The radio receiver's final stage, commonly known as the audio stage, is a critical, low-power circuit responsible for converting the detected electrical signal into an audible output. This stage must accomplish two primary, and often conflicting, objectives with efficiency: signal amplification (if necessary) and effective power transfer to the transducer (the speaker or earpiece). The successful operation of this stage hinges largely on the principles of simple power transfer and, most importantly, impedance matching.

The Imperative of Impedance Matching

A detected audio signal, even after initial low-level amplification, typically exists as a high-impedance source. To achieve maximum power transfer to a load, the load impedance should ideally equal the source impedance, a principle known as the Maximum Power Transfer Theorem. In the context of a low-power audio stage, the load can be one of two common devices:

1. High-Impedance Earpiece (Headphone): Vintage crystal and early transistor radios often utilized high-impedance magnetic earpieces (typically 1–2 kΩ or more) to maximize the meager power available. This simplifies the final stage because the earpiece's high impedance is often a closer match to the output impedance of a simple transistor (like a common-emitter stage) or a transformer coupled stage, which is necessary to deliver the limited signal power effectively.
2. Initial Amplifier Stage: For driving a modern, low-impedance loudspeaker (typically 4–8 Ω), the low-power stage must act as a buffer or pre-amplifier. The output from the detector or the initial audio stage is still a high-impedance signal, and attempting to directly drive a low-impedance load would result in severe power loss and signal degradation, primarily due to the vast impedance mismatch.

Low-Power Stage Implementation and Considerations

For a stable and efficient transition from the high-impedance signal to a working load, the circuit designer considers several topologies:

• Transformer Coupling: This classic technique uses an audio transformer to step down the high output impedance of the drive circuit (e.g., 10 kΩ) to match a lower-impedance load (e.g., 1 kΩ earpiece or the input of a push-pull stage). This method is highly effective for impedance transformation and DC isolation, although the size and non-linearity of the transformer introduce cost and performance trade-offs.
• Emitter Follower (Common Collector): the emitter follower configuration excels as an impedance buffer. It offers high input impedance and very low output impedance, making it ideal for driving subsequent amplifier stages. While it provides no voltage gain, its current gain and impedance transformation capabilities make it a strong choice for coupling a high impedance detector/pre amp to a low impedance base of a power transistor stage.
• Direct-Coupled Transistor Stages: Simple common emitter or common source stages, while offering voltage gain, often have a relatively high output impedance. Their use is best suited for driving other high impedance stages or the high impedance earpieces where the impedance match is close enough for acceptable results.

In all designs, the goal is not merely to transmit the signal, but to transfer power efficiently. The low-power stage, therefore, is an exercise in fundamental electronic design, balancing transistor selection, biasing, and the often-necessary impedance transformation to ensure the faint whisper of the detected radio signal is robustly converted into an audible acoustic wave.