Wireless Transmitter
FM Audio Wireless Transmitter Circuit
Here’s is a simple audio wireless transmitter for rebroadcast audio of fm radio or any others audio source. The wireles transmitter circuit consists of a frequency modulated oscillator, an audio preamplifier with pre emphasis to supply the frequency modulating signal, and a buffer amplifier to drive the antenna connector.
Wireless transmitter is easy to build and use components that are widely available in the market. You just need to tune the working frequency after power supply is installed in accordance with the desired frequency on the VHF band II.
Rebroadcast Audio Wireless Transmitter
Audio Wireless Transmitter Circuit Diagram
Oscillator’s frequency of the FM wireless transmitter is determined by L1 resonating with the 10 pf capacitor and the total capacitance across it. The collector-base capacitance of the transistors Q3, Q4, and Q5 is a function of their revers bias. This is basically a poor man’s (or lazy man’s) varactor. The voltage across Q3 is set by a voltage divider and is then modulated by an AC coupled audio signal from the pre amplifier, causing the reverse bias to vary with the audio signal, which changes the resonant frequency of L1′s circuit, causing the frequency of the oscillator to vary with the audio signal.
Audio Wireless Transmitter Schematic
The capacitance of Q4 and Q5 is adjusted by DC bias from the tuning adjustment potentiometer, and this capacitance sets the center frequency of the oscillator. All of the transistors in the oscillator -Q1 through Q5, are 2N4401.
The purpose of the buffer amplifier is to minimize frequency shift as loading on the antenna is changed. It was specifically designed to reduce the signal amplitude to the antenna. Wireles transmitter should not use any more power than is necessary to achieve the task at hand, and lightly coupling the RF into the buffer’s base with a gimmick capacitor did the trick. The transistor is an MPSH34.
The audio pre amplifier of the FM wireless transmitter that drives the frequency modulation state is a single inverting transistor. The open loop gain of Q7 is about 150 (mostly set by the voltage across the collector resistor) and the closed loop gain of the stage is about 20 (set by the ratio of the 100k feedback resistor to the 4.7k input resistor). A pre emphasis network is flat from about 3 Hz to 350 Hz, then the response increases at 6 db/octave until it levels off around 3 kHz. Visit FM Audio Wireless Transmitter
See also: Wireless Video Circuit
