This circuit is an op-amp-driven Class AB audio power amplifier with a single-supply configuration. The amplifier block itself is configured for a voltage gain of 100 (40dB)
1. Circuit Component Identification
The design is divided into four main functional sections:
Virtual Ground / DC Biasing (Left): Two 1k resistors form a voltage divider that splits the single +5V supply down to a stable +2.5V virtual ground reference.
AC Coupling (Input & Output): The 100uF capacitors blocks DC bias voltages, allowing only the pure alternating audio signal to enter the amplifier and exit to the load.
Voltage Gain Stage (Center): An operational amplifier set up in a non-inverting amplifier topology.
Current Output Stage (Right): A push-pull transistor pair (NPN top, PNP bottom) configured as common-collector (emitter-followers). The two series diodes provide a small forward bias voltage between the transistor bases to prevent crossover distortion
2. Principles of Operation
1. DC Biasing: Because the circuit runs on a single rail instead of a dual supply, the op-amp's non-inverting input is held at a mid-rail baseline. This allows the incoming AC wave to swing freely above and below the center point without clipping against the ground wire.
2. Voltage Amplification: The op-amp samples the low-power audio source via the non-inverting pin and multiplies the voltage according to its negative feedback loop resistors.
3. Current Amplification: Standard op-amps cannot drive heavy, low-impedance loads (like an speaker) because they lack current output capacity. The output push-pull stage acts as a current buffer. The top NPN transistor handles the positive half of the waveform cycle, while the bottom PNP transistor handles the negative half.
4. Negative Feedback Integration: Notice that the global feedback path (the resistor) is taken after the transistor output stage, rather than directly from the opamp output pin. This helps correct non-linearities, thermal drift, and any residual crossover distortion introduced by the transistors.
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