# Multiple Choice Questions of Electronics 18

1. With voltage - divider bias, the base voltage is

The voltage divider bias is shown in the below circuit. In that circuit, the resistances R1 and R2 are the voltage divider. Here, the input voltage is base supply voltage and the output voltage is base voltage. In a voltage divider, the output base voltage = Input base supply voltage * (R2 / (R1 + R2)). This equation is clearly shows that the base voltage is less than the base supply voltage (VCC).

2. For the emitter bias, the voltage across the emitter resistor is the same as the voltage between the emitter and the

In an emitter bias, the emitter to ground voltage is the summation of voltage drop across the emitter resistance and the voltage drop across the input internal impedance of the transistor. Here, the input impedance is very less and it is negligible. Hence, the voltage drop across the emitter resistance is same as the voltage between emitter and ground.

3. The majority carriers in the emitter of a pnp transistor are

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

4. The current gain of a pnp transistor is

Typically for a PNP transistor, the current gain (β) is the ratio of the DC collector current to the DC base current.

5. Which is the largest current in a pnp transistor?

Theoretically in a PNP transistor, the Emitter is more positive with respect to the Base and also with respect to the Collector. Hence, the emitter current is largest than the other currents in PNP transistor. In other words, the below PNP transistor schematic shows that the emitter current is the summation of collector current and the base current. So, the emitter current is largest than the collector current and the base current.

6. The currents of a pnp transistor are

The currents of a PNP transistor are opposite to the currents of an NPN transistor. The current relationships of both transistors are shown in symbolic diagram.

7. The function of snubber circuit connected across the SCR is to

Snubber circuit is basically series connected resistor and capacitor connected across the power electronics switch like thyristor, power transistor, and etc. The impedance of the device abruptly jumps to a very high level and it is blocking the current when a switching device changes its state from ON-state to OFF-state. But the current still tends to keep flowing through the switch, which induces a high voltage across the switch because if the current decreases faster, then the induced voltage (dv/dt) becomes higher. It may reach to sufficiently high level to destroy the switch. Snubber circuit is required in order to suppress the rate of rise of forward voltage dv/dt across the switch.

8. In a circulating-current type of dual converter, the nature of the voltage across the reactor is

In circulated current type of dual converter, the right side of the thyristor switches are allowing the positive alternating voltage and the left side of the thyristor switches are allowing the negative alternating voltage to the reactor. Hence, resultant voltage across the reactor is alternating voltage. But, at midpoint of the reactor, the resultant voltage is DC because of the positive and negative alternating quantities are cancelling with each other and thereby we get the DC magnitude supply.

9. Practical way of obtaining static voltage equalization in series connected SCRs is by the use of

By connecting the resistors of the same value across each SCR, then we can be eliminated the unequal distribution of voltage across each SCRs. Hence, we can obtain the static voltage equalization in series connected SCRs by doing the above thing.

10. A single-phase full bridge inverter can operate in load-commutation mode in case load consists of

In a load commutation mode single phase full bridge inverter, if the load is not connected to the inverter, the SCRs would never be tuned off and after half cycle a short circuit would develop through SCR1 and SCR4. If it is not possible to guarantee that a load will always provide the proper counter voltage for commutation. This counter voltage can be provided by the RLC under damped loads for achieving the proper commutation.