Radio magazine №4, 1984
This is a very sensitive envelope detector for AM radio. The circuit (see Fig.1) provides linear detection of weak signals with modulation depth of 80...85%
Fig.1. A simple sensitive envelope demodulator circuit.
VT1, VT2 - KT312V (Transition frequency (ft) = 120 MHz, forward current transfer ratio (hFE) min = 50 ).
The transistor KT312V can be replaced by almost any RF NPN transistor including 2N2222
The first stage (VT1) is a common-emitter amplifier circuit for input signals, the second stage is an emitter-follower circuit (VT2). The transistor VT2 operates at low current because of the high resistance of the resistor R5. The process of AM demodulation occurs at the lower bend of the volt-ampere curve. A high linearity of demodulation provided by the deep negative feedback in the emitter-follower circuit.