# Multiple Choice Questions of Electronics 17

1. If a transistor operates at the middle of the load line, a decrease in the base resistance will move the Q point

Whenever the base resistance decreases, the base current increases according to the base resistance decrement. We know that the base current is directly proportional to the output collector current. That small change in the base current gives the big increment in the output current. It means that the Q point move UP from the previous level.

2. If a transistor operates at the middle of the load line, a decrease in the current gain will move the Q point

Reduction in current gain leads to reduce the output current because both are proportional to each other. Obviously the Q point goes down, if the output current decreases.

3. If the base supply voltage increase, the Q point moves

According to the ohms law, the voltage is directly proportional to current. Hence, the base current also increases due to the increases in base voltage. This small change in the base current gives the big increment in the output current. It means that the Q point move UP from the previous level.

4. Suppose the base resistor is open. The Q point will be

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. It leads to the output current as very much as lower but there can be a small leakage current due to the collector diode. So, the Q point goes lower end of the load line.

5. If the base supply voltage is disconnected, the collector-emitter voltage will equal

In a transistor, if the base supply voltage is zero then the base current is also zero. We know that the base current and collector current are directly proportional. So, the collector current is also zero. In a transistor, collector current (IC) = 0 = (VCC - VCE) / RL => (VCC - VCE) =0, which gives VCC = VCE. Here, the VCC is collector supply voltage and the VCE is collector emitter voltage.

6. If the base resistor has zero resistance, the transistor will probably be

If the base resistance is zero, 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 and finally it is being destroyed.

7. If the collector resistor opens in a base-biased circuit, the load line will become

When the collector resistor opens, then the collector current is zero. If the collector current (IC) = 0 = (VCC - VCE) / RL => (VCC - VCE) =0, which gives VCC = VCE. Hence, the load line permanently reaches the horizontal as shown in below characteristics graph of a transistor. That means the transistor is in cut-off region.

8. The collector current is 1.5 mA. If the current gain is 50, the base current is

We know that the collector current IC = β * IB. Therefore, Base current IB = IC / β = 1.5 * 10-3 / 50 = 3 * 10-5 A = 30 * 10-6 = 30 μA.

9. When the Q point moves along the load line, VCE decreases when the collector current

The below characteristics curve of transistor shows that when the Q point moves along the load line, VCE decreases when the collector current increases.

10. When there is no base current in a transistor switch, the output voltage from the transistor is

In a transistor, if the base current is also zero, then the collector current is also zero. In a transistor, collector current (IC) = 0 = (VCC - VCE) / RL => (VCC - VCE) =0, which gives VCC = VCE. Hence, the output voltage from the transistor is highs.

11. The first step in analyzing emitter-based circuits is to find the

The first step in analyzing the emitter based circuit is to find the emitter current. This is given clearly in the below analysis of emitter biased circuit.

12. If the current gain is unknown in an emitter-biased circuit, you cannot calculate the

In an emitter-biased circuit, the emitter current (IE) = Current gain (β) * Base current (IB). So, if we don’t know the current gain in an emitter biased circuit, we can’t calculate the base current.

13. If the emitter resistor is open, the collector voltage is

If the emitter resistor is open, then the full supply voltage is going to the output load because the current chooses the low resistance path to flow and so it does not flow through the emitter resistance. Hence, the collector voltage is high.

14. If the collector resistor is open, the collector voltage is

If the collector resistor is open, then there is no way to flow the current to load. Hence, the output voltage is low at load side.

15. When the current gain increases from 50 to 300 in an emitter-biased circuit, the collector current

In an emitter biased circuit, there is no relation between collector current and the current gain. Hence, it remains almost the same.

16. If the emitter resistance increases, the collector voltage

If the emitter resistor increases, then the most of the supply voltage is going to the output load because the current chooses the low resistance path to flow and so it does not flow through the emitter resistance. Hence, the collector voltage also increases.

17. If the emitter resistance decreases, the

If the emitter resistor decreases, then the flow of collector current is more because the current chooses the low resistance path to flow. Hence, the current gain also increases. That means the Q point moves up from the previous level.

18. When testing an npn transistor using an ohmmeter, the collector-emitter resistance will be low when

The collector emitter resistance is nothing but the output impedance. Typically, for an NPN transistor, the output impedance is around 100000 Ω. So, when testing an NPN transistor if it shows the low output impedance, then that means the transistor is faulty.

19. The major advantage of a photo transistor as compared to a photo diode is its

Most phototransistors have outer casing which is either transparent or has a clear lens to focus the light onto the base junction for increased sensitivity. Also, in phototransistor, the current gain is too high and this is nothing but the sensitivity of a phototransistor. But this is not available in photo diodes. That is why the sensitivity of the photo transistor is greater than the photo diode.

20. For emitter bias, the voltage at the emitter is 0.7 V less than the

In the emitter bias, the base-emitter terminal is connected with the base resistor. So, according to the KVL, the total base voltage is the summation of the voltage drop across the base resistance and the base emitter voltage. Hence, the base emitter voltage is always less than the base voltage.

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