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The point of intersection of d.c. and a.c. load lines represents______.

Operating point

Current gain

Voltage gain

None

Correct Answer

⚙️ TE • Technical Concept & PrincipleElectricalElectronics

Option A

Operating point

Quick Summary:

The point of intersection of the d.c. (direct current) and a.c. (alternating current) load lines represents the operating point, also known as the Q-point or quiescent point. It specifies the steady-state collector voltage ( $V_{CEQ}$ ) and collector current ( $I_{CQ}$ ) of a transistor amplifier when no input signal is applied. Under signal conditions, the dynamic response of the transistor oscillates along the a.c. load line centered around this operating point.

⚙️TETechnical SolutionConcept & Principle

💡 Explanation

🔢 Key Formulas

$V_{CE} = V_{CC} - I_C R_C$ — D.C. Load Line Equation

$i_c = -\frac{v_{ce}}{R_{ac}}$ — A.C. Load Line Slope ( $-\frac{1}{R_{ac}}$ )

$R_{ac} = R_C \parallel R_L$ — Effective A.C. Load Resistance

⚙️ Working Principle

When a transistor circuit is biased without an a.c. signal, its state is governed solely by the d.c. load line based on d.c. power supply and circuit resistors. When an a.c. signal is fed into the circuit, the effective load impedance changes due to coupling and bypass capacitors, resulting in a steeper a.c. load line. Because the a.c. signal swings sinusoidally above and below the zero-signal equilibrium, the a.c. load line must pass through the fixed d.c. operating point.

📌 Key Points

▸
The d.c. load line is determined purely by d.c. circuit parameters ( $V_{CC}$ , $R_C$ , $R_E$ ).
▸
The a.c. load line accounts for parallel load impedance ( $R_L$ ) introduced via coupling capacitors.
▸
The slope of the a.c. load line ( $-\frac{1}{R_{ac}}$ ) is steeper than the d.c. load line slope ( $-\frac{1}{R_{dc}}$ ) because $R_{ac} < R_{dc}$ .
▸
The operating point (Q-point) is common to both lines, representing zero-signal d.c. bias values.

✅ Advantages

▸
Ensures maximum symmetrical peak-to-peak output swing without clipping when positioned centrally.
▸
Maintains stable linear amplifier operation.

❌ Disadvantages / Limitations

▸
Improper placement of Q-point causes thermal runaway or signal distortion (saturation/cutoff clipping).

🛠️ Applications / Uses

▸
Design of Class A, Class B, and Class C transistor amplifiers.
▸
Biasing network analysis for BJT and FET amplification circuits.

📄 Additional Information

▸
Option B (Current gain) is the ratio of output current change to input current change ( $stroke \beta = \frac{\Delta I_C}{\Delta I_B}$ ).
▸
Option C (Voltage gain) is the ratio of output voltage change to input voltage change ( $A_v = \frac{v_{out}}{v_{in}}$ ).

📊 Diagram / Illustration

✅

A is correct — The intersection of the d.c. and a.c. load lines defines the quiescent operating point (Q-point) where both zero-signal and signal equilibrium conditions are satisfied.

Core Concepts Used

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Operating Point (Q-Point) D.C. Load Line A.C. Load Line

💡 EXAM TIP

For maximum symmetrical output voltage swing in a Class A common-emitter amplifier, set the Q-point at the midpoint of the a.c. load line rather than the midpoint of the d.c. load line.

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