Two Transistor Radio Receivers
Here are two simple circuits of radio receivers for Middle Wave band, working with headphones.
The circuit diagram shown in figure 1 is quite unusual. Both transistor stages are connected without an inter-stage coupling capacitor. The bias voltage at the base of the transistor T1 is set by the resistor R1, the bias current goes through the coil L1, its resistance for DC is almost zero. This coil and the variable capacitor C1 are resonant tank elements of the ferrite rod antenna that receive RF signals of radio stations. The transistor T1 amplifies the RF signal.
Fig. 1.
T1, T2 - KT315A (ft = 250 MHz, h21e = 20..90), can be replaced with 2N3904;
C1 - 5..180pF; C2 - 10nF; C3 - 1nF;
C4, C5 - 10μF;
R1 - 36k; R2 - 4.7k; R3 - 1.8k;
LS1 - headphones with R ≥ 1500 Ω;
L1 - see description in the text;
B1 - 4.5 V battery.
The amplified signal voltage from the collector T1 goes to the base of the transistor T2. This stage not only amplifies the signal, but also demodulates the AM radio signals.
This receiver works with headphones, the summary resistance of LS1 must be not less than 1500 Ω.
The coil L1 is wound in one layer on a ferrite rod of 8 mm in diameter (permeability μe=400), with the length of about 70 mm. The coil L1 has 90 turns of 26...32 AWG enameled copper wire, tapped at the 10th turn.
The adjustment of the circuit is quite simple - match the value of R1 to get the sound in headphones as loud as possible.
The second circuit of the radio receiver is shown in figure 2. It works with the lower power supply of 1.5 Volts. The output of this receiver is connected to stereo headphones with the summary impedance not less than 32 Ω, so the quality of the sound is better.
Fig. 2.
T1, T2 - KT315A (ft = 250 MHz, h21e = 20..90), can be replaced with 2N3904;
D1 - KD503 (1N4148);
C1 - 5..180pF; C2, C3, C6 - 10nF;
C4 - 10μF; C5 - 1nF;
R1 - 20k; R2 - 33k;
LS1 - headphones with R ≥ 32 Ω;
L1, L2 - see description in the text;
B1 - 1.5 V cell.
The resonant tank connected to the RF amplifier (transistor T1) in the same way as it connected in the previous circuit (fig. 1) in the bias network. The RF choke L2 is connected in collector circuit of transistor T1.
The amplified RF voltage goes from the first stage through the capacitor C3 to the envelope detector with the diode D1, and the audio signal goes from the envelope detector to the audio amplifier based on the transistor T2. Note, the diode D1 is connected into bias circuit of the transistor T2, such configuration increases the sensitivity of the detector because of a DC current passing through the diode D1. This design uses the same ferrite rod antenna as the previous one. The RF choke L2 is wound on a ferrite toroidal core of 7 mm in diameter. The choke has 50 turns of 32..38 AWG enameled wire.
Lyzhin R, Radiokonstruktor 2003, November.