Objective Questions on Electronics page 16

1. In a normally biased npn transistor, the electrons in the emitter have enough energy to overcome the barrier potential of the

The flow of the electrons from the collector to emitter is controlled by the base to emitter electrons which has a certain potential energy. We know that the electron from the collector to emitter is very strong. There is a chance of affecting the base emitter junction by the heavy potential energy from the collector electron. So, the electrons in the emitter should have enough energy to overcome the barrier potential of the base emitter junction.

2. What is the most important fact about the collector current?

According to the KCL, the emitter current is the summation of the base current and the collector current. Since, the base current is very less than the collector current. So, it is neglected. Hence, the emitter current is approximately equal to the collector current.

3. In a pnp transistor, the major carriers in the emitter are

In an NPN transistor, the majority current carriers are electrons. In a PNP transistor, the majority current carriers are holes.

4. If the current gain is 100 and the collector current is 10 mA, the base current is

We know that the collector current IC = β * IB. Therefore, IB = IC / β = 10 * 10-3 / 100 = 1 * 10-4 A = 100 * 10-6 = 100 μA.

5. The base - emitter voltage is usually

The required base emitter voltage for a transistor is 0.7 Volts. Since, it is typically less than the input voltage. Here, the input voltage is base supply voltage. So, base emitter voltage is always less than the base voltage.

6. The collector - emitter voltage is usually

There is a voltage drop across the collector resistance. It draws the voltage drop from the input voltage or collector supply voltage. So, the collector emitter voltage is subtraction of the collector supply voltage and the voltage across the collector resistance. Thus, the collector emitter voltage is always less than the collector supply voltage.

7. The power dissipated by a transistor approximately equals the collector current times

The power dissipated by a transistor is calculated by multiplying the collector current and the collector emitter voltage. The power dissipation is nothing but the output power than can be consumed by the transistor. Thus, the output power (power dissipation) is calculated by the above method.

8. A small collector current with zero base current is caused by the leakage current of the

This thing happens in the cut-off condition. During this time, the base current is zero. In this period, the emitter depletion region is very much narrower than the collector depletion region because of forward bias at the collector and higher doping levels. Hence, there would be a small leakage current flow via collector diode. By this leakage current, the small collector current is generated.

9. If the base current is 100 mA and the current gain is 30, the emitter current is

The emitter current (IE) = Base current (IB) + Collector current (IC). Here, Collector current = Base current (IB) * Current gain (β) = 100 * 10-3 * 30 = 3 Amps. Now, emitter current (IE) = 3 + (100 * 10-3) = 3.1 Amps.

10. The base-emitter voltage of an ideal transistor is

Ideally, a saturated transistor acts as a closed switch contact between collector and emitter. Hence, there is no need of base emitter voltage to enable the transistor to work in active region. So, it is zero in the ideal transistor.

11. In the active region, the collector current is not changed significantly by

Because, the collector resistance is constant at all time. So, that the collector current is not changed significantly by the collector resistance in the active region.

12. The base-emitter voltage of the second approximation is

For the silicon transistor, the approximate normal operating voltage is 0.7 Volts. The operating voltage is nothing but the base emitter voltage.

13. If the base resistor is open, what is the collect current?

If the base resistance is open, there is no path to flow the current from the source to the base of the transistor. Hence, the base current is “0” Amps.

14. When the collector current increases, what does the current gain do?

The current gain is β = 1 / JC. Here, JC is collector current density. Hence, the value of current gain decreases or increases with respect to the collector current.

15. The current gain of a transistor is defined as the ratio of the collector current to the

The current gain is represented by beta (β). It is approximately the ratio of the DC collector current to the DC base current in forward active region.

16. The graph of current gain versus collector current indicates that the current gain

The below graph shows that the current gain varies slightly with respect to the collector current because, at one stage it achieves the saturation point. It says that how much current gain you we expect from the device.

17. When the base resistor increases, the collector voltage will probably

If the resistance increases, then the voltage drop across resistance also increases. So, it does not provide the exact voltage to the load. Hence, according to that increase in collector resistance, we have to increase the collector voltage to provide the rated voltage to the load.

18. If the base resistor is very small, the transistor will operate in the

If the base resistance is very small, then the base current is very high. Since, we know that the base current is directly proportional to the collector current with the multiplying factor of current gain. So, the value of collector current is enormously getting high. In this time, the transistor operates in saturation region.

19. Ignoring the bulk resistance of the collector diode, the collector-emitter saturation voltage is

By ignoring the resistance of the collector diode, there can be a huge flow of collector current. It assumes direct short circuit in the transistor. Hence, in the short circuit condition, the collector emitter saturation voltage is zero always.

20. Three difference Q points are shown on a load line. The upper Q point represents the

The upper Q point represents the high output current and that means the Q point represents the maximum current gain.