In the early days of radio, many of the common components used in receivers today were not available. The simplest of designs provided no amplification, using only the signal induced on the antenna to drive the earpiece. The detectors used in these early devices required a probe made of fine wire to be connected to a sensitive point of a quartz crystal. This component was later replaced by a diode as advances in semiconductor physics were made. The development of vacuum tubes and later transistors, provided a method to amplify the signal.

The circuit diagram of a basic crystal set is shown in figure 1.

Figure 1

The signal induced in the antenna can be modelled as a voltage source across the terminals of the variable capacitor. The capacitor and inductor together act as a bandpass filter which can be tuned to different frequencies by adjusting the variable capacitor. When the filter is tuned into a radio station a high impedance is presented by the filter and the diode becomes forward biased, the capacitor and earpiece together act as a low pass filter to provide demodulation.

Additional audio frequency (AF) amplification stages can also be added as shown in figure 2.

Figure 2