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 receiver is modeled as a tuned LC front end followed by RF gain, detection and preamp control, and a complementary transistor audio output stage driving a speaker load. This first version focuses on what each block does, how it was approximated in simulation, and what still needs to be measured against the physical build.

Physical Build

The physical front end uses a hand-wound coil on a ferrite rod and a 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.

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

The first simulation pass 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.

Debugging Process

The debugging plan follows the same layered structure as the page: prove each block before blaming the whole receiver. The most useful checks are node-by-node comparisons between the physical build and the LTspice model.

1. Start at the LC tank: verify continuity, tuning capacitor range, and approximate resonant behavior.
2. Check transistor bias: confirm each transistor is biased into the intended operating region before tracing signals.
3. Inject known signals: use the simulated 1 MHz AM source as a reference for bench testing where possible.
4. Trace stage outputs: compare Q1, Q2, detector/preamp, and speaker-node behavior against the simulation.
5. Separate RF and audio faults: determine whether a failure occurs before detection or after the audio driver/output stage.

Modeling Assumptions and Limitations

This simulation is intentionally simplified so the circuit layers can be understood and tuned quickly. The assumptions are useful for a first pass, but they also define where the model may differ from the hardware.

Files / Next Steps

This first portfolio version documents the architecture and simulation approach. The next version can add downloadable LTspice files, measured bench results, and a deeper schematic walkthrough as the physical build is refined.