Friday, November 14, 2014
AM RECIEVER CIRCUIT
It can use general purpose transistors, and in this example there are 3 BC109C transistors. The schematic and BOM show a 200µH inductor and a trimmer capacitor 150-500pF, though these parts can be salvaged from an old AM radio, to preserve the directional nature of a tuning coil, and an adjustment knob (plate capacitor) that work well for radio reception.
The 120k resistor is for regenerative feedback between the Q2 NPN transistor and the input to the tank circuit. The value of this resistor is important to the performance of the entire AM receiver. In fact, it may be better to replace the fixed value with a variable resistor paired with a fixed resistor to adjust the oscillation and sensitivity. All the connections should be short to minimize interference.
Performance will vary depending on stray capacitance in your layout, the inductor winding/core/length, etc. Changing values of some of the capacitors, or adding them, as well as a potentiometer in the feedback loop can help with the performance of the receiver. With such a small circuit that is affected so much by its construction and its environment, a lot of hand tuning and experimentation will be fun, instructive, and possibly necessary to make it work best.
Circuit Diagram
Parts List
Symbol Name Value
C1 VARIABLE 500pF
C2 NON POLARIZED 0.1uF
L1 AIR CORE 200uH
Q1 NPN
Q2 NPN
Q3 NPN
R2 RESISTOR 1K
R3 RESISTOR 330K
C3 NON POLARIZED 0.1uF
R1 RESISTOR
R4 RESISTOR 560K
R5 RESISTOR 10K
C4 NON POLARIZED 0.1uF
Application
A small, simple AM receiver project with only 3 transistors. This circuit can pick up medium wave stations in your area.
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