Questions

1. Determine the collector current \(I_{C}\) and collector-emitter voltage \(V_{CE}\). Neglect small base-emitter voltage. Given that \(\beta = 50\). If \(R_{B}\) in this circuit is changed to \(50 \,\text{k}\Omega\), find the new operating point.

   

2. A silicon transistor with \(\beta = 100\) is biased by base resistor method. Draw the d.c. load line and determine the operating point. What is the stability factor?

   

3. A germanium transistor is to be operated at zero signal \(I_{C} = 1 \,\text{mA}\). If the collector supply \(V_{CC} = 12 \, V\), what is the value of \(R_{B}\) in the base resistor method? Take \(\beta = 100\). If another transistor of the same batch with \(\beta = 50\) is used, what will be the new value of zero signal \(I_{C}\) for the same \(R_{B}\)?
4. Calculate the values of three transistor currents in the circuit.

   

5. Design base resistor bias circuit for a \(CE\) amplifier such that operating point is \(V_{CE} = 8\, \text{V}\) and \(I_{C} = 2 \, \text{mA}\). You are supplied with a fixed \(15 \, V\) d.c. supply and a silicon transistor with \(\beta = 100\). Take base-emitter voltage \(V_{BE} = 0.6 \, \text{V}\). Calculate also the value of load resistance that would be employed.

   

6. A fixed bias circuit in figure is subjected to an increase in temperature from 25 °C to 75 °C. If \(\beta = 100\) at 25 °C and 150 at 75 °C, determine the percentage change in \(Q\)-point values (\(V_{CE}\) and \(I_{C}\)) over this temperature range. Neglect anychange in \(V_{BE}\) and the effects of any leakage current.

   

7. For the emitter bias circuit, find \(I_{E}\), \(I_{C}\), \(V_{C}\) and \(V_{CE}\) for \(\beta = 85\) and \(V_{BE} = 0.7\, \text{V}\)

   

8. Determine how much the \(Q\)-point of question \(8\), will change over a temperature range where \(\beta\) increases from \(85\) to \(100\) and \(V_{BE}\) decreases from \(0.7 \,\text{V}\) to \(0.6 \,\text{V}\).
9. Draw the d.c. load line and determine the operating point. Assume the transistor to be of silicon.

   

10. Determine the operating point of the circuit shown in the previous question by using Thevenin's theorem.
11. A transistor uses potential divider method of biasing. \(R_{1} = 50 \, \text{k}\Omega\), \(R_{2} = 10 \, \text{k}\Omega\) and \(R_{E} = 1\, \text{k}\Omega\). If \(V_{CC} = 12 \,\text{V}\), find : (i) the value of \(I_{C}\) ; given \(V_{BE} = 0.1 \,\text{V}\), (ii) the value of \(I_{C}\) ; given \(V_{BE} = 0.3\,\text{V}\).
12. Calculate the emitter current in the voltage divider circuit shown in figure. Also find the value of \(V_{CE}\) and collector potential \(V_{C}\).

    

13. For the circuit shown in Fig., find the operating point. What is the stability factor of the circuit? Given that \(\beta = 50\) and \(V_{BE} = 0.7 \,\text{V}\).

    

14. The circuit shown in Fig. uses silicon transistor having \(\beta = 100\). Find the operating point and stability factor.

    

15. In the circuit shown in Fig., the operating point is chosen such that \(I_{C} = 2 \,\text{mA}\), \(V_{CE} = 3\,\text{V}\). If \(R_{C} = 2.2 \,\text{k}\Omega\), \(V_{CC} = 9 \,\text{V}\) and \(\beta = 50\), determine the values of \(R_{1}\), \(R_{2}\) and \(R_{E}\). Take \(V_{BE} = 0.3 \,\text{V}\) and \(I_{1} = 10 \,I_{B}\)

    

16. An \(npn\) transistor circuit has \(\alpha = 0.985\) and \(V_{BE} = 0.3\,\text{V}\). If \(V_{CC} = 16\,\text{V}\), calculate \(R_{1}\) and \(R_{C}\) to place \(Q\)-point at \(I_{C} = 2\, \text{mA}\), \(V_{CE} = 6 \,\text{V}\).

    

17. Determine whether or not the circuit shown in Fig. is midpoint biased.

    

18. Determine whether or not the circuit shown in Fig. is midpoint biased.

    

Answers

1. \(I_{C} = 1 \, \text{mA}\), \(V_{CE} = 7 \, \text{V}\) and
   \(I_{C} = 2 \, \text{mA}\), \(V_{CE} = 5 \, \text{V}\)
2. \(I_{C} = 1 \, \text{mA}\), \(V_{CE} = 4 \, \text{V}\) and \(S = 101\)
3. \(R_{B} = 1170 \, \text{k}\Omega\), \(I_{C} = 0.5 \, \text{mA}\)
4. \(I_{B} = 0.0091 \, \text{mA}\), \(I_{C} = 0.91 \, \text{mA}\), \(I_{E} = 0.919 \, \text{mA}\)
5. \(R_{C} = 3.5 \, \text{k}\Omega\), \(R_{B} = 720 \, \text{k}\Omega\)
6. \(I_{C} = 11.3 \, \text{mA}\), \(V_{CE} = 5.67 \, \text{V}\) at \(25~ ^{\circ}\text{C}\) and
   \(I_{C} = 17 \, \text{mA}\), \(V_{CE} = 2.48 \, \text{V}\) at \(75 ~ ^{\circ}\text{C}\).
   Percentage change in \(I_{C} = 50 \,\% (\text{increase})\) and
   Percentage change in \(V_{CE} = -56.3 \,\% (\text{decrease})\).
7. \(I_{C} = 1.73 \, \text{mA}\), \(V_{C} = 11.9 \, \text{V}\), \(V_{CE} = 14.6 \, \text{V}\)
8. \(I_{C} = 1.76 \, \text{mA}\), \(V_{CE} = 14.1 \, \text{V}\).
   Percentage change in \(I_{C} = 1.7 \,\% (\text{increase})\) and
   Percentage change in \(V_{CE} = -3.5 \,\% (\text{decrease})\).
9. \(I_{C} = 2.15 \, \text{mA}\), \(V_{CE} = 8.55 \, \text{V}\)
10. Same as the answer to question 9.
11. \(I_{C} = 1.9 \, \text{mA}\), at \(V_{BE} = 0.1 \, \text{V}\) and
    \(I_{C} = 1.7 \, \text{mA}\), \(V_{BE} = 0.3 \, \text{V}\)
12. \(I_{E} = 2 \, \text{mA}\), \(V_{CE} = 8 \, \text{V}\), \(V_{C} = 18 \, \text{V}\)
13. \(I_{C} = 1.2 \, \text{mA}\), \(V_{CE} = 3.72 \, \text{V}\), \(S = 18.4\)
14. \(I_{C} = 4.2 \, \text{mA}\), \(V_{CE} = 8.83 \, \text{V}\), \(S = 2.94\)
15. \(R_{E} = 800 \, \Omega\), \(R_{2} = 4.75 \, \text{k}\Omega \), \(R_{1} = 17.75 \, \text{k}\Omega\)
16. \(R_{1} = 54.4 \, \text{k}\Omega\), \(R_{C} = 3 \, \text{k}\Omega\)
17. Yes. \(I_{C} = 2.028 \, \text{mA}\), \(V_{CE} = 3.94 \, \text{V}\)
18. Yes. \(I_{C} = 6.33 \, \text{mA}\), \(V_{CE} = 4.94 \, \text{V}\)

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