Common Base Amplifier

The common base amplifier is the only BJT configuration that provides current gain less than one but delivers excellent voltage gain and extremely wide bandwidth. It is the core of RF front-end circuits and high-frequency current-to-voltage converters.

Core Concept

In a common base amplifier, the base terminal is the reference (AC ground). The input signal drives the emitter, and the output is taken from the collector. Current flows from emitter to collector, with almost no change since alpha is close to 1. This means current gain is slightly below unity, around 0.98 for a BC547 with beta = 50.

The input impedance of the common base stage is very low, approximately equal to the transconductance reciprocal: Rin = 1/gm = VT/IC. At IC = 1 mA, Rin ≈ 26 Ω. This matches well with coaxial cable (75 Ω) or transmission line inputs in RF systems.

The output impedance is very high, close to rO (the Early resistance). This makes the common base stage an excellent current buffer and a natural fit for cascode amplifiers, where it stacks above a common emitter stage to extend bandwidth to gigahertz frequencies.

Key Equations

Current gain: AI = -alpha = -IC/IE, magnitude just below 1. Alpha = beta/(beta+1). For beta=100, alpha=0.99.

Voltage gain: AV = -alpha * RC / re where re = VT/IE = 26mV/IE. With IC=1mA, re=26Ω, and RC=3.3kΩ, AV ≈ 126.

Input impedance: Rin = re = VT/IC. At 1 mA, Rin = 26 Ω.

Output impedance: Rout ≈ rO = VA/IC where VA is the Early voltage, typically 50-200V for BJTs.

Given:
  BC547, VCC = 12V, RC = 3.3 kΩ, RE = 1 kΩ
  VBE = 0.7V, VB = 0V (base grounded via CB)
  beta = 100, VT = 26 mV

Why this formula:
  Find IC from DC bias, then compute re and voltage gain.

DC bias:
  VE = VB - VBE = 0 - 0.7 = -0.7V
  (With separate -VEE=-12V and RE to -VEE for proper biasing)
  IE = (VEE - VBE) / RE = (12 - 0.7) / 1000 = 11.3 mA
  (Use IC ≈ IE = 11.3 mA for alpha ≈ 1)

[Simpler example: VCC only, use fixed bias VB=2V]
  IE = (VB - VBE) / RE = (2 - 0.7)/1000 = 1.3 mA

Formula:
  re = VT / IE
  AV = RC / re   (magnitude, for common base)

Substitution:
  re = 26 mV / 1.3 mA = 26 / 1.3 = 20 Ω
  AV = RC / re = 3300 / 20

Calculation:
  AV = 165

Final Answer:
  re = 20 Ω, Voltage gain AV = 165 (non-inverting in common base)
  Input impedance Rin = re = 20 Ω

Exam Tip: The common base amplifier does NOT invert the output signal. This is the opposite of common emitter. GATE questions often ask for phase relationship: common emitter gives 180° phase shift; common base gives 0° (non-inverting). Also, current gain for common base is alpha (less than 1), not beta. Confusing alpha and beta in the gain formula is the most common error.

Key Properties

• Current gain AI = alpha < 1 (typically 0.95 to 0.99). Power gain is still positive because voltage gain is large.
• Input impedance is very low: Rin = re = 26 Ω at 1 mA. This matches transmission line impedances in RF circuits.
• Output is non-inverting: a positive signal at the emitter produces a positive signal at the collector.
• Bandwidth is the widest of all three BJT configurations. There is no Miller effect since the base is grounded.
• Used in cascode amplifiers (CE + CB stacked) to achieve high gain with wide bandwidth in RF and video amplifiers.
• Output impedance is high (≈ rO), making the stage look like a current source to the load.

Quick Revision

• Input: emitter. Output: collector. Reference: base (AC ground).
• Current gain: alpha (less than 1). No current amplification.
• Voltage gain: AV = RC/re, same magnitude as common emitter.
• Phase shift: 0° (non-inverting).
• Input impedance: very low (~26 Ω at 1 mA).
• Best bandwidth of all BJT configurations: no Miller capacitance.
• Used in RF amplifiers, cascode stages, and photo-detector transimpedance circuits.
• Exam trap: Students write AV as negative (inverting) for common base, confusing it with common emitter. Common base voltage gain is positive.