Project: Custom-made Power Supply and Amplifier

Objective

As a member of a 4-person group in an electronics course, I contributed to the end-to-end design, simulation, and construction of a custom audio amplifier and its power supply from discrete components (BJTs, MOSFETs, resistors, capacitors). The project required meeting strict specifications for gain, output power, load current, and thermal performance.

Technical Approach & My Contributions

Our team divided responsibilities, with my primary focus on the power supply design and simulation and significant contributions to the amplifier's frequency response analysis.

Power Supply Design & Selection

The requirement was to convert 120V AC wall power to a stable DC voltage between 5V and 12V with high efficiency and low regulation (minimal voltage drop under load).

My Role: I took the lead in designing and simulating two distinct power supply architectures in LTSpice to determine the optimal solution.

• Design 1: Zener-MOSFET Regulator: This design used a Zener diode to set a reference voltage and a MOSFET as a pass transistor. While simple, simulation revealed its output voltage varied significantly with changing load (poor load regulation) and was sensitive to temperature fluctuations.

• Design 2: LM317 Linear Regulator: This design leveraged the integrated LM317 voltage regulator IC. My LTSpice simulations demonstrated its superior line and load regulation, providing a much more stable 10V DC output under all required conditions.

Outcome: Based on the quantitative data from my simulations, the team selected the LM317-based design. I then led the soldering of this supply onto a perfboard, ensuring a robust and reliable power source for the amplifier stage.

Two-Stage Amplifier Design

The amplifier needed to provide high voltage gain while being able to drive a low-impedance load like a speaker.

• Architecture: We designed a two-stage amplifier:

• Common-Emitter (CE) Stage: This stage provides the primary voltage gain.

• Common-Collector (CC) Stage: This stage provides current gain and a low output impedance, enabling it to drive the speaker without loading down the CE stage.

My Contribution: I assisted in calculating the DC biasing conditions for the BJTs to ensure they operated in the active region. Furthermore, I was responsible for simulating the amplifier's frequency response in LTSpice. I verified that the gain was flat across the audio spectrum and identified the -3dB cutoff points to ensure it met the bandwidth requirements.

Prototyping, Testing, and Integration

Implementation: We first constructed the amplifier circuit on a breadboard for easy debugging. Using a function generator and an oscilloscope, we validated the circuit's functionality independently of our custom power supply.

Testing: A critical test I performed was determining the maximum unclipped output voltage. By increasing the input signal, I identified the point where the output waveform began to clip, establishing the amplifier's maximum usable output swing at 500 mV peak-to-peak.

Integration: The final step was connecting the breadboarded amplifier to our custom soldered power supply. We conducted comprehensive tests to ensure the entire system met all project specifications under load.

Results 

The project was a complete success, with the final product meeting or exceeding all course requirements.

• Quantitative Results: The amplifier achieved the target voltage gain and delivered the required current to the load. The power supply maintained a stable 10V DC output with excellent regulation, as predicted by the initial simulations.

• Validation of Design Process: The close correlation between our LTSpice simulation results and the measured performance on the physical breadboard underscored the value of simulation-driven design.

Technical Lessons Learned:

• Trade-off Analysis: I gained practical experience in evaluating design trade-offs, as demonstrated by the power supply selection process where we prioritized performance over circuit complexity.

• Theoretical to Practical: The project solidified my understanding of amplifier biasing and the real-world implications of concepts like output impedance and clipping.

• Team Collaboration: Working effectively in a team to delegate tasks, integrate subsystems, and troubleshoot complex circuits was an invaluable experience.

Conclusion: This project served as a comprehensive application of analog electronics principles. It honed my skills in circuit simulation (LTSpice), practical prototyping (breadboarding, soldering), and systematic validation using lab equipment. Successfully navigating the challenges from initial design to a fully functional integrated system demonstrated my strong foundation in electronic design and implementation.