03 Class B Push-Pull Amplifiers

Topic 3: Class B Push-Pull Amplifiers & The 78.5% Proof

Exam-Focused Concept Note

Core Concepts (Short Note): A Class B amplifier is biased exactly at cutoff (0 V), meaning the transistor only conducts for exactly 180° (one-half) of the input signal cycle. While this zero-bias approach drastically reduces wasted standby power, a single Class B transistor heavily distorts the output because half of the waveform is missing. To solve this, a “Push-Pull” configuration is utilized. It uses two transistors operating on alternating half-cycles—one pushes the positive half of the signal to the load, and the other pulls the negative half—combining them to reproduce a full 360° output. This alternating arrangement significantly boosts the maximum power efficiency to 78.5% while canceling out even-harmonic distortion.

Key Differences: Class A vs. Class B Push-Pull Amplifiers

Parameter	Class A Transformer-Coupled	Class B Push-Pull
Operating Cycle	360° (Constantly ON)	180° per transistor (Alternating)
Maximum Efficiency	50%	78.54%
No-Signal Power Waste	High (Maximum heat dissipation at 0 input)	Zero (Draws 0 A when there is no signal)
Distortion Components	Very Low (but contains all harmonics if overdriven)	Even harmonics are cancelled out completely (leaves mainly 3rd harmonic)
Power Supply Draw	Constant average current from the supply	Current draw fluctuates with the signal size

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1. The “Push-Pull” Concept

Core Concepts: A single Class B transistor cannot provide a faithful reproduction of the input signal. The push-pull circuit acts as a team of two complementary halves:

• Why is it called “Push-Pull”? During the positive half-cycle of the input signal, the first transistor is driven into conduction and “pushes” current into the load. During the negative half-cycle, the first transistor turns off and the second transistor turns on, “pulling” current from the load.
• Harmonic Cancellation: The symmetrical nature of the push-pull operation has a massive mathematical advantage: it balances out and completely eliminates all even harmonics in the output, leaving only the odd harmonics (like the third harmonic) as the principal source of distortion.

Exact PYQs to Master:

• Why is the push-pull power amplifier called so? (Asked heavily in: 2020, 2019, 2018, 2017, 2015)

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2. Power & Dissipation Formulas for Class B

Core Formulas: In a Class B push-pull amplifier, the current drawn from the supply is a rectified signal.

• DC Input Power: The total power drawn from the supply uses the average current ($I_{dc}=\frac{2}{\pi }{I}_{(p)}$). $P_{i(dc)}=V_{CC}\times I_{dc}=V_{CC}\left[\frac{2}{\pi }{I}_{(p)}\right]$
• AC Output Power: The power delivered to the load ($R_L$) using peak voltage ($V_{L(p)}$) is: $P_{o(ac)}=\frac{V_{L(p)}^2}{2R_L}$
• Power Dissipated by Transistors: The power wasted as heat by both transistors combined is the difference between input and output power: $P_{2Q}=P_{i(dc)}-P_{o(ac)}$ (Note: To find the dissipation of a single transistor, just divide $P_{2Q}$ by 2).

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3. The 78.5% Maximum Efficiency Proof (Must-Master)

The Core Derivation: This is the single most heavily tested mathematical proof in your syllabus. You must memorize this step-by-step deduction.

1. Define Efficiency: $\eta =\frac{P_{o(ac)}}{P_{i(dc)}}\times 100\%$
2. Substitute the Base Formulas: $\eta =\frac{V_{L(p)}^2/2R_L}{V_{CC}[(2/\pi )(V_{L(p)}/R_L)]}\times 100\%$ Simplifying this gives the general efficiency equation: $\eta =\frac{\pi }{4}\frac{V_{L(p)}}{V_{CC}}\times 100\%$
3. Apply the Maximum Condition: Maximum efficiency occurs when the peak output voltage swing reaches the supply voltage limit. Set $V_{L(p)}=V_{CC}$.
4. Final Calculation: Substitute $V_{CC}$ into the equation: $maximum\eta =\frac{\pi }{4}\frac{V_{CC}}{V_{CC}}\times 100$ $maximum\eta =\frac{\pi }{4}\times 100$

Exact PYQs to Master:

• Show that the maximum efficiency of push pull power amplifier is 78.5%. (Asked in: 2019)
• Deduce the expression for maximum efficiency of push-pull power amplifier. (Asked in: 2018, 2017, 2015)

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4. The Class B Design Numerical (PYQ Blueprint)

Core Process: Class B numericals are direct “plug-and-chug” questions. They usually ask for the maximum power conditions. When the prompt asks for “maximum input/output power,” you must instantly assume the maximum voltage swing ($V_{L(p)}=V_{CC}$) and use these specific limit formulas:

• Maximum AC Output Power: $maximum{P}_{o(ac)}=\frac{V_{CC}^2}{2R_L}$
• Maximum DC Input Power: $maximum{P}_{i(dc)}=\frac{2V_{CC}^2}{\pi R_L}$

Exact PYQ to Master (The 2021 Clone):

• For a class B amplifier using a supply of $V_{CC}=30V$ and driving a load of $R_L=16\Omega$, determine the maximum input power and output power. (Asked in: 2021) (Solution check based on Boylestad Example 12.8: Max Output Power = $(30^2)/(2\times 16)=28.125\text{ W}$. Max Input Power = $2(30^2)/(\pi \times 16)=35.81\text{ W}$).