Aprendiendo desde cero sobre amplificadores clase AB | Teoría, Diseño, Práctica

How to Build a Simple Class AB Amplifier

Introduction to the Project

• The speaker introduces a simpler and cheaper alternative to a previously shown 100W class A amplifier, which used expensive MOSFETs.

• The goal is to create an amplifier that performs well without requiring large power supplies.

Amplification Stages Explained

• An operational amplifier (op-amp) in non-inverting configuration is used for voltage amplification but cannot deliver sufficient current for an 8-ohm speaker.

• To amplify current, a common collector amplifier (emitter follower) is introduced, which has a voltage gain of approximately one but increases current output.

Push-Pull Configuration

• A second emitter follower using PNP transistors is added symmetrically, creating a push-pull configuration that formalizes the class B amplifier design.

• This setup allows both NPN and PNP transistors to work together: NPN provides current into the speaker while PNP takes it out.

Addressing Distortion Issues

• The output voltage from the op-amp minus the base-emitter junction voltage leads to distortion at low signal levels due to insufficient activation of transistors.

• This distortion occurs around zero crossing points where neither transistor conducts properly, leading to signal clipping.

Transitioning to Class AB Amplifier

• To mitigate zero crossing distortion, two resistors biasing diodes are introduced, ensuring adequate base-emitter voltage for both transistors.

• This modification transitions the design from class B to class AB by maintaining some bias in the transistors even when no input signal is present.

Enhancements and Feedback Mechanism

• With zero volts at output during idle conditions, coupling capacitors can be eliminated. Operational feedback helps adjust gain and manage DC levels effectively.

• By applying feedback directly from the output, nonlinearities caused by diode drops are compensated for improved performance.

Current Source Implementation

• When analyzing circuit behavior under load conditions with maximum output signals, it becomes evident that base currents may become insufficient.

Amplifier Design and Thermal Management

Current Stability in Amplifiers

• The output signal's location ensures a consistent current for both the NPN base and the diode, allowing for greater stability despite variations from the rectifier bridge's ripple.

Addressing Thermal Avalanche

• A common issue with amplifiers is thermal avalanche, where increased current through diodes leads to higher emitter currents (quiescent current), typically around tens of milliamps.

• As temperature rises, the voltage-current graph of a diode shifts left, causing fixed junction voltages to allow more current flow, which further increases temperature and can damage transistors.

Mitigation Strategies

• To combat thermal avalanche, resistors are added between BJT emitters. Increased current results in higher voltage drops across these resistors, reducing base-emitter junction voltage but not eliminating the problem entirely.

• Finding a balance in resistor values (typically between 0 and 1/5 ohm) is crucial; too much resistance reduces output signal strength while dissipating power as heat.

Quiescent Current Control

• Adding emitter resistors necessitates additional diodes to compensate for voltage loss; however, this solution is flawed due to unequal voltage drops across junctions.

• A base-emitter voltage multiplier stage can be employed to control quiescent current effectively by adjusting resistor values like a voltage divider, ensuring approximately 0.7V drop consistently.

Final Adjustments and Circuit Design

• Using a trimmer resistor allows fine-tuning of quiescent current control in push-pull configurations. This adjustment process is referred to as "vias adjustment," meaning polarization in amplifier context.

• Connecting operational outputs requires careful consideration of symmetry; adjustments may lead to clipping occurring unevenly across half cycles depending on connection points.

Project Overview and Components

• The final amplifier diagram includes enhancements such as low-pass filters with specific cutoff frequencies and protective capacitors that limit input signals exceeding source voltages.

• The design integrates variable gain capabilities (2 to 69), high-pass filters at 10 Hz, and low-pass filters at 23 kHz while maintaining operational amplifier performance close to these frequencies.

• Additional components include heatsinks for transistors and protection diodes; although optional for functionality, they enhance reliability under load conditions.

• Designed for an 8-ohm speaker powered by ±17V sources from a center-tap transformer with decoupling capacitors aiding peak current delivery during operation.

How to Manufacture a Class AB Amplifier

Preparing the Gerber File for Manufacturing

• The silkscreen on the lower layer indicates how to connect components in the project. The Gerber file is located in the project folder, named "Gerber class AB amplifier deck" and is in zip format.

• To send it for manufacturing, visit jlcpcb.com, click on "Instant quote," and drag the Gerber file onto the page. Select your desired quantity (5 or 10 plates recommended) and color (e.g., red).

Order Placement Process

• After selecting options, click "save to Card." If you have an account, log in; if not, create one easily using Google login. This will redirect you to enter shipping and billing information.

• The assembled board can be viewed live during a broadcast available in the project's playlist linked in the video description. Minor changes were made to component values and positions but retain core functionality.

Assembly Considerations

• Ensure that the TL071 operational amplifier is not yet mounted on the board for calibration purposes. Transistors of the Push-Pull stage should be mounted on a heatsink; a larger heatsink is recommended if no fan is used.

• Use thermal insulators for transistors while power supply transistors do not require plastic separators due to their housing material. Adjust screws fully left for minimum gain and quiescent current before connecting power sources.

Testing Procedures

• A 12V transformer with a central tap at 1 amp will be used for testing despite needing 2 amps for maximum power output; this setup suffices for initial tests with connected rectifier bridges from previous projects.

• Measure voltage across emitter resistors of Push-Pull stage using a multimeter; aim for 10 mV which corresponds to approximately 33 mA of quiescent current after circuit stabilization (about 10–15 seconds). Adjustments depend on temperature conditions during testing.

Final Adjustments and Performance Evaluation

• Connect TL071 once initial adjustments are complete; use a function generator set at 1 kHz sine signal as input while monitoring output via oscilloscope to observe clipping behavior near source limits due to voltage drop issues with rectifiers affecting performance under load conditions.