This article presents a simple AM radio circuit built using the CMOS digital IC CD4011, which is known for its high sensitivity to radio signals. The CD4011 contains four independent NAND gates, making it a versatile component for various digital and analog applications. While NAND gates are typically used as switching elements—producing either a high or low output—they can also be operated in an amplification mode under certain conditions.
The key principle behind this circuit lies in the behavior of the NAND gate during signal transitions. Normally, the output switches between high and low levels, but there's a transitional region where the output behaves more linearly. By carefully designing the feedback network, each NAND gate can be biased into this linear region, allowing it to function as a voltage amplifier rather than just a logic gate. This unique property enables the CD4011 to be used in a simple yet effective radio receiver.
The circuit diagram (shown above) illustrates the basic layout of the AM radio. The receiving antenna is connected to a resonant circuit composed of inductor L1 and capacitor C1, which tunes the circuit to the desired frequency. Resistor R1 provides DC negative feedback to stabilize the operation, while capacitor C2 acts as an AC bypass, ensuring that only the RF signal is processed.
The incoming signal is first amplified by gate A of the CD4011. It then passes through a double-voltage detection circuit made up of diodes D1 and D2, resistor R2, and capacitor C4. This stage demodulates the AM signal, extracting the audio information. The resulting audio signal is further amplified by gates B, C, and D before being sent to the headphone output.
For optimal performance, C1 should be a variable capacitor, and L1 can be constructed by winding 80 to 100 turns of wire around a 50 mm bar magnet. It’s important to use a high-impedance earphone for better sound quality. For improved volume and clarity, you can add an audio amplifier like the LM386 or use a triode for impedance matching. Adjusting the number of turns on the coil allows you to fine-tune the receiving frequency within the AM band.
This project is a great way to explore the versatility of CMOS logic gates beyond their traditional digital applications and understand how they can be creatively used in analog circuits. It’s both educational and fun for electronics enthusiasts looking to build a simple radio from scratch.
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