Modes of operation of BJT
Definition
The modes of operation of a BJT are the different operating regions determined by the biasing condition of the emitter-base junction and collector-base junction. These modes describe whether the transistor is OFF, in active amplification, fully ON, or operating under special conditions such as reverse biasing.
Main Content
1. Cut-off Region
- In the cut-off mode, both the emitter-base junction and the collector-base junction are reverse biased.
- Ideally, no collector current flows except for a very small leakage current, so the transistor behaves like an open switch.
In this region, the base-emitter voltage is below the turn-on value, so the transistor does not conduct properly. For a silicon BJT, the base-emitter voltage is usually less than about 0.7 V for conduction. Since the base current is nearly zero, the collector current is also almost zero. This mode is widely used when the transistor is required to be completely OFF in digital switching circuits.
Example: In a transistor switch controlling an LED, when there is no input signal at the base, the transistor remains in cut-off and the LED stays OFF.
2. Active Region
- In the active mode, the emitter-base junction is forward biased and the collector-base junction is reverse biased.
- The transistor works as an amplifier, where a small base current controls a much larger collector current.
This is the most important region for linear amplification. The collector current is approximately given by , where is the current gain. In this mode, the transistor is not fully ON or OFF; instead, it operates in a controlled manner. Small changes in base current produce proportional changes in collector current, making this region suitable for audio amplifiers, voltage amplifiers, and signal processing circuits.
Example: In a microphone amplifier circuit, weak input signals are applied to the base, and the transistor amplifies them at the collector output.
3. Saturation Region
- In saturation mode, both the emitter-base junction and the collector-base junction are forward biased.
- The transistor is fully ON, and the collector-emitter voltage becomes very small.
In this condition, increasing the base current further does not produce a significant increase in collector current because the transistor has reached its maximum conduction state. The collector current is no longer controlled in the same proportional manner as in the active region. This mode is extremely useful in switching applications, where the transistor must act like a closed switch and allow maximum current through a load.
Example: In a relay-driving circuit, when enough base current is supplied, the transistor saturates and energizes the relay coil.
Working / Process
1. Apply biasing to the junctions
- The operating mode of the BJT is decided by the voltage applied across the emitter-base and collector-base junctions.
- Forward bias on the emitter-base junction encourages current flow, while reverse bias on the collector-base junction restricts it.
2. Observe the resulting current flow
- If both junctions are reverse biased, the transistor is in cut-off and current is nearly zero.
- If the emitter-base junction is forward biased and the collector-base junction is reverse biased, the transistor enters active mode.
- If both junctions are forward biased, the transistor goes into saturation.
3. Determine the transistor behavior
- In cut-off, the transistor behaves as an open switch.
- In active region, it behaves as an amplifier.
- In saturation, it behaves as a closed switch.
Advantages / Applications
- Used as a switch in digital circuits, where cut-off and saturation modes are very important.
- Used as an amplifier in analog circuits, especially in the active region.
- Helps in controlling large currents with a small base input, making transistor circuits efficient and compact.
Summary
- BJT operation depends on how its junctions are biased.
- Cut-off, active, and saturation are the main modes of operation.
- These modes decide whether the transistor acts as an amplifier or a switch.