Discrete Transistor AM Radio Receiver

Overview

This project is a 9 V battery-powered AM radio receiver built around discrete transistor stages rather than an integrated radio IC. The page documents the physical antenna and tuning build, the LTspice model, and the signal path from RF pickup through audio output.

The completed receiver combines a tuned LC front end, two RF gain stages, transistor detection and volume control, an audio driver, and a complementary BD139/BD140 output stage. This case study presents the design as a complete engineering record: architecture, construction, simulation, measured operating points, troubleshooting method, and downloadable project files.

Physical Build

The physical front end uses a hand-wound coil on a 9.5 mm diameter, 100 mm long ferrite rod and an approximately 200 pF mechanical variable capacitor for tuning. Moving the capacitor changes the resonant point of the LC tank and determines which part of the AM band is emphasized before amplification.

Signal Chain

The receiver is divided into layered functional blocks. Each layer has a simple job: select the station, increase RF level, extract the audio envelope, control the audio level, and drive the speaker.

Circuit Overview

The schematic is treated as a set of layers instead of one large mystery circuit. Open each block below to inspect the role of that part of the receiver.

Bill of Materials

The physical receiver uses the following discrete parts. Grouped reference designators share the listed quantity and value.

LTspice Simulation Setup

The LTspice source is configured as a controlled AM test signal rather than a real antenna environment. A 1 MHz carrier is amplitude-modulated by a 1 kHz tone, making it easier to verify that the receiver passes RF energy and recovers the audio envelope.

.step param Ctune 150p 260p 10p
.tran 0 20m 10m 50n
.options reltol=0.01

Results

Bench DC checkpoints

Representative stable operating points from the completed breadboard provide a concise electrical baseline for the receiver.

The simulation shows the expected low-frequency audio tone appearing at later nodes and at the speaker load after the RF and detector stages. The variable capacitor sweep gives a way to compare response near resonance and verify that tuning has a measurable effect.

Measured simulation snapshot

Values below are taken from the 15–19.9 ms measurement window of the supplied transient run.

Debugging Process

Debugging followed the receiver from input to output, using the schematic as a map and separating RF faults from audio faults. Each stage was checked independently before the complete signal chain was evaluated.

1. Validate the tuned input: confirm coil continuity, variable-capacitor wiring, and sensitivity to ferrite-rod orientation.
2. Establish DC operating points: verify the battery rail and collector voltages before introducing a signal.
3. Separate RF and audio paths: determine whether signal loss occurs before or after envelope recovery.
4. Use a controlled reference: compare stage behavior against the 1 MHz carrier and 1 kHz modulation used in LTspice.
5. Observe the final node: probe the physical speaker output to confirm recovered audio reaches the load.

Modeling Assumptions and Limitations

The simulation is intentionally scoped as a circuit-analysis model rather than a full electromagnetic or acoustic model. Its value is in controlled comparison of tuning, stage transfer, and waveform behavior.

Project Files & Conclusion

The completed project package includes a portable LTspice schematic and the transistor model library used for BF199, BC560C, BC549C, BD139, and BD140 devices.

The project demonstrates a complete analog signal chain using discrete devices: resonance and selectivity at the input, RF voltage gain, envelope recovery, audio amplification, and low-impedance speaker drive.