3.2 Transistor Circuit Configurations

3.2 Transistor Circuit Configurations

Because a BJT has three terminals, one terminal must always be shared in common between the input and output circuits (often referenced to AC ground). This leads to the three fundamental BJT circuit configurations:

1. Common Emitter (CE)
2. Common Base (CB)
3. Common Collector (CC), universally known as the Emitter Follower

Each topology exhibits distinct characteristics regarding input impedance ($Z_{in}$), output impedance ($Z_{out}$), voltage gain ($A_v$), current gain ($A_i$), and phase relationship ($\Delta\theta$), governing its role in avionics systems.

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1. Common Emitter (CE) Configuration

In the Common Emitter topology, the AC input is applied to the Base, the output is extracted from the Collector, and the Emitter is shared in common (connected to AC ground via a bypass capacitor).

Key Characteristics

• Input Impedance ($Z_{in}$): Moderate, typically $1\text{ k}\Omega\text{ to }5\text{ k}\Omega$ ($Z_{in} \approx \beta \cdot r'_e$).
• Output Impedance ($Z_{out}$): Moderate to high, typically $20\text{ k}\Omega\text{ to }100\text{ k}\Omega$ (determined by $R_C \parallel r_o$).
• Voltage Gain ($A_v$): High, typically $-50\text{ to }-500$ ($A_v \approx -R_C / r'_e$).
• Current Gain ($A_i$): High, equal to $\beta$ ($50\text{ to }300$).
• Power Gain ($A_p$): Highest of all three configurations. Because $A_p = A_v \times A_i$, combining substantial voltage gain with high current gain produces power gains of $40\text{ to }47\text{ dB}$.
• Phase Shift: $180^\circ$ Phase Inversion between input and output AC voltages.

Mechanism of Phase Inversion

The $180^\circ$ phase inversion is a central EASA Part-66 exam concept:

1. When the input voltage at the base swings positive, forward bias $V_{BE}$ increases.
2. Base current $I_B$ increases, causing collector current $I_C = \beta I_B$ to rise.
3. The increased $I_C$ causes a larger voltage drop across collector resistor $R_C$ ($V_{RC} = I_C R_C$).
4. Because collector voltage is $V_C = V_{CC} - I_C R_C$, the collector voltage drops toward ground.
5. Thus, a positive-going input peak produces a negative-going output peak, creating a $180^\circ$ phase inversion.

[!WARNING] Multistage Feedback Hazards: In aircraft autopilot servos and interphone amplifiers, cascading two CE stages creates two sequential $180^\circ$ shifts ($180^\circ + 180^\circ = 360^\circ = 0^\circ$). If stray capacitive coupling feeds output signal back to the input in phase, it produces regenerative positive feedback, causing unwanted high-frequency oscillation.

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2. Common Base (CB) Configuration

In the Common Base topology, the AC input is injected into the Emitter, the output is taken from the Collector, and the Base is connected directly to AC ground.

Key Characteristics

• Input Impedance ($Z_{in}$): Very low, typically $20\ \Omega\text{ to }50\ \Omega$ ($Z_{in} \approx r'_e = 25\text{ mV} / I_E$).
• Output Impedance ($Z_{out}$): Very high, typically $500\text{ k}\Omega\text{ to }1\text{ M}\Omega$.
• Voltage Gain ($A_v$): High, comparable to Common Emitter ($A_v \approx +R_C / r'_e$).
• Current Gain ($A_i$): Slightly less than unity ($A_i = \alpha \approx 0.95\text{ to }0.998$). There is no current gain.
• Power Gain ($A_p$): Moderate ($A_p = A_v \times \alpha$).
• Phase Shift: $0^\circ$ (Non-inverting / In-Phase).

Miller Effect Suppression & RF Applications

In Common Emitter amplifiers, collector-base capacitance ($C_{cb}$) is amplified by voltage gain ($C_{Miller} = C_{cb}(1 + |A_v|)$), creating high-frequency feedback that degrades bandwidth.

In the Common Base configuration, the base is tied to AC ground, acting as an electrostatic shield between emitter and collector. This eliminates Miller multiplication of capacitance. Consequently, CB stages provide superior high-frequency bandwidth and stability, making them the standard choice for aircraft VHF/UHF communications front-ends, radar IF preamplifiers, and matching to $50\ \Omega$ antenna feedlines.

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3. Common Collector (CC) / Emitter Follower

In the Common Collector topology, the AC input is applied to the Base, the output is taken from the Emitter, and the Collector is tied to AC ground (via the DC supply rail $V_{CC}$).

Key Characteristics

• Input Impedance ($Z_{in}$): High, typically $20\text{ k}\Omega\text{ to }500\text{ k}\Omega$ ($Z_{in} \approx \beta R_E$).
• Output Impedance ($Z_{out}$): Very low, typically $10\ \Omega\text{ to }100\ \Omega$ ($Z_{out} \approx r'_e + [R_S / \beta]$).
• Voltage Gain ($A_v$): Slightly less than unity, typically $0.98\text{ to }0.99$ (never exceeds 1.0).
• Current Gain ($A_i$): High, equal to $\beta + 1$.
• Power Gain ($A_p$): Moderate ($A_p \approx A_i$).
• Phase Shift: $0^\circ$ (Non-inverting / In-Phase).

The Emitter Follower in Avionics

The circuit is known as the Emitter Follower because the emitter output voltage directly "follows" the base input voltage, offset only by the constant $0.7\text{ V}$ base-emitter diode drop:

$V_E = V_B - 0.7\text{ V}$

Because it transforms high impedance to low impedance with unity voltage gain, the Common Collector is universally used as an impedance-matching buffer. It interfaces delicate, high-impedance sensors (such as piezoelectric engine vibration pickups and cockpit microphones) to low-impedance cables and downstream processing stages without signal loading or attenuation.

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Summary Comparison Matrix

The table below summarizes the core parameters tested in EASA Part-66 Module 04:

[!NOTE] Cascaded Architectures: Typical aircraft radar and communications receivers cascade these stages: a Common Base or Common Collector input stage matches antenna or sensor impedance, followed by multiple Common Emitter stages for high voltage and power amplification, terminating in an Emitter Follower output stage to drive transmission lines.