JFET

This presentation is about JFET in Electronics.
It was presented at Daffodil International University

1. Welcome!! To our Presentation JEFET

2. Group Members: Taminul Islam ID: 181-15-11116 1 Novana Nasir Nova ID: 181-15-10865 2 Akhi Mony ID: 181-15-11130 3 Rishalatun Jannat Lima ID: 181-15-11120 4

3. Introduction

4. FET (Field Effect Transistor) It’s a transistor where output current is controlled by electric field. Junction FET Metal Oxide Semiconductor FET

5. JFET N Channel JFET N-Channel JFET is a JFET whose channel is composed of primarily electrons as the charge carrier. It means that when the transistor is turned on, it is primarily the movement of electrons which constitutes the current flow.

6. JFET P Channel JFET  P-Channel JFET is a JFET whose channel is composed primarily of holes as the charge carrier. This means that when the transistor is turned on, it is primarily the movement of holes which constitutes the current flow.

7. Constructional Details

8. Constructional Details A JFET consists of a p-type or n-type silicon bar containing two pn junctions at the sides Fig : 1 Fig : 2

9. JFET Polarities

10. Fig : 1 Fig : 2 The input circuit ( i.e. gate to source) of a JFET is reverse biased. This means that the device has high input impedance . The drain is so biased w.r.t. source that drain current ID flows from the source to drain . In all JFETs, source current I S   is equal to the drain current i.e I S  = I D . JFET Polarities

11. Principle & Working of JFET

12. Principle of JFET The two PN junctions at the sides form two depletion layers. The current conduction by charge carriers (i.e. electrons) is through the channel between the two depletion layers and out of the drain. The width and hence resistance of this channel can be controlled by changing the input voltage V GS . The greater the  reverse voltage V GS , the wider will be the depletion layer and narrower will be the conducting channel. The narrower channel means greater resistance and hence source to drain cu rrent decreases. Reverse will happen when V GS  decreases. Thus JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage V GS . In other word, the magnitude of drain current I D   can be changed by altering V GS .

13. Principle of JFET Thus JFET operates on the principle that width and hence resistance of the conducting channel can be varied by changing the reverse voltage V GS . In other word, the magnitude of drain current I D   can be changed by altering V GS .

14. Working of JFET Case-1 When a voltage V DS  is applied between drain and source terminals and voltage on the gate is zero as shown in fig the two pn junctions at the sides of the bar establish depletion layers. The electron will flow from source to drain through a channel between the depletion layers.

15. Working of JFET Case-2 When a reverse voltage V GS  is applied between gate and source terminals, as shown in fig 2 the width of depletion layer is increased. This reduces the width of conducting channel, thereby increasing the resistance of n-type bar. Consequently, the current from source to drain is decreased. On the other hand, when the reverse bias on the gate is decreased, the width of the depletion layer also decreases. This increases the width of the conducting channel and hence source to drain current.

16. Symbol of JFET

17. Importance of JFET

18. Importance of JFET A JFET acts like a voltage controlled device i.e. input voltage (VGS) controls the output current. This is different from ordinary transistor (or bipolar transistor) where input current controls the output current. Thus JFET is a semiconductor device acting like a vacuum tube. The need for JFET arises because as modern electronic equipment became increasingly transistorized, it became apparent that there were may functions in which bipolar transistors were unable to replace vacuum tubes. Owing to their extremely high input impedance.

19. Difference between JFET & BJT 1. JFET is called as unipolar transistor . 2. High noise level. 3. Low input impedance (due to forward bias ) 4. Gain is characterized by voltage gain. JFET 1. BJT is called as bipolar transistor . 2. Low noise level. 3. High input impedance (due to reverse bias ). 4. Gain is characterized by trans conductance. BJT

20. MOSFET over JFET

21. MOSFET over JFET JFETs can only be operated in the  depletion mode whereas MOSFETs can be operated in either depletion or in   enhancement mode. MOSFET have input impedance much higher than that of JFET. This is due to negligibly small leakage current. When JFET is operated with a reverse bias on the junction, the gate current IG is larger than it would be in a comparable MOSFET.  The output characteristics of JFET is flatter than the MOSFET.  because the drain resistance in the JFET (1 MΩ) is higher than the MOSFET (50kΩ)

22. Basic JFET amplifier Common source Common gate Each Common drain

23. Disadvantage of JFET

24. Disadvantage of JFET The main disadvantage of the junction field effect transistor  (JFET) is the relatively low gain bandwidth product. The performance of JFET go downs as frequency increases due to feedback by internal capacitance . Grater sensibility damage in its handling. Costly

25. Any Question?

26. Thank you Good Luck!