File:Single transistor radio circuit.svg

English: A single transistor TRF receiver circuit, one of the simplest amplifying radio receivers. The radio signal from the antenna is applied to a tuned circuit (resonant circuit) consisting of inductor L and capacitor C1 in parallel. At its resonant frequency

${\displaystyle f={1 \over 2\pi }{\sqrt {1 \over L\cdot C1}}\;}$

the tuned circuit has high impedance so the radio station at that frequency is passed on to the transistor Q, while at all other frequencies it has low impedance so signals at other frequencies are routed to ground. The transistor both amplifies and rectifies the amplitude modulated (AM) radio signal. Since the base-emitter junction acts as a diode it conducts only on the positive half-cycles of the carrier wave, blocking the negative half cycles, rectifying the carrier to extract the audio modulation signal from the radio wave. The collector current is an amplified copy of the base current, consisting of pulses of current with amplitude proportional to the audio modulation. The capacitor (C2) functions as a lowpass filter, smoothing the current, removing the radio frequency pulses leaving the audio signal, which is converted into sound waves by the speaker (SP).