During normal working of transistor as amplifier, the emitter junction is _______.
Forward biased
Transistor Amplifier Biasing Explanation
For a transistor to function correctly as an amplifier, it needs to be biased into a specific operating mode known as the active region.
A bipolar junction transistor (BJT) has two p-n junctions:
- The junction between the emitter and the base (Emitter-Base junction or emitter junction).
- The junction between the collector and the base (Collector-Base junction or collector junction).
The way these two junctions are biased (either forward biased or reverse biased) determines how the transistor operates.
There are generally three main operating regions for a transistor:
- Cut-off Region: In this region, both the emitter-base and collector-base junctions are reverse biased. The transistor acts like an open switch, and very little current flows.
- Saturation Region: Here, both the emitter-base and collector-base junctions are forward biased. The transistor acts like a closed switch, allowing maximum current flow limited by the external circuit.
- Active Region: This is the crucial region for amplification. In the active region, the emitter-base junction is forward biased, while the collector-base junction is reverse biased.
When a transistor is biased in the active region, a small change in the input signal at the base (or emitter) can control a much larger current flowing between the collector and emitter. This controlled current allows the transistor to amplify signals.
The table below summarizes the biasing conditions for the different operating regions:
| Operating Region | Emitter-Base Junction | Collector-Base Junction | Typical Application |
|---|---|---|---|
| Cut-off | Reverse Biased | Reverse Biased | Switch (OFF state) |
| Saturation | Forward Biased | Forward Biased | Switch (ON state) |
| Active | Forward Biased | Reverse Biased | Amplifier |
Therefore, for a transistor to operate normally as an amplifier, the emitter junction must be forward biased.
Important Questions from Configuration of BJT
BC147 is the transistor used for:
- What is the relationship between the common-emitter current gain ($\beta$) and the common-base current gain ($\alpha$) of a bipolar junction transistor (BJT)?
Which of the following is NOT true for a common collector transistor?
The other name for the common collector amplifier is -
Match List I with List II:
List I
(Bias Configuration of BJT)
List II
(Stability factor equation)
(A) Fixed Bias Configuration (I) S(V BE ) = \(\rm −\frac{\beta/R_E}{\beta+R_{TH}/R_E}\) (B) Emitter Bias Configuration (II) S(V BE ) = −β/R E (C) Voltage Divider Configuration (III) S(V BE ) = \(\rm −\frac{\beta/R_C}{\beta+R_E/R_C}\) (D) Feedback Bias Configuration (IV) S(V BE ) = \(\rm −\frac{\beta/R_E}{\beta+R_B/R_E}\) Choose the correct answer from the options given below :
