voltage , the current will stay constant which will be I_DSS and its physical formula can be given by :

I_D = I_DSS (1-VGS/VP)^2

There are three types of connecting a JFET in a Circuit to do a certain function.

a.Common gate

This configuration provides a low input impedance while provides a high output impedance, although the voltage is high, but the current gain as well as the power gain are low, there is an advantage in this configuration is that the input and the output are in phase.  

b. Common drain

This configuration is known as source follower, as the source voltage follows the gate input voltage, that offers high input impedance and low output impedance, it is used as a buffer just to separate the input from the output,
The voltage gain here is one, the input voltage and the output one seems to have the same voltage, although the current gain is high, and the input and the output signals are in phase.

c.Common Source

This configuration is the most widely used as the common source circuit in general provides a medium input and output impedance, and both current and voltage gains can be described as medium, but the output signal is shifted by 180 degree (the out is the inverse of the input signal).