What are the 3 conditions for the BJT to be biased in its linear or active operating region ? Explain it with a Output characteristic of CE Transistor configuration.
The correct answer and explanation is:
Correct Answer:
The three conditions for a Bipolar Junction Transistor (BJT) to operate in its linear or active region are:
- Base-Emitter Junction is Forward Biased.
- Collector-Base Junction is Reverse Biased.
- Collector voltage is greater than base voltage and emitter voltage.
Explanation (Approx. 300 words):
In a Common Emitter (CE) transistor configuration, the BJT functions as an amplifier when operating in the active region. For proper amplification, the transistor must be biased correctly so that it remains in the active region during signal variations.
1. Forward Biased Base-Emitter Junction:
This condition ensures that charge carriers (electrons in NPN or holes in PNP) are injected from the emitter into the base. A typical silicon transistor requires about 0.7V across the base-emitter junction for it to conduct. This biasing allows the transistor to respond to small input changes at the base.
2. Reverse Biased Collector-Base Junction:
This ensures that the minority carriers in the base region move swiftly toward the collector, enabling efficient current flow from collector to emitter. It prevents unwanted saturation and ensures linearity in amplification.
3. Collector Voltage > Base Voltage > Emitter Voltage (for NPN):
This condition maintains both previous junction biases. If this voltage hierarchy is not maintained, the transistor may enter saturation or cutoff, losing its linear response.
Relation with Output Characteristics (CE Configuration):
In the output characteristic curves of a CE transistor, the collector current (I_C) is plotted against the collector-emitter voltage (V_CE) for various base currents (I_B). The active region appears as a nearly horizontal part of the curve for each I_B value, indicating that I_C remains almost constant despite changes in V_CE. This region lies between the saturation region (left side, low V_CE) and the cutoff region (near zero I_C).
Only in this active region is the transistor able to provide faithful amplification with minimal distortion, making it crucial in analog applications such as audio amplifiers and signal processing.