DIY IRF510 AM Transmitter Circuit

Are you interested in building your own AM transmitter? This blog post dives into a detailed DIY IRF510 AM transmitter circuit using the IRF510 MOSFET, perfect for radio enthusiasts and electronics hobbyists. The IRF510 MOSFET-based AM transmitter circuit offers an excellent balance of power output, stability, and ease of construction.  Let’s explore the design based on the attached schematic.

Overview of the IRF510 AM Transmitter

This AM (Amplitude Modulation) transmitter circuit is designed to operate at 3.9 MHz and 7.2 MHz, making it suitable for amateur radio projects. The IRF510 MOSFET serves as the core amplification stage, delivering a robust signal to the antenna.

IRF510 AM Transmitter –Key Components

Power Stage:

• Q2 (IRF510 MOSFET): The heart of the transmitter, providing RF amplification
• R2 (47Ω, 0.5W): Gate bias resistor for proper MOSFET operation
• R3 (2.2kΩ): Drain bias resistor

Oscillator Stage:

• Q1 (2N2222 NPN Transistor): Crystal oscillator driver
• CR  Crystal: Frequency-determining element 3.9mhz or 7.2mhz
• C1 (330pF): Oscillator timing capacitor
• R1 (100kΩ): Base bias resistor

Filtering and Coupling:

• C2 (10nF): AC coupling capacitor
• C3 (100nF): Power supply decoupling
• C4 (470pF, 100V): Output coupling capacitor
• C5 (1200pF, 100V): Harmonic filtering
• C6 (10nF): Additional filtering
• C7 (100nF): Modulation circuit decoupling

Inductors and Transformers

RF Chokes and Inductors:

• L1: Primary oscillator inductor (40 turns for 3.9 MHz, 23 turns for 7.2 MHz)
• L2: Flyback inductor (24 turns of AWG #20 or #22)
• L3: Output matching inductor (16 turns for 3.9 MHz, 11 turns for 7.2 MHz)
• L4: Antenna coupling inductor (20 turns for 3.9 MHz, 15 turns for 7.2 MHz)

Modulation Transformer:

• U1: 6V or 9V modulation transformer for AM modulation

How the Circuit Works

The 2N2222 transistor (Q1) forms a crystal-controlled oscillator. The crystal resonator determines the transmission frequency, while C1 and L1 provide the necessary feedback for sustained oscillation. The IRF510 MOSFET (Q2) amplifies the RF signal from the oscillator stage. The gate bias is set by R2, while R3 provides proper drain bias. This configuration ensures efficient RF amplification with minimal distortion.

The L3-L4 inductor network, combined with capacitors C4 and C5, creates an impedance matching circuit that efficiently transfers RF power to the antenna while providing harmonic suppression.The modulation transformer U1 allows audio signals to modulate the RF carrier, creating the AM transmission. C7 provides decoupling for the modulation circuit.

Frequency Bands and Coil Specifications

This transmitter design supports dual-band operation:

3.9 MHz  (80 meters)

• L1: 40 turns, AWG #30, T50-2 core
• L3: 16 turns, AWG #20, T50-2 core
• L4: 20 turns, AWG #20, T50-2 core

7.2 MHz  (40 meters)

• L1: 23 turns, AWG #30, T50-2 core
• L3: 11 turns, AWG #20, T50-2 core
• L4: 15 turns, AWG #20, T50-2 core

IRF510AM Transmitter –Performance Specifications

• Power Output: Approximately 5 watts
• Frequency Stability: Crystal-controlled accuracy
• Modulation Depth: Up to 95% with proper audio input
• Harmonic Suppression: >40dB with proper filtering
• Operating Voltage: +12V DC

80 meter 1W QRP Transmitter with Ceramic Resonator Control

Building Steve Quest’s Simple AM Transmitter

About The Author