In active mode, what are the biasing conditions for the BJT junctions?

In active mode, what are the biasing conditions for the BJT junctions?

• A. Base-Emitter forward, Collector-Base reverse
• B. Base-Emitter reverse, Collector-Base forward
• C. Both forward
• D. Both reverse

Answer: (A)

Active mode requires the base–emitter junction to be forward biased and the base–collector junction to be reverse biased. The forward bias on the emitter–base junction drives charge carriers from the emitter into the base, while the reverse bias on the collector–base junction widens the depletion region and efficiently sweeps those carriers into the collector. This arrangement lets the collector current be controlled mainly by the base current, with Ic ≈ β·Ib, and makes the transistor act as a current amplifier within a range of voltages. If the emitter–base junction were reverse biased, the transistor wouldn’t operate in this normal amplifying manner; it could operate in inverse-active mode, where gains are much smaller. If both junctions were forward biased, the device would be in saturation and lose the current amplification behavior. If both were reverse biased, the transistor would be essentially off, with very little current flow.

Active mode requires the base–emitter junction to be forward biased and the base–collector junction to be reverse biased. The forward bias on the emitter–base junction drives charge carriers from the emitter into the base, while the reverse bias on the collector–base junction widens the depletion region and efficiently sweeps those carriers into the collector. This arrangement lets the collector current be controlled mainly by the base current, with Ic ≈ β·Ib, and makes the transistor act as a current amplifier within a range of voltages.

If the emitter–base junction were reverse biased, the transistor wouldn’t operate in this normal amplifying manner; it could operate in inverse-active mode, where gains are much smaller. If both junctions were forward biased, the device would be in saturation and lose the current amplification behavior. If both were reverse biased, the transistor would be essentially off, with very little current flow.