GATE Electrical – Analog Electronics
Page 10: BJT Characteristics
The behavior of a Bipolar Junction Transistor (BJT) can be understood using its characteristic curves. These curves show the relationship between currents and voltages in the transistor.
For analysis, the Common Emitter (CE) configuration is most widely used.
1. Input Characteristics
The input characteristic shows the relationship between:
Base Current (IB) vs Base–Emitter Voltage (VBE)
Important observations:
- Input characteristic resembles a PN junction diode.
- Base current increases rapidly after threshold voltage.
- Typical VBE value for silicon transistor ≈ 0.7 V.
Input resistance is defined as:
Rin = ΔVBE / ΔIB
2. Output Characteristics
The output characteristic shows the relationship between:
Collector Current (IC) vs Collector–Emitter Voltage (VCE)
This curve is drawn for different values of base current (IB).
Three important regions appear in this graph:
- Cutoff Region → IB = 0
- Active Region → Amplifier operation
- Saturation Region → Transistor fully ON
3. Transfer Characteristics
Transfer characteristics represent the relationship between:
Collector Current (IC) vs Base Current (IB)
This relationship gives the current gain of the transistor.
IC = β IB
Where:
- β = current gain
- Typical β value = 50 to 300
Important BJT Parameters
- Input resistance (Rin)
- Output resistance (Rout)
- Current gain (β)
- Voltage gain
Important GATE Points
- CE configuration is most commonly used.
- Input characteristic is similar to diode curve.
- Active region is used for amplification.
- Saturation region is used for switching.
Next Page → Load Line Analysis of BJT
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