Basic Electronic Devices and Circuits (10B11EC211)Tutorial Sheet -5 (BJT)

1. A BJT is connected in common-base configuration and is biased such that IE = 2 mA. If its dc alpha is 0.98, calculate the values of collector current IC and base current IB

[Ans. IC = 1.96 mA, IB = 40 μA]

2. In a given transistor circuit, the base current is 100 μA and the collector current is 2.9 mA. Calculate the dc alpha (α0) of the transistor. [Ans. α0 = 0.97]

3. A BJT is connected in CB configuration. It is biased such that IE = 2mA. If its dc alpha (α0) is 0.985, calculate the collector current and the base current.

[Ans. IC = 1.97 mA, IB = 30 μA]

4. A transistor in a circuit is connected in CE configuration. The dc voltages are such that IB = 40 μA. If the dc beta (β0) of the transistor is 80 and the collector leakage current ICBO is 5 μA, calculate the value of emitter current. [Ans. 3.645 mA]

5. For a transistor connected in CB mode, α0 = 0.995, IE = 10 mA, leakage current ICO = 0.5 μA. Determine the values of the collector current IC and the base current IB. Also determine β0 and ICEO for the transistor.

[Ans. IC = 0.9505 mA, IB = 49.5 μA, β0 = 199, ICEO = 100 μA]

6. A transistor having β0 = 140 is connected in a circuit in CE configuration. Its collector current is 5 mA and base current is 35 μA. Calculate the leakage current ICO·

[Ans. ICO = 0.71 μA]

7. A transistor has ICBO = 0.1 μA and ICEO = 16 μA. Find its α0· [Ans. 0.99375]

8. In a certain transistor, 99.5 % of the carriers injected into the base, cross the collector-base junction. If the collector leakage current is 5.0 μA and the collector current is 22 mA, find (a) the exact value of α0, (b) the emitter current, and (c) the approximate value of α0 when ICBO is ignored. [Ans. (a) 0.995; (b) 22.1055mA; (c) 0.9952]

9. The dc alpha (α0) of a transistor is 0.99 and its collector reverse leakage current is 3 μA. Determine the value of collector current and base current, if the emitter current is 4 mA.

[Ans. 3.963 mA, 37 μA]

10. The emitter current in an NPN bipolar junction transistor is 8.8 mA. It is known that only 0.85 % of the minority carriers (electrons) injected into the base recombine, and the collector reverse leakage current is 200 nA. Find (a) the base current, (b) the collector current, (c) the exact value of dc alpha, and (d) the approximate value of dc alpha, neglecting ICO. [Ans. (a) 74.8 μA; (b) 8.7252 mA; (c) 0.991477; (d) 0.9915]

11. A BJT has α0 = 0.987 and ICBO = 52 nA. Find (a) β0 and ICEO, (b) the exact collector current when IB = 40 μA, (c) approximate collector current, neglecting the collector leakage current. [Ans. (a) 75.92,4 μA; (b) 3.0408 mA; (c) 3.0368 mA]

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12. When the emitter voltage of a transistor connected in CB configuration is changed by 200 m V, its emitter current changes by 5 mA. Calculate the dynamic input resistance of the transistor. [Ans. ri = 40 Ω]

13. In the circuit shown in Fig. 1, find (a) IC when VCB = 10 V, and (b) VCB when IC = 1.4 mA. [Ans. (a) 1.515 mA; (b) 10.38 V]

Fig. 1 Fig. 2 Fig. 3 Fig. 4

14. Determine the coordinates of the operating point of the fixed-bias circuit of Fig. 2. Given: RC = 1 kΩ, RB = 100 kΩ, β0 = 60. [Ans. (4 V, 6 mA)]

15. In the circuit of Fig. 4, the NPN silicon transistor has its VBE = 0.6 V, and β0 = 100. If VCE

= 7 V for RC = 4 kΩ, find RB and IC. [Ans. 765 kΩ, 1.75 mA]

16. The NPN transistor used in a fixed-bias circuit has β0 = 100. If VCC = 10 V, RC = 1 kΩ, and RB = 200 kΩ, calculate VCE. [Ans. 5 V]

17. A PNP transistor with β0 = 200 is used in the circuit of Fig 6.39. A dc supply of 9 V and RC of 1.5 kΩ are used. The operating point is to be fixed at IC = 2 rnA. Calculate the value of RB and the voltage VCE. [Ans. 900 kΩ, –6 V]

18. A PNP germanium transistor with β0 = 100 and VBE = 0.2 V is used in Fig. 5. If VCC = 16 V, RB = 790 kΩ, RC = 5 kΩ, compute the Q-point. [Ans. VCE = –6 V, IC = 2 mA]

19. Determine the Q-point for a fixed-bias circuit using an NPN transistor with β0 = 40, VCC

= 9 V, RC = 1 kΩ and RB = 100 kΩ. [Ans. 5.4 V, 3.6 mA]

20. Determine the Q-point for the collector-to-base bias circuit of Fig. 6, if VCC = 12 V, RC = 3 kΩ, RB = 60 kΩ, and β0 of the transistor is 60. Ignore the drop VBE.[Ans. 3 V, 3 mA]

21. Compute the value of RB in Fig. 5, so as to set up the biasing condition such that VCE = 0.5VCC, if VCC = 30 V, RC = 5 kΩ, and β0 = 40. [Ans. 200 kΩ]

22. In the collector to base bias circuit of Fig. 5, VCC = 15 V, RC = 5 kΩ, RB = 215 kΩ, β0 = 100, VBE = 0.7 V. (a) Determine the Q-point. (b) If the transistor is replaced by another one with β0 = 300, again determine the Q-point.

[Ans. (a) VCE = 5 V, IC = 2 mA; (b) VCE = 2.5 V, IC = 2.5 mA]

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Fig. 5 Fig. 6 Fig. 7

23. In the biasing circuit of Fig. 6, VCC = 9 V, RC = 250 Ω, RB = 50 kΩ, RE = 500 Ω and β0 = 80. Determine the Q-point. [Ans. 3 V, 8 mA]

24. Compute the Q-point of the transistor in Fig. 6, if VCC = 20 V, RC = 2 kΩ, RB = 400 kΩ, RE

= 1 kΩ and β0 = 100. [Ans. 8 V, 4 mA]

25. The NPN transistor in the biasing circuit of Fig 6 has β0 = 56 and VBE = 0.7 V. Determine its Q-point, if VCC = 18 V, RC = 500 Ω, RB = 50 kΩ, RE = 0.75 kΩ.

[Ans. 4.31 V, 10.95 mA]

26. The dc current gain in CE mode of a germanium BJT is 100. It is to be biased at IC = 5 mA and VCE = 3.8 V, by using the emitter resistor bias circuit of Fig. 6. If VCC = 10 V and RC = 500 Ω, calculate the values of RB and RE needed.

[Ans. RE = 740 Ω, RB = 120 kΩ]

27. With the following circuit components and BJT specifications for the potential divider biasing circuit of Fig. 7, calculate the collector current and the collector-to-emitter voltage. Make necessary assumptions to simplify calculations.

RC = 5 kΩ Rl = 40 kΩ VCC = 12 VRE = 1 kΩ R2 = 5 kΩ β0 = 60 VBE = 0.3 V

[Ans. IC = 1 mA, VCE = 6 V]

28. In the above question, make use of Thevenin's theorem to find the accurate values of the collector current and the collector-to-emitter voltage.

[Ans. IC = 0.93 mA, VCE = 6.42 V]

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