Differentiate between NPN and PNP transistor operation and typical biasing configurations (CE, CB, CC).

• A. NPN conducts when base is higher than emitter; PNP conducts when base is lower than emitter; common-emitter is one configuration; common-base and common-collector are other configurations.
• B. NPN conducts when base is lower than emitter; PNP conducts when base is higher than emitter; only common-emitter exists.
• C. NPN requires emitter tied to ground; PNP requires collector tied to Vcc; all configurations have identical bias.
• D. NPN and PNP behave identically; biasing doesn't matter.

Answer: (B)

The key idea is how the base-emitter junction is forward biased and how that controls the current paths through the transistor, plus the different ways you can connect the transistor to use it as an amplifier.

For an NPN transistor, forward biasing the base-emitter junction means the base must be about 0.6 to 0.7 volts higher than the emitter. When this happens, electrons are injected from the emitter into the base and, because the collector-base junction is reverse-biased, most of those carriers are swept into the collector. A small base current controls a much larger collector current, with conventional current flowing from the base into the emitter and from the collector into the emitter.

For a PNP transistor, the polarity is reversed. The base must be about 0.6 to 0.7 volts lower than the emitter to forward-bias the base-emitter junction. In this case, holes move from the emitter into the base, and the conventional current flows from the emitter toward the collector. Again, a small base current governs a larger collector current, but the directions are opposite to the NPN case.

Beyond polarity, the three typical biasing configurations describe how the transistor is used in circuits:

• Common-emitter: the emitter is the reference (common) point for input and output. The input is between base and emitter, the output is between collector and emitter. This arrangement gives good voltage and current gain but inverts the signal phase.

• Common-base: the base is the common point. The input is between emitter and base, and the output is between collector and base. It provides high voltage gain, low input impedance, and no phase inversion.

• Common-collector (emitter follower): the collector is the common point. The input is between base and collector, and the output is between emitter and collector. It offers high input impedance, low output impedance, and near-unity voltage gain (useful as a buffer), but no substantial voltage amplification.

In short, NPN and PNP differ mainly in the required base-emitter polarity for conduction, and the three biasing configurations describe how the transistor can be wired to meet different amplification and impedance goals.