Transistor as a Switch - UNSOLVED PRACTICE SET
Chapter: Semiconductor Electronics | Topic: Transistor as a Switch
TRANSISTOR AS A SWITCH - UNSOLVED PRACTICE SET
Topic: Transistor as a Switch
Multiple Choice Questions
Q1. A transistor acts as a switch when it operates in:
- Active region only
- Cut-off and saturation regions
- Active and saturation regions
- Cut-off region only
Q2. When a transistor is in cut-off:
- Both junctions are forward biased
- Both junctions are reverse biased
- The emitter-base junction is forward biased and the collector-base junction is reverse biased
- The emitter-base junction is reverse biased and the collector-base junction is forward biased
Q3. When a transistor is in saturation:
- V_CE โ 0.2 V and I_C is maximum
- V_CE โ V_CC and I_C is zero
- V_CE โ V_CC/2
- I_C = 0 and V_CE = V_CC
Q4. In the cut-off state of a transistor switch:
- The switch is ON
- The switch is OFF
- The transistor is amplifying
- The transistor is oscillating
Q5. The switching speed of a transistor is limited by:
- The resistance of the load
- The time taken for charge carriers to move across the junctions
- The supply voltage
- The base current only
Q6. A transistor is preferred over a mechanical switch because:
- It is cheaper
- It has no moving parts, longer life, and faster switching
- It consumes less power
- It is easier to manufacture
Short Answer Questions
Q7. Explain how a transistor can be used as a switch. What are the two states of the switch?
Q8. Why is the collector-emitter voltage very small (โ 0.2 V) when the transistor is in saturation?
Q9. Draw a simple transistor switch circuit and explain how a small base current controls a large collector current.
Q10. What is the difference between a transistor operating as an amplifier and the same transistor operating as a switch?
Q11. Why does a transistor switch dissipate very little power in both cut-off and saturation states?
Q12. Explain why a transistor switch is much faster than a mechanical relay switch.
Long Answer Questions
Q13. Explain the working of a transistor as a switch. Draw the circuit diagram and explain the cut-off and saturation states with relevant values of V_BE, V_CE, and I_C.
Q14. Discuss the advantages of using a transistor as a switch compared to a mechanical switch. Explain why transistor switches are essential for digital electronics.
Q15. Explain the switching characteristics of a transistor. What is switching time, and what factors affect it? How can the switching speed be improved?
Numerical & Application-based Problems
Q16. A transistor switch circuit has V_CC = 12 V, R_C = 1 kฮฉ, and ฮฒ = 100. The load requires 10 mA to operate.
(a) Calculate the minimum base current required to saturate the transistor.
(b) If the base resistor R_B = 10 kฮฉ, calculate the base current when the input voltage is 5 V.
(c) Determine whether the transistor is in saturation.
(d) Calculate the power dissipated in the transistor in saturation.
Q17. An NPN transistor is used to switch an LED. The LED requires 20 mA and has a forward voltage drop of 2 V. V_CC = 5 V and ฮฒ = 50.
(a) Calculate the value of the collector resistor R_C.
(b) Calculate the minimum base current needed.
(c) If the input to the base is 3.3 V from a microcontroller, calculate the required base resistor R_B (assume V_BE = 0.7 V).
(d) Calculate the power dissipation in the transistor when ON.
Q18. In your school robotics club, students are using transistor switches to control motors.
(a) A student wants to control a 12 V DC motor drawing 500 mA using an NPN transistor with ฮฒ = 100. Calculate the minimum base current required and choose an appropriate base resistor if the control signal is 5 V.
(b) The student finds that the motor doesn't turn off completely when the base voltage is removed. Explain why this happens (due to leakage current) and how adding a base-emitter resistor can ensure complete cut-off.
(c) She then builds a simple NOT gate using a transistor. Draw the circuit and explain how it inverts the input signal (HIGH input gives LOW output and vice versa).
(d) A classmate suggests using a transistor switch to control a 230 V AC bulb. Explain why this is dangerous and what component (like a relay or TRIAC) should be used instead.
(e) In modern India, smart home systems use millions of transistor switches in IoT devices. Explain how a transistor switch controlled by a microcontroller can turn on an LED bulb, and why MOSFETs are often preferred over BJTs for this application.