Amplitude Modulation

 Amplitude Modulation Using High-Level Modulation Circuit

• Introduction

Amplitude modulation is a way of sending information like audio or voice through the air using radio waves. Imagine if there is a message you need to send to someone, like your voice message or  music then you need a carrier wave with a high frequency that can travel long distances. A common circuit for implementing amplitude modulation uses a transistor-based configuration known as the high-level modulation circuit.

• Circuit components 

1. Radio frequency carrier source - generates high frequency carrier signal normally in kHz to MHz range.
2. Radio frequency class c amplifier - a class C amplifier is a electronic amplifier that only works part of the time.
3. Modulating signal - amplifies the low-frequency audio signal. 
4. Modulation transformer - couples the modulated output to the next stage.
5. Power supply - provides the DC (direct current) voltage to the circuit.
6. Other components - Resistors and capacitors to build a better circuit.

• Basic operation

In collector or high-level modulation, it combines two signals to create an amplified modulation radio wave. The high-frequency carrier signal that comes from an oscillator goes into the base of the transistor, we commonly call that the control input. This transistor amplifies the carrier signal and acts as working class-C mode, meaning it only turns on in some moments, like less than half of each cycle, and provide power efficiency. In this moment, the audio signal, like your voice message or music is connected to the collector circuit through a special transformer. This transformer allows the audio signal to be added on top of the DC supply without interfering with it. This is special because a normal circuit connection might lead to a short-circuit. 

After this, when the audio signal goes to the positive side (above 0 volts), it adds to the collector’s power supply voltage and making it stronger. When the audio signal goes to the negative side (below 0 volts), it subtracts from the power supply, and then the signal gets weak.

This continuously changing power supply causes the transistor to generate a carrier wave that is strong in the loud audio and weak during the quiet audio signals. This leads to the result that we hope during this process, the result is an amplitude-modulated signal where the carrier wave’s strength rises and falls in perfect sync with the audio signal that we provide. After this process the audio signal is ready to be transmitted through the air.

• Modulation process

1.When the modulation signal is at zero 

•         Collector voltage = DC (direct current) voltage.
•         Output is just a carrier signal with constant amplitude. 

2.When the modulation signal is positive

•         Audio signal voltage adds to the supply voltage.
•         Carrier amplitude increases, and the output signal gets strong.

3.When the modulating signal is negative

•         The audio signal voltage is subtract from the supply voltage.
•         Carrier amplitude decreases, and the output signal gets weaker.

• Circuit diagram 

• Circuit advantages

1. High efficiency - Class C amplifiers provide power efficiency of 70% - 85%.
2. High power output - Commonly used for high-level modulation.
3. Simple design - Needs few components compared to other modulation circuits.

• Real-world usage

Radio broadcasting transmitters are used for medium waves and short waves.

Aviation communication system.

Point-to-point communication links.

• Conclusion

The collector modulation circuit provides an efficient and practical usage for amplitude modulation. This circuit achieves amplitude modulation by maintaining high power efficiency using a class C amplifier. This circuit will help to working with amplitude-modulated transmission systems.