Encyclopedia of radio electronics and electrical engineering / radio reception
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This two-transistor circuit is called a "mini-radio". It uses only two transistors and few passive components, making this circuit easy to build. And although this circuit is very simple, nevertheless it works quite well even without an external antenna and ground. Transistor T1 works as a high-frequency amplifier with adjustable feedback, while simultaneously acting as a detector. The sensitivity of the receiver depends on the position of the potentiometer R1, which controls the amount of feedback.
The detected signal comes from the collector of transistor T1 to the base of a low-frequency amplifier made on transistor T2. To prevent the residual detection products from entering the input of this amplifier, capacitor C3 is used, so that only a low-frequency signal is fed to the base of transistor T2. The amplified low-frequency signal is fed to the headphones included in the collector circuit of the transistor T2 through the separating capacitor C5, which prevents the passage of the constant component.
The magnetic antenna coil is wound on a ferrite rod with an initial magnetic permeability of 2000, with a diameter of 10 mm and a length of 10 cm, it contains two windings of 5 and 60 turns, connected in series, wound with copper enameled wire PEV-0,2 or similar. The conductors leading to this coil should be as short as possible.
The circuit can use transistors of the 2SC3112, 2SC2675, KT3102V, KT3102E series. Headphones must have an impedance of 32 ohms or more, if loud-speaking reception is required, then you can use some kind of low-frequency amplifier, made for example, on operational amplifiers.
If you connect an external antenna to this receiver, then its sensitivity will increase significantly. To do this, a capacitor with a capacitance of 4,7 pF is connected to the upper output according to the circuit.
The radio receiver is powered by a 9 V battery, while the current consumption is 1 mA.
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