AM Radio Receiver – ECE 342

Overview

Designed and simulated a complete AM radio receiver with a microphone input option for ECE 342 (Spring 2026). The system takes a signal from an antenna, amplifies and selects an AM station, demodulates the audio, and drives an 8 Ω speaker through a Darlington power stage, all on a custom Altium PCB.

The design is organized into four cascaded stages: RF pre-amplifier → AM demodulator + LPF → audio pre-amplifier → Darlington power output. A DPDT slide switch allows toggling between the AM radio input and a 3.5 mm microphone jack.

Signal Chain

Stage 1 – RF Pre-amplifier (2N3904, Common Emitter)

An LC tank circuit (L = 300 µH, C_var = 18–300 pF) selects the AM station by resonating at the desired carrier frequency. The 2N3904 is biased at I_C ≈ 2.12 mA for linear amplification:

• V_B = 3.24 V, V_C = 5.0 V, V_CE = 2.46 V (active region confirmed)
• R₁ = 27 kΩ, R₃ = 10 kΩ (voltage divider bias, R₁ ≈ 2.83 × R₃)
• R₄ = 1.2 kΩ emitter degeneration; C₄ = 100 nF bypass capacitor
• C₂ = 1 nF output coupling; 1206 SMD capacitors throughout

Stage 2 – AM Demodulator + Low-pass Filter

The demodulator stage biases a second 2N3904 near cut-off (V_B ≈ 0.77 V) to act as a half-wave rectifier, extracting the audio envelope from the AM carrier.

• R₅ = 680 kΩ, R₉ = 47 kΩ → V_B ≈ 0.77 V (just above cut-off)
• C₆ (330 pF, variable) + R₈ = 10 kΩ: strips carrier, retains audio envelope
• R₇ = 1 kΩ, C₈ = 47 nF: final audio low-pass filter before next stage
• Reducing C₆ increases gain; removing it maximizes gain at the cost of more carrier noise

Stage 3 – Audio Pre-amplifier (2N3904)

Self-biased common-emitter stage providing the bulk of voltage gain before the power transistor. A 1 kΩ potentiometer (volume control) sits between this stage and the output.

• Feedback resistor R₁₃ = 1.2 MΩ sets bias: V_C ≈ 6.4 V (≈ V_CC/2 for max swing)
• I_C ≈ 1 mA; R₁₂ = 6.8 kΩ load; R₁₄ = 100 Ω emitter resistor
• β = 200 assumed → I_B = 5 µA, confirming active region operation
• C₁₂ = 100 pF optional noise filter at base

Stage 4 – Darlington Power Output (2N3904 + TIP31C)

A Darlington pair (Q3 drives Q4) was chosen after a single common-emitter stage proved insufficient to drive an 8 Ω speaker. The configuration provides extremely high current gain (β_total ≈ β_Q3 × β_Q4) with V_BE,total = 2 × 0.7 V = 1.4 V.

• Q3: 2N3904 driver transistor
• Q4: TIP31C (100 V, 3 A) output transistor with TO-220 heatsink
• C₉ = 470 µF output coupling; C₁₁ = 1000 µF supply bypass
• R₁₅ = 10 Ω emitter resistor on Q4 for stability
• Microphone input option via DPDT slide switch and 3.5 mm jack (J1)

PCB Layout

LTspice Simulation Results

The full signal chain was verified in LTspice using a transient analysis (.tran) and operating point check (.op).

Operating Point (confirmed active region for all transistors)

Transient Simulation (signal path)

• Antenna input: AM-modulated signal ±15 mV (1 kHz audio on ~1 MHz carrier)
• After RF pre-amp: amplified RF signal riding on ~5.1 V DC bias
• Demodulated audio: clean ~8 mV peak recovered audio signal
• Speaker voltage: ±1.5 V swing → ~330 mW peak into 8 Ω

Design Notes & Key Decisions

• RF pre-amp added: downstream stages alone produced insufficient gain to drive the 8 Ω speaker; adding the LC-tuned common-emitter stage resolved this.
• Darlington pair: a single common-emitter output stage lacked current drive; the Q3 (2N3904) + Q4 (TIP31C) Darlington gives β_total ≈ 25,000+, enabling clean speaker drive.
• Ground plane clearance: ground pours were removed around the LC tank on the PCB to prevent capacitive coupling that would inject noise into the tuned circuit.
• C₆ trade-off: lower capacitance on the demodulator filter increases audio gain but allows more carrier ripple through; the chosen value (330 pF) balances both.
• Microphone option: a DPDT slide switch routes either the AM demodulated signal or a microphone input (biased via R₁₆ = 10 kΩ, +9 V) into the audio pre-amp stage.
• Test points: 19 orange 5013 test point markers placed throughout the PCB for each stage's key nodes.