This is a simple two transistor AM radio using regenerative feedback to enhance the received radio frequency signal. The variable capacitor (VC1) in parallel with coil (L1) act as a tunable resonant circuit. This “tank” circuit can be tuned to select the desired AM frequency converting it from an electromagnetic wave to an oscillating electric current within the resonant circuit.
Removing the RF component
The modulated RF voltage developed across the tuned circuit is fed to first transistor (Q1) through the coupling capacitor (C1). Q1 amplifies and demodulates the signal. At the collector of Q1 the RF component is shorted to ground through capacitor C2.
Amplification of the Audio & Regeneration
leaving only the audio to be amplified by transistor Q2. At the emitter of Q1 a portion of the signal is fed back to the tap on L1. The amount of this regenerative (in-phase) feedback is adjustable with VR1. The amplified audio at the collector of Q2 is fed to the Speaker through C4. The signal at the emitter of Q2 charges C3. The voltage across C3 is fed back as an auto.matic gain control to the base of Q1
R1 = 1M
R2 = 22K
R3 = 4.7k
R4 = 1K
VR1 = 4.7K
C1 = .1 uF
C2 = 470pF
C3,C4 = 10uF/25V
VC1 = Variable Capacitor
T1,T2 = 2N2222
L1 = Tank coil
SPEAKER = 2 inches, 8 ohm
The commercial broadcast band is
from 540 to 1600 kHz. Resonant
frequency equals one divided by
two pi times the square root of LC.
