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General Description of the Amplifier
The circuit corresponds to a class AB audio power amplifier, fully discrete, designed with emphasis on high open loop gain, low noise, and good power supply immunity. It uses an input differential pair with dynamic loads implemented by current mirrors, a voltage amplification stage with active load, and drivers powered from actively regulated intermediate supply nodes, decoupled from the main rails. The output stage is a complementary emitter follower with global feedback.
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Input Stage: Differential Pair with Current Mirror Load
The input signal is applied to a BJT differential pair biased by a constant current source.
The collectors of the differential pair are connected to current mirrors, which act as dynamic loads.
This configuration provides:
• Differential to single ended conversion
• High gain from the first stage
• Excellent common mode rejection ratio (CMRR)
• Improved linearity compared to resistive loads
Global feedback is applied to one transistor of the differential pair, closing the overall control loop.
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Differential Pair Current Source
The differential pair is supplied by an active current source, referenced by diodes.
Its purpose is to set a stable operating current for the differential stage, reducing sensitivity to power supply and temperature variations, and improving both PSRR and thermal stability.
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Voltage Amplification Stage with Active Load
The signal amplified by the differential pair drives the voltage amplification stage (VAS).
This stage also operates against an active load, maximizing open loop gain while allowing large voltage swing with moderate current.
Stability is ensured through distributed frequency compensation, using small value capacitors that shape the open loop bandwidth without introducing excessive dominant pole limitation.
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Active Supply Nodes for the Driver Stage
The transistors surrounding the VAS and the driver stage do not function as classical cascodes.
Their role is to generate intermediate, actively regulated supply nodes that feed the driver stage.
This approach:
• Isolates the drivers from main rail fluctuations
• Reduces power supply modulation caused by output current
• Improves PSRR and linearity under dynamic conditions
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Class AB Driver Stage
The signal from the VAS drives a complementary driver stage, responsible for supplying base current to the output transistors.
Biasing is set by a thermally compensated network, ensuring stable class AB operation and minimizing crossover distortion.
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Output Stage
The output consists of a complementary emitter follower, with emitter resistors for current sharing and thermal stability.
An output network including a series inductor with resistor and a Zobel network ensures stability with reactive loudspeaker loads and long cables.
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Global Feedback
Global negative feedback is taken from the output and applied to the input differential pair.
Thanks to the high open loop gain provided by the dynamic loads and current sources, the amplifier achieves low distortion, low output impedance, and good loudspeaker control without excessive feedback.
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Advantages of the Design
• High open loop gain
• Excellent CMRR and PSRR
• Low harmonic and intermodulation distortion
• Good dynamic behavior and high slew rate
• Driver stage isolated from main power rails
• Stable operation with real loudspeaker loads
Conclusion
This amplifier is clearly oriented toward high fidelity audio, prioritizing linearity, stage isolation, and controlled dynamic behavior. In exchange, it demands careful component selection, PCB layout, and thermal design, making it better suited for demanding audio projects than for low cost or mass production implementations.
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