The Commando AM transmitter circuit demonstrates a multi-stage approach to amplitude modulation transmission using three transistors (Q1, Q2, Q3) in distinct functional roles. This design, originally published on Free Radio TX Blog (http://freeradiotx.blogspot.com/), provides excellent performance for amateur radio applications with proper component selection and circuit layout.
Three-Stage Transistor Configuration
The commando AM transmitter employs three transistors working in sequence to achieve reliable AM transmission. Q1 serves as the oscillator stage, generating the carrier frequency with crystal control for stability. This transistor operates in an oscillator configuration with feedback components that determine the operating frequency.
Q2 functions as the buffer/driver amplifier, isolating the oscillator from the final amplifier while providing additional gain. This stage prevents loading effects on the oscillator and ensures stable frequency operation. The buffer stage uses careful biasing with resistors R2 and R4 to maintain proper operating conditions.
Q3 operates as the final RF power amplifier, delivering the modulated signal to the antenna. This transistor handles the highest power levels in the circuit and requires proper heat dissipation. The collector circuit includes impedance matching components to efficiently transfer power to the antenna system.
Audio Modulation System
The audio transformer (U2) introduces the modulating signal into the commando AM transmitter circuit. The modulation occurs by varying the supply voltage or bias conditions of the final amplifier Q3. This creates amplitude variations in the RF carrier corresponding to the audio input signal.
Coupling capacitors C11 and C12 handle audio frequency coupling while blocking DC components. The 47pF capacitor provides high-frequency rolloff to prevent audio distortion, while the 100pF capacitor shapes the modulation bandwidth for clean AM transmission.
Power Supply
The circuit operates from 12V DC through connector P1. Multiple bypass capacitors (100nF, 10nF) filter the power supply lines and prevent RF feedback between stages.
The power distribution system ensures each transistor receives clean, stable operating voltage. Proper decoupling prevents oscillations and maintains stable operation across the entire frequency range of the commando AM transmitter.
RF Output and Antenna Matching
The final stage Q3 drives the antenna through impedance matching components including the 1.5mH inductor L1. This network transforms the transistor’s output impedance to match standard antenna impedances (typically 50 ohms). Proper matching ensures maximum power transfer and minimizes reflected power.
The output filtering components suppress harmonics and spurious emissions, maintaining spectral purity required for amateur radio operation. The antenna connection ANT1 provides the interface to your radiating system.
Circuit Heritage: This commando AM transmitter design was originally published on Free Radio TX Blog (http://freeradiotx.blogspot.com/), a valuable resource for amateur radio circuit designs and RF experimentation.
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