AM Transmitter using Transistor with Electret Microphone

Amplitude modulation is the oldest broadcast technique still on the air, and it remains the clearest way to teach what modulation actually means: a high-frequency carrier whose amplitude is shaped, moment by moment, by the audio signal. This project builds a complete AM transmitter from discrete parts. A BC547 transistor wired as an LC oscillator sustains a carrier in the medium-wave band (design target around 1 MHz, set by a hand-wound coil and tuning capacitor); an electret microphone feeding a simple preamp captures voice; the audio is applied to modulate the transistor's operating point so the carrier amplitude follows the sound; and a whip antenna radiates the result. The demonstration needs no special receiver — any household MW radio tuned to the carrier frequency demodulates it. The student learns the modulation process end to end: carrier generation, resonance, modulation depth, and why the received audio distorts when the modulation is pushed too far. It is deliberately a low-power, room-scale lab demonstration, not a broadcast station.

1. The student winds the tank coil to the specified turns, connects it with the tuning capacitor, and verifies the oscillator starts — a carrier the radio can find.
2. The MW radio is tuned across the band until the quiet carrier is located (heard as a silent spot or whistle), marking the transmit frequency.
3. The electret microphone stage is powered and its audio output confirmed; speaking into the mic produces the modulating signal.
4. The audio is coupled to the oscillator transistor's bias network; voice peaks drive the operating point up and down, so the carrier amplitude follows the audio envelope — amplitude modulation.
5. The whip antenna is attached and the student walks the radio a few meters away, hearing their own voice reproduced by the radio's detector.
6. The modulation preset is adjusted while listening: too low and the audio is faint, too high and it distorts — demonstrating modulation depth and over-modulation.
7. Touching the tank area shows hand-capacitance frequency pulling, and the student re-tunes the radio — a memorable lesson in oscillator stability.

Tech stack:

• AM modulation theory
• LC oscillator design
• Transistor biasing (BC547)
• Electret microphone interfacing
• Tank circuit resonance
• Antenna basics
• Receiver demodulation
• RF prototyping practices

• [BC547 LC oscillator] A single-transistor oscillator with a hand-wound tank coil generates the medium-wave carrier — the classic discrete RF stage.
• [Hand-wound tank circuit] The coil is wound by hand on a former with a tuning capacitor, so resonance is built and understood, not bought as a module.
• [Electret microphone stage] An electret mic capsule with a preamp provides the audio input, biased and coupled to the modulator.
• [Amplitude modulation] The audio varies the transistor's operating point so the carrier envelope follows the voice waveform — modulation made visible on a CRO if available.
• [Whip antenna] A short whip antenna radiates the modulated carrier for room-scale reception experiments.
• [MW-band carrier] The carrier sits in the medium-wave band (design target ~1 MHz), so any ordinary radio receives it with no special equipment.
• [Battery powered] A 9 V battery supply keeps the transmitter portable and isolates it from mains for bench experiments.

• Assembled transistor AM transmitter with tank coil and mic stage
• Hand-wound coil on former with tuning capacitor
• Electret microphone capsule and preamp
• Whip antenna
• Tuning and demonstration procedure notes
• Project report PDF (background, AM theory, circuit design, demo observations)
• PPT presentation for final review
• Viva Q&A preparation document (modulation, resonance, modulation depth, antenna)

• Range is room-scale by design — a few meters indoors; this is a lab demonstration, not a broadcast station.
• The carrier frequency drifts with temperature and hand capacitance; the receiver needs re-tuning during demos.
• Modulation depth is set by a preset — over-driving it distorts the audio, which is itself part of the lesson.
• The medium-wave band is shared spectrum; transmissions are kept brief and low-power, strictly as a lab demo.
• The whip is a short antenna, not a tuned MW antenna, which is one reason range stays short.
• Performance is verified by ear on a standard radio — no spectrum-analyzer or field-strength claims are made.