1. Satish Pradhan Dnyanasadhana College, Thane
( Academic Year 2017 – 2018 )
SEM-VI
T. Y. B. Sc.
Electronics
Unit 1 by
Dr. S. R. Bhagat
Dr. S. R. Bhagat

2. Field Effect Transistors
The current flow is controlled by electric field
Unipolar
Switch faster
Dr. S. R. Bhagat

3. Field Effect Transistors
JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
FET
Enhancement MOSFET
Depletion MOSFET
n-Channel EMOSFET
n-Channel DMOSFET
p-Channel DMOSFET
p-Channel EMOSFET
Dr. S. R. Bhagat

4. FET and BJT Unipolar Bipolar High input resistance No offset voltage
Immune to radiations
Less noisy
Greater thermal stability
Simple to fabricate less space
Dr. S. R. Bhagat

5. Basic Ideas
There are three terminals: Drain (D) and Source (S) are connected to n-cha
Dr. S. R. Bhagat

6. Normal Biasing of JFET
Dr. S. R. Bhagat

7. Schematic Symbols Gate Drain Source Gate Drain Source Gate Drain
n-channel JFET
Offset-gate symbol
n-channel JFET
p-channel JFET
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8. Drain Characteristics
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9. Drain Characteristics
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10. Transconductance Curve
IDSS
VGS (off)=VP
Figure: Transfer (or Mutual) Characteristics of n-Channel JFET
Dr. S. R. Bhagat

11. FET Parameters Drain Resistance (rd) Transconductance (gm)
The dynamic a.c. resistance is defined as the ratio of infinitesimal change in VDS to the corresponding change in drain current ID at a constant value of VGS
Transconductance (gm)
The mutual conductance is defined as the ratio of the change in drain current to the corresponding change in VGS at a constant value of VDS
Dr. S. R. Bhagat

12. FET Parameters The voltage amplification factor ()
It is the ratio of change in VDS to the corresponding change in drain current VGS at a constant value of ID
where
Dr. S. R. Bhagat

13. Gate Bias
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14. Self Bias
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15. Voltage Divider Bias
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16. Common Source Amplifier
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17. JFET Analog Switch
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18. Shunt Switch
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19. Series Switch
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20. JFET Analog Switch Multiplexer
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21. FET as VVR
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22. Enhancement MOSFET
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23. Enhancement MOSFET
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24. Biasing of MOSFET Depletion Type
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25. Biasing of MOSFET Enhancement Type
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26. MOSFET Switch The MOSFET switch is most popular type of switch.
It is good for transmitting low level voltage signals (as opposed to high current).
Output swing depends critically on RD (ID=IDSS for VGS=0).
Current flows at all times.
Dr. S. R. Bhagat

27. A JFET has a drain current 5 mA
A JFET has a drain current 5 mA. If IDSS = 10 mA and VGS(off) =  6V, find the value of VP and VGS
In a n-channel JFET potential divider biased circuit, it is desired to set the operating point at ID = 25 mA and VGS = 8 V. If VDD = 30 V, R1 = 1 M and R2 = 500 k, find the value of RS Given ] IDSS = 10 mA and VP =  5V
The transconductance of a JFET used in a voltage amplifier circuit is 3000 mho and the load resistance is 10 k , calculate the voltage amplification factor assuming that rd >> RL
For a JFET, IDSS = 9 mA and VP =  35V. Determine ID when VGS = 0 V and  2 V.
Dr. S. R. Bhagat

28. A JFET amplifier employs voltage divider bias
A JFET amplifier employs voltage divider bias. The resistances are of value R1 = 1 M and R2 = 1 M. . If VDD is 20 V and the drain current is found to be 2 mA for RS = 15 k find VGS. If VDS is one half VDD, what is the value of RD?
Dr. S. R. Bhagat

29. SCR
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30. Working of SCR
When Gate is open
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31. Working of SCR
When Gate voltage is zero
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32. Working of SCR When Gate voltage is positive with respect to cathode
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33. Equivalent circuit of SCR
When Gate is open and V < VBr (Breakover voltage)
When gate voltage is positive
Dr. S. R. Bhagat

34. Important terms for SCR
Forward Breakover Voltage (VBr)
It is the minimum forward voltage, gate being open, at which SCR start conducting heavily i.e. turned on.
Holding Current (IH)
It is the value of current below which the SCR switches from the conduction state to the forward blocking region under specified conditions
Peak Reverse Voltage (PRV)
It is the maximum reverse voltage that can be applied to an SCR without conducting in reverse direction
Dr. S. R. Bhagat

35. Important terms for SCR
Forward Current Rating
It is maximum anode to cathode current that SCR is capable of passing without damage
Circuit Fusing Rating
It is the product of square of forward surge current and the time of duration of the surge
Dr. S. R. Bhagat

36. V-I Characteristics of SCR
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37. SCR as a Switch
Advantages of SCR switch over mechanical or electromechanical switches
No moving parts, hence noiseless operation at high efficiency
The switching speed is very high upto 109 operations/sec
It allows control over large current upto 100 A in the load by means of small gate current
It is solid state device and has small size, hence gives trouble free long service
Dr. S. R. Bhagat

38. SCR as a Switch DC gate trigger SCR turns on when switch S is closed
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39. SCR as a Switch AC gate trigger SCR turns on when IG  IGT
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40. SCR as a Switch Anode current interruption to make SCR off
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41. SCR as a Switch
Forced Communication
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42. SCR as a Half wave Rectifier
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43. SCR as a Half wave Rectifier with firing angle upto 180
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44. Triac
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45. Triac Construction
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46. Triac Operation
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47. Triac Characteristics
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48. Applications of Triac Intensity control of high power lamp
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49. Applications of Triac Electronic changeover of transformer taps
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50. The Diac
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51. Applications of Diac Light Dimmer
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52. A half wave rectifier circuit with SCR is adjusted to have a gate current of 1 mA. The VBRF of SCR is 100 V for Ig = 1 mA If a sinusoidal voltage of 200 V peak is applied find ( i ) firing angle ( ii ) conduction angle ( iii ) average current. Assume load resistance 1 k and the holding current to be zero
An ac voltage of v = 240 sin 314t is applied to an SCR half wave rectifier. If the SCR has a forward breakdown voltage of 180 V, find the time during which SCR remains off
An SCR has a circuit fusing rating of 60 A2s. Determine the highest surge current value that SCR can withstand for a period of 20 ms.
Dr. S. R. Bhagat