Variable capacitor C5 and coil L1 are the tuned circuit. R5 generates some negative feedback, which improves the audio quality. This has the effect of greatly increasing the sensitivity and selectivity of the receiver. The idea is to adjust potentiometer R6 to a point where oscillation is about to start and back off a bit. This will have the effect of starting to oscillate. Some of the demodulated signals are fed back as positive feedback through R4 into the tap of L1 via the regeneration control R6. Capacitor C3 removes any remaining RF components. Transistors Q1 and Q2 can be any NPN transistor. Rectification of the AM signal takes place inside Q2, and R3. A popular choice was the regenerative receiver shown below. Transistor Amplified AM Radioīefore the advent of easy-to-use amplifier IC’s such as the LM386, receivers were made from designs using discreet components. By changing the capacitance of C2 (the resonant point), the tuning can be varied across the medium wave band. The diode D1 rectifies and recovers the modulation and the capacitor C3 bypasses the radio (RF) part-leaving the original modulated audio. (C1 is a small capacitor as well as the tap to prevent the antenna from damping the Q of the tuned circuit). If the selectivity is low, you would hear other neighboring stations at the same time. This increase in selectivity enables the circuit to tune into the station you want. The greater the Q, the more selective the circuit becomes. The ratio of this dynamic impedance to any loss resistance present is called Q. If they are in parallel (as in our circuit), the combined impedance is very high, and if they are in series, resonance also occurs but the combined impedance is very low. When the reactance (AC resistance) of capacitor C2 is the same as the reactance of the coil L1, resonance occurs at the frequency f=1/2π√(LC).įor example, with L1 equal to 300 uH and C2 equal to 100 pF:į = 1/2π√((0.0003 H)*(0.000000000001 F)) = 919kHz. What’s truly amazing here is that this circuit will work without any batteries and provide hours of AM listening fun. Use magnet wire to wind the coil if possible.įor this radio work well, you will need a good earth connection and at least 20m of wire as high as possible outdoors as an antenna. The tap is at about 5 to 10 turns to be coupled to the antenna. L1 is a coil wound on a ferrite rod with about 50 to 60 turns. The LS is a high-impedance piezoelectric headphone like the vintage 2000Ω “crystal earpiece”.Ĭapacitor C2 is a variable capacitor of about 300pF to 500pF. But any diode will work, only with less volume. The diode is ideally a germanium diode, like the OA81, as it has a lower forward volt drop. Building this is an excellent start in understanding how AM radios work: Shown below is a simple (and magical if you have ever built one) crystal radio circuit. AM is also used on all aircraft radios from 108 to 136MHz. This means that the transmitter radio (or carrier) signal is modulated with the music or speech content in a manner that the amplitude of the carrier is varied in relation to the incoming speech or music. Radio stations in the medium wave and short wave bands transmit their signals using Amplitude Modulation (AM). AM receivers are also simple and easy to build. But they are still popular as they have more channel space than FM stations and offer longer range coverage, especially at night via the ionosphere. Medium wave and short wave were the main broadcast radio bands until the advent of FM. On the other hand, short wave extends beyond that and up to about 30MHz. Medium wave is a band of radio frequencies extending from 530kHz to 1700kHz.
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