AM Transmitter Using BC547b 

Transmitters that transmit AM signals are known as AM transmitters. These transmitters are used in medium Wave (MV) and short wave (SW) frequency bands for AM broadcast. The MW band has frequencies between 550 KHz and 1650 KHz: and the SW band has frequencies ranging from 3 MHz to 30 MHz .

Circuit Diagram :

AM transmitter takes the audio signal as an input and delivers amplitude modulated wave to the antenna as an output to be transmitted. The block diagram of AM transmitter is shown in the following figure.

The working of AM transmitter can be explained as follows:

• The audio signal from the output of the microphone is sent to the pre-amplifier T1 BC547b, which boosts the level of the modulating signal.
• The RF oscillator generates the carrier signal.
• Both the modulating and the carrier signal is sent to AM modulator.
• Power amplifier T2 BC547b is used to increase the power levels of AM wave. This wave is finally passed to the antenna to be transmitted.

There is no need to wind the inductor as it is a readily available RF choke. To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors are used instead of it. To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency. T1 is biased with a 1MOhm resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.

Component list :

R1 = 1 MΩ

R2 = 4.7 kΩ

R3 = 1 kΩ

R4 = 100 kΩ

R5 = 1 kΩ

C1 = 100 nF

C2 = C3 = 220pF

C4 = 150pF

T1 = T2 = BC547b

6 V BATTERY

L = Inductor 150µH

MIC

Advantages Of Amplitude Modulation (AM).

• AM transmitters are less complex 
• AM receivers are simple detection is easy 
• AM waves can travel long distance low bandwidth

Disadvantages of Amplitude Modulation (AM)

• Power wastage takes place
• Amplitude Modulation gets affected due to noise 
• Amplitude Modulation needs larger bandwidth 

Applications of  Amplitude Modulation (AM) 

• Radio broadcasting 
• Picture transmission in TV system