Micro Power AM Receiver Circuit Diagram 9V

Circuit Diagram

This circuit represents a simple crystal radio incorporating an audio amplifier, demonstrating notable sensitivity and the ability to pick up numerous strong broadcast stations within the Los Angeles region. A minimal antenna length of 15 feet is sufficient for operation, though extending the antenna will boost the signal strength but can negatively impact selectivity, potentially leading to overlapping audio from adjacent stations. Connecting the wire antenna to one of the coil's taps, instead of the junction of the capacitor and coil, can enhance selectivity. Grounding is essential, and a practical solution involves positioning the antenna outside on a concrete surface, allowing the extended headphone leads to rest upon it. This configuration successfully enabled listening to the local news station, KNX 1070. The inductor was fabricated by winding 200 turns of #28 enameled copper wire upon a 7/8 diameter, 4-inch-long PVC pipe, resulting in an approximate inductance of 220 uH. The inductor featured taps spaced every 20 turns, affording flexibility for selecting diode and antenna connections for optimal performance. The diode, ideally a germanium (1N34A type), provides the best results, although silicon diodes can also be used if the signal is robust. The carrier frequency is eliminated from the rectified signal at the diode's cathode via the 300 pF capacitor, while the audio frequency is permitted through the 0.1 uF capacitor to the non-inverting input of the first operational amplifier, functioning as a high-impedance buffering stage. The second operational amplifier stage amplifies the voltage approximately 50 times, DC-coupled to the first via the 10K resistor. Imbalances in the values of the 100K and 1 Meg resistors may necessitate employing closely matched resistor values or introducing a capacitor in series with the 10K resistor to maintain the DC voltage at the transistor emitter within a range of 3 to 6 volts. Alternatively, reducing the overall gain can be achieved by utilizing a smaller feedback resistor (470K). High-impedance headphones are recommended for best results, and walkman stereo headphones can also be employed. The circuit draws approximately 10 mA from a 9-volt power supply. Germanium diodes (1N34A type) are readily available from Radio Shack, #276-1123.

Operational Amplifiers

Operational amplifiers, or op-amps, are integrated circuits designed to amplify linear electrical signals. In this circuit, two op-amps are used. The first op-amp, acting as a high-impedance buffer stage, takes the signal from the diode and provides a stable input to the second stage. This stage then boosts the signal's voltage level, approximately 50 times. Op-amps are characterized by their high input impedance, which means they draw very little current, and their high gain, which allows them to amplify signals significantly. These characteristics are crucial for maintaining signal integrity and achieving the desired amplification level. The precise values of the resistors involved – 10K, 100K, 1 Meg, and 470K – are carefully chosen to shape the amplification curve and ensure optimal performance. They enable precise control over the gain and the overall behavior of the amplifier stage. The circuit utilizes common op-amp models like the LM358, which is a versatile and widely used op-amp suitable for a variety of applications. Further details regarding op-amp selection and characteristics can be found in datasheets provided by manufacturers such as Texas Instruments or Analog Devices. These components are frequently employed in audio amplifiers, instrumentation, and control systems due to their flexibility and wide range of applications. The circuit relies on the ability of the op-amps to deliver high gain and provide a stable reference point for the amplified audio signal. The selection of the op-amps is important for the overall performance of the circuit, and their specifications should be considered when designing and implementing this type of amplifier. The integration of these components enables a highly adaptable and controllable amplification process, resulting in a versatile audio amplifier design.