To find out the qualitative response to changes in Vgs and Vds, consider Vgs =0. In response to a small applied voltage Vds, a n-channel JFET acts as a simple semiconductor resistor and Id increases linearly with Vds. With increasing current, the ohmic voltage drop between the source and the channel region reverse biases the junction and the conducting portion of the channel begins to constrict. because of the ohmic drop along the length of the channel itself, the constriction is not uniform but is more pronounced at distances father from the source. Eventually, a Vds is reached at which the channel is "pinched off". At this voltage, Id begins to level off. and approach a constant value. It is not possible for the channel to close completely and reduce Id to zero. If that were the case, then the ohmic drop required to provide the back biasing would be lacking.
Id is not equal to zero at Pinch off voltage.
Thus, pinch off voltage can be defined as the gate reverse voltage that removes all the free charge from the channel.
A Jfet works by applying voltage to the drain of the jfet. A jfet will then conduct across from drain to source.
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BJT is Bipolar junction transistor FET is Field effect Transistor It is a current controlled device It is voltage controlled device
to made reverse bias junction between gate to source
No, jfet works only in depletion mode.
A Jfet works by applying voltage to the drain of the jfet. A jfet will then conduct across from drain to source.
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• High Input Impedance Amplifier. • Low-Noise Amplifier. • Differential Amplifier. • Constant Current Source. • Analog Switch or Gate. • Voltage Controlled Resistor. • JFET as a Switch • JFET as a Chopper • JFET as a Current source • JFET as a Amplifier • JFET as a Buffer
absolutelly
amplification factor
BJT is a example for current controll device. And JFET is a voltage controlled device.
JFET is a unijunction transistor.
An advantage of JFET is stable high current operation. A disadvantage of JFET is low capacitance. An advantage of BJT is constant voltage operation. A disadvantage of BJT is low thermal conductance.
BJT is Bipolar junction transistor FET is Field effect Transistor It is a current controlled device It is voltage controlled device
1-BJT is bipolar while JFET is unipolar. 2-BJT has low input impedence while JFET has high input impedence. 3-JFET has low power discipation as compared to BJT. 4-JFET has low noise as compared to BJT. 5-BJT is current controlled while JFET is voltage controlled. 6-JFET is mostly used in digital circuits.
JFET as a VVR-Voltage Variable ResistorFET is a device that is usually operated in the constant-current portion of its output characteristics. But if it is operated on the region prior to pinch-off (that iswhere VDS is small, say below 100 mV[ohmic region of JFET]), it will behave as a voltage-variable resistor (WE). It is due to the fact that in this region drain-to-source resistance RDS can be controlled by varying the bias voltage VGS. In such applications the FET is also referred to as a voltage-variable resistor or volatage dependent resistor. It finds applications in many areas where this property is useful.Figure shows the drain characteristic curves for a 2N 5951 in the ohmic region (i.e. for low VDS). From the characteristic curve it can be seen that RDS varies with VGS. For example, when VGS = 0, RDS = 133 ohm and when VGS = - 2 V, RDS = 250 ohm. Because of this a JFET operating in the ohmic region with small ac signals acts as a voltage-controlled resistance.Note that the drain curves shown in figure,extend on both sides of the origin. This means that a JFET can be employed as a voltage-variable resistorfor small ac signals, typically those less than 100 mV. When it is employed in this way, it does not require a dc drain voltage from the supply. All that is required is an ac input signal.
In a JFET the only insulation between the gate and the channel is a reverse biased diode junction, if this junction becomes forward biased then the gate and channel are effectively shorted and the device no longer acts as a transistor (it will act as a forward biased diode instead). In the n-channel JFET, the gate is the P-side of this diode and the channel is the N-side of this diode. To keep this diode reverse biased (and the device operating as a transistor) therefor the gate MUST always be maintained at a voltage more negative than the most negative section of the channel.