Common Base Configuration of BJT

Introduction

• Common application of BJT: Used as an amplifier.
• Configurations of BJT: common emitter, common base, common collector.

Common Base Configuration

• In this configuration, Base terminal is common between input and output.
  • Input: between Base and Emitter terminals.
  • Output: between Collector and Base terminals.
• Device operates in active region:
  • Base-emitter junction: Forward biased
  • Collector-base junction: Reverse biased

NPN and PNP Transistors

• NPN Transistor:
  • Emitter current goes away from the transistor.
  • Base and collector currents enter into the transistor.
• PNP Transistor:
  • Emitter current enters into the transistor.
  • Base and collector currents go away from the transistor.
• Emitter current:
  • Equal to summation of base and collector currents.

Characteristics of Common Base Configuration

• Characterized by Input and Output Characteristics.
• Input characteristics show relationship between Vbe (input voltage) and Ie (input current).
  • Similar to forward characteristics of PN diode.
  • As Vcb increases, the curve shifts left and Ie increases due to decreasing depletion width.

Input Impedance

• Input impedance (Zin):
  • Determined from slope of input characteristics.
  • Z_in = (\frac{\Delta V_{be}}{\Delta I_e})
  • Very low resistance, typically in the ohm range.

Output Characteristics

• Output characteristics define relationship between Ic (output current) and Vcb (output voltage).
• Three regions of operation:
  1. Active Region:
     • Collector current (Ic) remains constant irrespective of Vcb change.
     • (I_c = \alpha \cdot I_e)
  2. Saturation Region:
     • Collector current reduces as Vcb becomes negative.
     • Both junctions forward biased.
  3. Cut-off Region:
     • Emitter current is zero; collector current remains almost zero.
     • Only due to minority charge carriers, i.e., Icbo (reverse saturation current).

Amplification in Common Base Configuration

• Simplified analysis without DC biasing voltages.
• Example:
  • Input impedance (R_in) = 10 ohm
  • Input voltage (V_i) = 5 mV
  • Input current (I_e) = (\frac{5 mV}{10 ohm} = 0.5 mA)
  • Assuming (\alpha = 1), (I_c \approx I_e)
  • Output impedance (R_o) = Typical in hundreds of kΩ
  • Load resistor (R_L) = 1 kΩ
  • Output voltage (V_o):
    • (V_o = I_o \times R_L = 0.5 mA \times 1 kΩ = 0.5 V)
    • Voltage gain = (\frac{V_o}{V_i} = \frac{0.5 V}{5 mV} = 100)

Current Gain

• Current gain (Ic/Ie) is less than 1 in this configuration.
• Voltage gain typically varies from 50 to 300.

Conclusion

• Common base configuration allows for voltage gain.
• Next topic: Common-Emitter Configuration.

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