02 Class A Amplifiers

Topic 2: Class A Amplifiers (Series-Fed & Transformer-Coupled)

Exam-Focused Concept Note

Core Concepts (Short Note): A Class A amplifier operates such that the output signal varies for a full 360° of the input cycle,. To achieve this, the transistor’s quiescent operating point (Q-point) must be biased directly in the center of the active region (load line) so that the signal can swing completely up and down without hitting cutoff or saturation. Because the transistor is constantly “on” and conducting DC bias current even when no AC input signal is present, basic Series-Fed Class A amplifiers inherently waste a lot of power, resulting in a poor maximum efficiency of 25%,. To improve this efficiency up to 50%, designers replace the direct collector resistor with an output transformer, creating a Transformer-Coupled Class A amplifier. This heavily changes how the load interacts with the circuit, virtually eliminating DC power waste in the load and allowing for impedance matching.

Key Differences: Series-Fed vs. Transformer-Coupled Class A

Parameter	Series-Fed Class A	Transformer-Coupled Class A
Load Connection	Load is connected directly as the collector resistor ($R_C$).	Load ($R_L$) is isolated and connected via an output transformer.
Maximum Efficiency	25% (highly inefficient).	50% (improved efficiency).
DC Load Line	Slanted straight line (slope dictated by $R_C$).	Pure vertical straight line (assuming ideal $0\Omega$ transformer primary winding).
Power Waste	High, because the quiescent DC current ($I_{CQ}$) constantly flows directly through the load resistor.	Reduced, as the DC current only flows through the primary winding, not the actual output load.
Impedance Matching	Poor.	Excellent, adjustable by changing the transformer’s turns ratio ($a=N_1/N_2$),.

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1. Series-Fed Class A Amplifier Formulas & Efficiency

Core Concepts & Formulas: In a series-fed configuration, the DC supply directly drives both the bias current and the output load.

• DC Input Power: The total power drawn from the supply is determined by the supply voltage and the quiescent collector current: $P_{i(dc)}=V_{CC}{I}_{CQ}$.
• AC Output Power: The power delivered to the load ($R_C$) is: $P_{o(ac)}=\frac{V_{CE(p-p)}{I}_{C(p-p)}}{8}$ or $P_{o(ac)}=\frac{V_{C(rms)}^2}{R_C}$.
• Efficiency: $\eta =\frac{P_{o(ac)}}{P_{i(dc)}}\times 100$.
• Maximum Efficiency Proof: By setting the maximum peak-to-peak voltage swing to $V_{CC}$ and maximum current swing to $V_{CC}/R_C$, the theoretical maximum efficiency limit is exactly 25%,.

Exact PYQs to Master:

• What are the maximum theoretical efficiencies of the class A series-fed and transformer coupled amplifier circuits? (Asked in: Open-Book Exam Questions / Theory).

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2. Transformer-Coupled Class A Amplifier & Impedance Matching

Core Concepts & Formulas: This amplifier uses a transformer to step the voltage/current up or down and to match the load impedance for maximum power transfer,.

• Reflected Load Resistance ($R_L^{\prime }$ or $R_{eff}$): The actual load ($R_L$) is transformed and “seen” by the primary side as $R_L^{\prime }$. It relies on the turns ratio ($a=N_1/N_2$): $R_L^{\prime }=a^2{R}_L=(\frac{N_1}{N_2}{)}^2{R}_L$.
• DC vs. AC Load Line: Because the primary winding of an ideal transformer has $0\Omega$ DC resistance, the DC load line is perfectly vertical at $V_{CEQ}=V_{CC}$. The AC load line passes through the Q-point with a slanted slope equal to $-1/R_L^{\prime }$.
• AC Output Power: Using the maximum and minimum swings from the AC load line: $P_{o(ac)}=\frac{(V_{CEmax}-V_{CEmin})(I_{Cmax}-I_{Cmin})}{8}$.
• Maximum Efficiency: The theoretical upper limit for this configuration is improved to 50%,.

Exact PYQs to Master:

• Find the effective input resistance seen at the primary of a 10:1 transformer with secondary terminals connected to a $8\Omega$ speaker. (Asked in: Open-Book Exam Questions).
• Draw the circuit diagram of a class A transformer-coupled amplifier using an npn transistor. (Asked in: Practice / Theory).

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3. The Class A Design Numerical (PYQ Blueprint)

Core Process: The examiners heavily test this topic via a specific mathematical design problem where you must use the transformer impedance matching formulas to achieve maximum efficiency.

• Step 1: Identify the given constraints (e.g., $V_{CC}$, $I_{CQ}$, Turns ratio $N_1:N_2$, Load $R_L$).
• Step 2: Calculate the reflected load $R_L^{\prime }$ using the formula $a^2{R}_L$.
• Step 3: Determine the DC input power using $P_{i(dc)}=V_{CC}\times I_{CQ}$.
• Step 4: Since the design calls for “maximum efficiency” (50%), the peak AC voltage swing will equal $V_{CC}$ and the peak AC current swing will be $V_{CC}/R_L^{\prime }$. Use these to find the AC output power and verify the exact design parameters.

Exact PYQs to Master:

• Design a class A amplifier for maximum efficiency with the following specification: $V_{CC}=15V$, $I_c=100mA$, a load resistance of $8\Omega$, and a turn ratio of 5. (Asked in: 2023).