1. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Subject Code : ECE – 101/102 BASIC ELECTRONICS COURSE MATERIAL For 1st & 2nd Semester B.E. (Revised Credit System) DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

2. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Bipolar Junction Transistor Solid state transistor was invented by a team of scientists at Bell laboratories during 1947-48 Advantages of solid state transistor over vacuum devices: –Smaller size, light weight –No heating elements required –Lower power consumption and operating voltages –Low price

3. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Different transistor packages

4. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Introduction Bipolar Junction Transistor (BJT) is a three layer, 2 junction semiconductor device It is a sandwich of one type of semiconductor material between two layers of another type Two kinds of BJT sandwiches are: NPN and PNP

5. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Introduction The three layers of BJT are called Emitter, Base and Collector Base is narrower compared to the other two layers Base is lightly doped, Emitter is heavily doped and Collector is moderately doped NPN – Emitter and Collector are made of N-type semiconductors; Base is P-type PNP – Emitter and Collector are P-type, Base is N-type BJT has two junctions – Emitter-Base (EB) Junction and Collector-Base (CB) Junction

6. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Introduction Note: Arrow direction from P to N (like diode) which indicates the direction of the flow of conventional current The device is called “bipolar junction transistor” because current is due to the motion of two types of charge carriers- free electrons & holes Transistor symbols:

7. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Operation Operation of NPN transistor and PNP is similar with roles of free electrons and holes interchanged Depending upon the bias condition (forward or reverse) of each of the two junctions, different regions of operation for the BJT are obtained Active region- transistor operating as an amplifier and saturation & cutoff region- Switching applications, e.g. in logic circuits Region

8. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Operation

9. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Operation Note the current directions in NPN and PNP transistors For both varieties: Collector current has two components: –One due to injected charge carriers from emitter –Another due to thermally generated minority carriers Therefore, C E B ICIC IEIE IBIB NPN C E B ICIC IEIE IBIB PNP

10. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations BJT has three terminals For two-port applications, one of the BJT terminals needs to be made common between input and output Accordingly three configurations exist: –Common Base (CB) configuration –Common Emitter (CE) configuration –Common Collector (CC) configuration InputOutput 2-port device

11. Department of Electronics and Communication Engineering Manipal Institute of Technology, Manipal, INDIA Transistor Configurations Common Base configuration Base is common between input and output –Input voltage: V EB Input current: I E –Output voltage: V CB Output current: I C

12. As the currents constituting the collector current are in the same direction, we can write I C as, Since I CBO value is very low, we can neglect it compared to I E and I C. Therefore, where α dc is the fraction of charge carriers emitted from emitter, that enter into the collector region This parameter α dc is called common base DC current gain Value of α dc varies from 0.90 to 0.998 and is defined for the majority carriers Therefore, Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Operation

13. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations CB Input characteristics –A plot of I E versus V EB for various values of V CB –It is similar to forward biased diode characteristics –As V CB is increased, I E increases only slightly

14. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Base Width modulation As the reverse bias voltage V CB is increased, the depletion region width at the C-B junction increases. Part of this depletion region lies in the base layer. So, effective base width decreases. Hence number of electron-hole combination at the base decreases. So base current reduces and collector current increases. Since I E ≈I C, emitter current also shoots up early. This phenomenon is also called as the Early effect

15. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations CB Output characteristics –A plot of I C versus V CB for various values of I E –Three regions are identified: Active, Cutoff, Saturation

16. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations In case of Active region E-B junction forward biased C-B junction reverse biased I C increases with I E For given I E, I C is almost constant; increases only slightly with increase in V CB. This is due to base-width modulation In the cut off region, E-B and C-B junctions of the transistor are reverse biased Collector current is 0A In the saturation region, the E-B and C-B junctions of the transistor are forward biased

17. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations Common Emitter configuration Emitter is common between input and output –Input voltage: V BE Input current: I B –Output voltage: V CE Output current: I C

18. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations CE Input characteristics Plot of I B versus V BE for various values of V CE. As V CE is increased, I B decreases slightly. This is due to base-width modulation

19. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations CE Output characteristics –A plot of I C versus V CE for various values of I B –Three regions identified: Active, Cut-off, Saturation V CESat

20. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Configurations Active region Linear region in the output characteristics E-B junction forward biased C-B junction reverse biased I C increases with I B For given I B, I C increases slightly with increase in V CE ; this is due to base-width modulation Saturation region I C decreases to zero at V CE =0 Cut off region I B = 0, hence I C = I CEO

21. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Operation Parameter β dc is common emitter DC current gain Therefore, collector current expression is: We have, Substituting for I E, we get where and Values of α dc and β dc vary from transistor to transistor = ==

22. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Problems 1.A BJT has alpha value as 0.998 and I CBO of 1μA. If emitter current is 5mA, calculate the collector and base currents. 2.An npn transistor has collector current 4mA and base current 10 μA. Calculate the alpha and beta value of the transistor neglecting the reverse saturation current I CBO. 3.In a transistor, 99% of the carriers injected into the base cross over to the collector region. If collector current is 4mA and I CBO is 6 μA, Calculate I E and I B. 4.A Ge transistor with β = 100 has I CBO = 5μA. If the transistor is connected in common-emitter operation, find the collector current for base current (a) 0 A (b) 40 µA. 5.A Ge transistor has collector current of 51 mA when the base current is 0.4 mA. If β = 125, then what is the value of I CEO ?

23. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing What is meant by biasing the transistor? –Applying external dc voltages to ensure that transistor operates in the desired region Which is the desired region? –For amplifier application, transistor should operate in active region –For switch application, it should operate in cut-off and saturation region What is meant by quiescent point (Q-point)? –The point we get by plotting the dc values of I C, I B and V CE on the transistor characteristics

24. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Transistor characteristics showing Q-point: Q-point is in the middle of active region.

25. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Types of biasing: –Fixed bias or base resistor bias –Self bias or voltage divider bias Fixed bias: –The value of I B is “fixed” by choosing proper value for R B –Equations to be considered are: Fixed bias circuit

26. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Load Line We have: This is an equation of straight line with points V CC /R C and V CC lying on y-axis and x-axis respectively This line is called “Load line” because it depends on resistor R C considered as “Load” and V CC Intersection of load line with the transistor characteristic curve is called Q-point or operating point for a particular value of I B, hence giving a common solution

27. Transistor Biasing If R B is varied, Q-point moves along the load line If R B is held constant and R C is varied, then slope of load line varies If R B & R C are held constant and V CC is varied, then load line shifts, maintaining same slope With reference to the graphs, with everything else held constant –If R B is increased, transistor goes towards cut-off and if R B is decreased, transistor goes towards saturation –If R C is increased, transistor goes towards saturation and if R C is decreased, transistor goes towards active region –If V CC is increased, transistor goes towards active region and if V CC is decreased, transistor goes towards saturation Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA

28. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Variation in load line with circuit parameters R B, R C and V CC

29. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Advantages of Fixed bias: –Simple circuit to analyze and design –Uses very few circuit components Disadvantages of Fixed bias: –Q-point is unstable i.e. if temperature increases, then β increases, hence I CQ and V CEQ varies. Effectively Q-point shifts –If the transistor is replaced with another transistor having different β value, then also Q-point shifts

30. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Problems on Fixed bias 1.For a fixed bias circuit using Si transistor, R B = 500 kΩ, R C = 2kΩ, V CC = 15 V, I CBO = 20 µA and β = 70. Find the collector current I CQ and V CEQ at Q-point. 2.A Si transistor is biased for a constant base current. If β = 80, V CEQ = 8 V, R C = 3 kΩ and V CC = 15 V, find I CQ and the value of R B required. Repeat the problem if the transistor is a germanium device. 3.For a fixed bias circuit, V CC = 12 V and R C = 4 kΩ. The Ge transistor used is characterized by β = 50, I CEO = 0 A and V CE sat = 0.2 V. Find the value of R B that just results in saturation 4. A fixed bias circuit has V CC = 20 V, R C = 5 kΩ and R B = 300 kΩ. The Si transistor has I CBO = 0 and β = 50. Find I CQ and V CEQ.

31. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Voltage divider bias or Self bias –Resistor R E connected between emitter and ground –Voltage-divider resistors R 1 & R 2 replace R B –Circuit can be analyzed in two methods: Exact method (using Thevenin ’ s theorem) Approximation method (neglecting base current) Self bias circuit

32. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Exact method: –Input side of self-bias circuit (Fig. a) is transformed into Thevenin’s equivalent circuit (Fig. b) where, R TH is the resistance looking into the terminals A & B (Fig. c)

33. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Biasing Equations to be considered: Since β >> 1 and (β+1)R E >> R TH Since I C is almost independent of β, Q-point is stable Self-bias circuit redrawn with input side replaced by Thevenin’s equivalent :

34. Transistor Biasing Advantages of Self bias: –The collector current and hence the Q-point is independent of variation in temperature and replacement of transistor Disadvantages of Self bias: –More circuit components are required –Analysis and design are complex Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA

35. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Problems on Self bias 1.For a self bias circuit, the transistor is a Si device, R E = 200 Ω, R 1 = 10R 2 = 10 kΩ, R C = 2 kΩ, β = 100 and V CC = 15 V. Determine the values of I CQ and V CEQ. 2.Suppose if the transistor used in problem 1 failed and was replaced with a new transistor with β = 75, is the new transistor still biased for active region operation? 3.A self bias circuit uses Silicon transistor with R C = 3.3K Ω, R E = 1KΩ, R 1 = 39K Ω, R 2 = 8.2K Ω, β = 120 and V CC = 18 V. Determine the values of I B, I CQ and V CEQ.

36. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier Amplifier –Circuit which increases the magnitude of the input signal applied –BJT basically amplifies current: Collector current equals beta times Base current –By suitably designing the transistor circuit, we can get voltage amplification and power amplification –For faithful amplification (with no distortion), BJT should operate in Active region throughout the input cycle –This is achieved by proper use of biasing circuit –Biasing circuit fixes the operating point in the middle of active region

37. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier As v in varies, i in varies, thus base current i B varies This variation in base current is amplified beta times to get variation in collector current i C Output voltage v out is V CC – i C R C If v in increases, there is proportional decrease in v out Similarly if v in decreases, v out increases proportionally Thus output voltage of CE amplifier is 180 o out of phase with input voltage CE amplifier circuit with fixed bias

38. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier Voltage gain or voltage amplification factor is v out /v in which is dependent on β, R C and other physical parameters of the transistor Figure shows input and output waveforms for the amplifier circuit shown previously Note the dc shift in the output voltage waveform. i.e., when v in = 0, v out = V CEQ

39. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier Gain of the amplifier is usually expressed in decibels (A V ) dB = 20 log 10 | A V | Usually a gain of 100 (i.e. 40 dB) can be obtained using single transistor. For higher gain requirement, two or more amplifier stages are to be cascaded Overall gain is product of individual gains, but when expressed in dB, overall gain is sum of individual gains (in dB)

40. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier RC Coupled Amplifier CE amplifier employing self bias

41. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier Additional components used are C C and C E –C C is called coupling capacitor – used to prevent dc component from entering or leaving amplifier stage –C E is called emitter bypass capacitor – used to bypass the ac emitter current – preventing it from flowing through R E –If ac emitter current is allowed to pass through R E, then v BE reduces and hence output voltage reduces Frequency response of amplifier –It is important to know the behavior of the amplifier at different frequencies –Gain is NOT constant at all frequencies – depends on various factors –Frequency response is a plot of gain versus frequency

42. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier Figure shows frequency response plot At lower and higher frequencies, gain is less Gain attains constant value at mid frequencies Bandwidth of amplifier is range of frequencies over which gain is not less than 3 dB of maximum gain

43. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Transistor Amplifier Analysis of frequency response curve –At very low frequencies, reactance of coupling capacitors is high, hence there is loss of signal voltage across capacitors, resulting in reduced gain –Also at low frequencies, emitter bypass capacitor does not fully bypass the ac emitter current, hence ac voltage drop develops across R E, resulting in reduced gain –At very high frequencies, shunt capacitances due to wiring and inter-layer junction capacitances will be prominent, hence resulting in signal loss –At mid frequencies, gain is maximum and constant

44. Transistor Amplifier Advantages Cost is low Offers high fidelity in audio frequency range (20 - 20KHz) Circuit is quite compact Disadvantages Tends to be noisier with age Gain is less Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA

45. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA Problems on Amplifiers 1.A three-stage amplifier circuit has first stage gain of 45 dB, second stage gain of 50 dB and third stage gain of –5 dB. What is the overall gain? If input to the first stage is 0.1mV, what is the output of final stage? 2.An amplifier has maximum gain of 200 and bandwidth of 500 kHz. If lower cutoff frequency is 50 Hz, what is the upper cutoff frequency and gain at this frequency? 3.The voltage amplifier has a voltage gain = 200 at the cut off frequencies. Find the maximum voltage gain?

46. Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal, INDIA End of Module 6