BJT Configurations: Characteristics of CB, CE, and CC Modes

Posted on Nov 19, 2025 in Industrial Electronics and Automation Engineering

Common Base (CB) Transistor Configuration

Circuit Description

In the Common Base (CB) configuration, the input is applied between the Emitter (E) and Base (B), and the output is taken between the Collector (C) and Base (B). The Base terminal is common to both the input and output. For active region operation, the emitter-base junction is forward-biased, and the collector-base junction is reverse-biased.

Input Characteristics

• Definition: A graph of Emitter Current (I_E) versus Emitter-Base Voltage (V_EB) for a constant Collector-Base Voltage (V_CB).
• Shape: Similar to the forward characteristic of a diode because the E-B junction is forward-biased.
• Input Resistance: R_in = ΔV_EB / ΔI_E, typically very low (20 Ω – 150 Ω).

Output Characteristics

• Definition: A graph of Collector Current (I_C) versus Collector-Base Voltage (V_CB) for a constant Emitter Current (I_E).
• Shape: Nearly flat in the active region, showing that I_C is almost independent of V_CB and approximately I_C ≈ αI_E.
• Output Resistance: R_out = ΔV_CB / ΔI_C, typically very high (≈ 1 MΩ).

Operating Regions

PN Junction Diode V-I Characteristics

Definition

A PN junction diode is formed by joining P-type and N-type semiconductor materials. It allows current to flow primarily in one direction—from the P-type to the N-type material—when it is forward-biased.

V-I Characteristics

The V-I (Voltage-Current) characteristic curve shows the relationship between the diode current (I_D) and the voltage across the diode (V_D).

Forward Bias (P positive, N negative)

• The depletion width decreases.
• Current is negligible until the voltage reaches the knee (or threshold) value.
• Knee Voltage (V_γ): Approximately 0.7 V for silicon and 0.3 V for germanium.
• Beyond the knee voltage, the current rises exponentially with a small increase in voltage.

Reverse Bias (P negative, N positive)

• The depletion width increases, and the junction resists current flow.
• Only a small reverse saturation current (I_S) flows, which is due to minority carriers.
• At a critical reverse voltage, a breakdown (either Zener or avalanche) occurs, causing a sharp increase in current.

Common Emitter (CE) Transistor Configuration

Circuit Description

In the Common Emitter (CE) configuration, the input is applied between the Base (B) and Emitter (E), and the output is taken between the Collector (C) and Emitter (E). The Emitter terminal is common. The CE mode is the most widely used because it provides high current and voltage gain.

Input Characteristics (I_B vs V_BE)

• The curve resembles that of a forward-biased diode.
• The base current (I_B) rises rapidly after the cut-in voltage (approximately 0.7 V for Si).
• The curve shifts slightly with changes in V_CE due to the Early effect (base-width modulation).
• Input resistance is moderate (approximately 1 kΩ).

Output Characteristics (I_C vs V_CE)

• Active Region: I_C ≈ βI_B. This region is used for amplification. Junctions: E-B (Forward), C-B (Reverse).
• Cut-off Region: I_B = 0, and both junctions are reverse-biased. The collector current (I_C) is very small (leakage current I_CEO). The transistor acts as an open switch.
• Saturation Region: V_CE is low (< 1 V), and both junctions are forward-biased. I_C is limited by the collector resistor (R_C). The transistor acts as a closed switch.

Common Collector (CC) Transistor Configuration

Circuit Description

In the Common Collector (CC) configuration, the input is applied between the Base (B) and Collector (C), and the output is taken between the Emitter (E) and Collector (C). The Collector terminal is common.

Key Features

• Current Gain (γ = I_E / I_B): Very high, where γ = 1 + β.
• Voltage Gain (A_v): Approximately 1 (unity). The output voltage closely follows the input voltage, hence the name “Emitter Follower.”
• Input Impedance: High.
• Output Impedance: Low.
• Application: Primarily used for impedance matching, connecting a high-impedance source to a low-impedance load.

Comparison of Transistor Configurations (CB, CE, CC)

Transistor Current Gain: Alpha (α) and Beta (β)

Alpha (α_dc)

α_dc = I_C / I_E. This is the ratio of the collector current to the emitter current in the CB configuration. It represents the fraction of emitter current that reaches the collector, so α is always less than 1.

Beta (β_dc)

β_dc = I_C / I_B. This is the ratio of the collector current to the base current in the CE configuration. It represents the current amplification factor, and β is usually large (typically 20 – 500).

Relationship Between Alpha and Beta

Starting from the basic transistor current equation: I_E = I_B + I_C

1. Expressing β in terms of α:
   β = α / (1 − α)
2. Expressing α in terms of β:
   α = β / (1 + β)

Transistor Circuit Problem Solutions

Problem 1

Given: I_C = 2.5 mA, I_E = 2.6 mA

Find: α_dc, I_B, β_dc

Solution:

• I_B = I_E − I_C = 2.6 mA − 2.5 mA = 0.1 mA
• α_dc = I_C / I_E = 2.5 / 2.6 = 0.9615
• β_dc = I_C / I_B = 2.5 / 0.1 = 25

Problem 2

Given: Initial I_C = 3 mA, Initial I_E = 3.03 mA, New β_dc = 70

Find: Initial α_dc and new I_C

Solution:

• Initial I_B = I_E − I_C = 3.03 mA − 3.00 mA = 0.03 mA
• Initial α_dc = I_C / I_E = 3 / 3.03 = 0.99
• New I_C = β × I_B = 70 × 0.03 mA = 2.1 mA

Problem 3

Given: α_dc = 0.98, I_B = 100 µA

Find: I_C, I_E, β_dc

Solution:

• β = α / (1 − α) = 0.98 / (1 − 0.98) = 0.98 / 0.02 = 49
• I_C = β × I_B = 49 × 100 µA = 4900 µA = 4.9 mA
• I_E = I_C + I_B = 4.9 mA + 0.1 mA = 5.0 mA

Problem 4

Given: I_C = 1 mA, I_B = 25 µA, Target I_C = 5 mA

Find: α_dc, β_dc, and the new I_B required

Solution:

• β = I_C / I_B = 1 mA / 25 µA = 1000 µA / 25 µA = 40
• α = β / (1 + β) = 40 / (1 + 40) = 40 / 41 = 0.9756
• New I_B = I_C / β = 5 mA / 40 = 0.125 mA or 125 µA