A High-Swing Bipolar Class-AB Amplifier for 8Ω Speaker Applications
This project, from the ECEN 489 Audio Engineering course at Texas A&M, presents the design and implementation of a high-swing bipolar audio amplifier with a class-AB output stage. The amplifier is capable of driving an 8Ω load with up to 4 WRMS output power and 0.035% total harmonic distortion (THD) at 1 WRMS. The design was verified both in simulation and experimentally using a Dayton Audio CE81PF-8 speaker.
Full Project Report
Theoretical Background
Audio amplifiers must deliver substantial current to low-impedance loads, such as 4–8Ω speakers, while maintaining low distortion and high efficiency. Common output topologies include class-A, class-B, and class-AB stages. A class-A amplifier conducts current through the entire waveform, providing excellent linearity but poor efficiency. Conversely, class-B amplifiers conduct for only half the waveform, doubling efficiency but introducing crossover distortion.
The class-AB configuration provides an optimal tradeoff: each output device conducts slightly more than 50% of the cycle by applying a small bias voltage to the transistor bases. This greatly reduces crossover distortion while retaining high efficiency. The amplifier also utilizes Darlington pair configurations to boost current gain while preserving input impedance and linearity, enabling better drive capability for the final power stage.
The overall system employs two cascaded amplifier stages: a low-noise preamplifier for volume control and a power output stage to drive the speaker. Negative feedback is used throughout the design to reduce distortion and stabilize the DC operating point.
Topology and Design Highlights
The amplifier consists of a preamp and a fixed-gain stage, followed by a class-AB emitter-follower output stage. The preamp uses a TI OPA134 op-amp in non-inverting configuration with a logarithmic potentiometer to allow fine volume control. The second stage uses a bipolar current-mirror operational transconductance amplifier (OTA) providing over 60 dB open-loop gain and a bandwidth above 50 kHz.
The output stage employs TIP122 (NPN) and TIP127 (PNP) Darlington power transistors in an emitter-follower configuration, delivering up to 0.5 A peak current. A bias network provides approximately ±1.4 V base bias to maintain class-AB conduction. Heat-sinking and parallel device configurations were used to mitigate thermal effects.
The current mirror bias circuit generates the supply currents required by the OTA and output stages, ensuring high output resistance and stable bias over temperature. The design operates from symmetrical ±10 V supplies, allowing a large signal swing close to 80% of the rail-to-rail limit.
Achieved Results
The amplifier was simulated in Multisim and verified experimentally. Simulations achieved excellent linearity and stability, with real-world measurements showing consistent qualitative performance despite breadboard limitations.
- Output Swing: ±8 Vpk
- Output Power: 4 WRMS (simulated), 0.8 WRMS (measured)
- Open-Loop Gain: 62.6 dB (simulated)
- Phase Margin: 104°
- Bandwidth: 51.5 kHz (simulated), 67.2 kHz (measured)
- THD @ 1 WRMS: 0.035% (simulated), 3.86% (measured)
- Quiescent Power: 4.3 W
- Input-Referred Noise: 8.52 µVRMS
Conclusion
The high-swing bipolar class-AB amplifier achieved the intended design objectives, demonstrating excellent simulated performance and successful hardware validation. Measurement deviations are attributed to breadboard parasitics, thermal drift, and coupling noise. Despite these limitations, the amplifier produced audibly clean output with low distortion at typical listening levels.
Future improvements could include implementing the circuit on a PCB or perfboard for reduced parasitics, improved heat dissipation, and more accurate characterization. The project effectively demonstrates practical analog design techniques for low-distortion audio amplification using discrete bipolar transistors.