In this circuit, there are two inputs to the FM signal. One is from the microphone, another is from the headphone jack. Does the the microphone input interfere with the headphone jack input? Can anyone explain this FM circuit further?
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2 Answers
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I wonder if the mic input interferes with the headphone jack input?
I've added a few words (in red) to the circuit image found in your link: -
Can anyone explain this FM circuit further?
The text in the link says this: -
The transmitter consists of three stages. The first stage is an audio pre-amplifier built around 2SC9014 transistor, second stage is an oscillator built using 2SC9018 transistor, and third stage is an RF amplifier built around 2SC9018 transistor. RF amplifier stage increases an output power, range and stability by separating the antenna from the oscillator.
And, I will add that the oscillator (formed around Q2) is a common-collector Colpitts oscillator that has \$C_{BC}\$ miller capacitance alteration due to the audio input voltage at C3. The alteration of the miller capacitance frequency modulates the output.
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The microphone signal cannot interfere with the headphone input. The headphone jack automatically switches between the two sources.
The schematic symbol for the headphone jack shows that then there's no headphone plugged in, the microphone is connected to the FM modulator. When the headphone in plugged in, the microphone is disconnected from the modulator.
