Radio Communication

Sound and radio waves are different phenomena. Sound consists of pressure variations in matter, such as air or water. Sound will not travel through a vacuum. Radio waves, like visible light, infrared, ultraviolet, X-rays and gamma rays, are electromagnetic waves that do travel through a vacuum. When
you turn on a radio you hear sounds because the transmitter at the radio station has converted the sound waves into electromagnetic waves, which are then encoded onto an electromagnetic wave in the radio frequency range (generally in the range of 500-1600 kHz for AM stations, or 86-107 MHz for FM stations). Radio electromagnetic waves are used because they can travel very large distances through the atmosphere without being greatly attenuated due to scattering or absorption. Your radio receives the radio waves, decodes this information, and uses a speaker to change it back into a sound wave. An animated illustration of this process is given below (mouse-over the images for animations).

• A sound wave is produced with a frequency of 5 Hz - 20 kHz.                                                                                                    
• The sound wave is equivalent to a pressure wave traveling through the air.

• A microphone converts the sound wave into an electrical signal.

•    The electrical wave traveling through the microphone wire is analogous to the original sound wave. 
• The electrical wave is used to encode or modulate a high-frequency "carrier" radio wave. The carrier wave itself does not include any of the sound information until it has been modulated.
• The carrier wave can either be amplitude modulated by the electrical signal, or frequency modulated.
• 
• The signal is transmitted by a radio broadcast tower.
• 
• Your radio contains an antennato detect the transmitted signal, a tuner to pick out the desired frequency, a demodulator to extract the original sound wave from the transmitted signal, and an amplifier which sends the signal to the speakers. The speakers convert the electrical signal into physical vibrations (sound).

Class B amplifier-Working

Unlike the Class A amplifier mode of operation above that uses a single transistor for its output power stage, the Class B Amplifier uses two complimentary transistors (an NPN and a PNP) for each half of the
output waveform. One transistor conducts for one-half of the signal waveform while the other conducts for the other or opposite half of the signal waveform. This means that each transistor spends half of its time in the active region and half its time in the cut-off region thereby amplifying only 50% of the input signal.

Class B operation has no direct DC bias voltage like the class A amplifier, but instead the transistor only conducts when the input signal is greater than the base-emitter voltage and for silicon devices is about 0.7v. Therefore, at zero input there is zero output. This then results in only half the input signal being presented at the amplifiers output giving a greater amount of amplifier efficiency as shown below.

Class B- Circuit Diagram

In a class B amplifier, no DC current is used to bias the transistors, so for the output transistors to start to conduct each half of the waveform, both positive and negative, they need the base-emitter voltage Vbe to be greater than the 0.7v required for a bipolar transistor to start conducting. Then the lower part of the output waveform which is below this 0.7v window will not be reproduced accurately resulting in a distorted area of the output waveform as one transistor turns "OFF" waiting for the other to turn back "ON". The result is that there is a small part of the output waveform at the zero voltage cross over point which will be distorted. This type of distortion is called Crossover Distortion

Class AB amplifier-Worrking

The Class AB Amplifier is a compromise between the Class A and the Class B configurations above. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is
present.

An input signal will cause the transistor to operate as normal in its Active region thereby eliminating any crossover distortion which is present in class B configurations. A small Collector current will flow when there is no input signal but it is much less than that for the Class A amplifier configuration. This means then that the transistor will be "ON" for more than half a cycle of the waveform. This type of amplifier configuration improves both the efficiency and linearity of the amplifier circuit compared to a pure Class A configuration.

Class AB- Circuit Diagram

Class AB Output Waveform

The class of operation for an amplifier is very important and is based on the amount of transistor bias required for operation as well as the amplitude required for the input signal. Amplifier classification takes into account the portion of the input signal in which the transistor conducts as well as determining both the efficiency and the amount of power that the switching transistor both consumes and dissipates in the form of wasted heat.

Class C amplifier - Working

In class C operation, collector current flows for less than one half cycle of the input signal.

The class C operation is achieved by reverse biasing the emitter-base junction,
which sets the dc operating point below cutoff and allows only the portion of the input signal that

overcomes the reverse bias to cause collector current flow. if an input signal amplitude is increased to the point that the transistor goes into saturation and cutoff, it is then called an OVERDRIVEN amplifier. 
Working:

During the positive period of the input signal (On stage)During the positiv period of the input signal the transistor will conduct (On-state). You can imagin that the transistor is a switch which connects the emitter with the collector. 
What will happend now is that the current I1 (red) flow through the coil and then into the transistor and down to ground. A magnetic field builds up in the coil depending on the magnitude of the current. At the same time the voltage over the capacitor discharge through the resistor making another current flow I2 (blue) also through the transistor. The I2 current passes the resistor (antenna) which radiate the energy.

During the negative period of the input signal (Off stage)
During the negativ period of the input signal the transistor will not conduct (Off-state). You can imagin that the transistor is an open switch. No current can pass through the collector to the emitter. 
The magnetic filed which was build upp in the coil will now collaps and generate a current I1 (red) which will flow through the capacitor and into the resistor (antenna).

Why all Digital Electronics Circuits use DC and Not AC?

The question will be little confusing.
But the answer is simple.

AC Vs DC

In Digital Electronics, Gates are the basic Elements.
Actually this Gates are made up of Transistors.

NAND gate using Transistors

Transistors are working as a Switch in Digital Electronics.

Transistor as a Switch

ie, When control signal is present, Transistor is ON, otherwise Transistor is OFF.

Now, What is this Control Signal?

That is the Signal Applied to the Base of the Transistor.

The Switch must be ON till the control signal is present and the Switch must be OFF till the control signal is absent. 

Switch with Control Terminal

Now consider applying AC signal as the control signal to the Base of the Transistor.

The AC signal will vary from Positive peak to Negative peak going through the 0V.

So how can we keep the Transistor ON and OFF as per our requirement?
It is not possible.

Now consider DC. It is Direct current and it is constant in value.
So if we apply DC to the Base of the Transistor as a control signal, we can keep the Transistor ON of OFF as per our wish.

That is In digital Electronics, We need only HIGH signal and LOW signal, not the intermediate values. 
Hence we cannot use AC in Digital.

Now consider Transistor working as an AMPLIFIER.
Here also, the transistor is working in DC (Power supply of the Transistor is DC), but the input is an AC signal.

Transistor as an Amplifier

Thus Amplification of AC signal is just an application of the Transistor and that doesn't mean that the Transistor is working in AC.

Why cant we power the Transistor with AC?
We can apply AC as a Power supply to the Transistor.
But the transistor will not give the desired operation.

Biasing of Transistor (a) NPN  (b) PNP

Because, for acting as a Switch or Amplifier, the transistor should be biased.
In order to keep the transistor in constant Biasing conditions, we need Constant current. ie DC.

If we apply AC as a power supply to the transistor, the Biasing conditions of the Transistor will be varying in each cycle of the AC signal.

Hence the transistor will not work properly.
This is the reason why we convert the AC signals into DC using Rectifiers in the Power supply section of the Electronic Devices.

(We can apply the same principle to MOSFET also.
MOSFET are used as switches in Digital Electronics as Switches.
Working principle of MOSFET is same as that of the Transistor.
However, MOSFET is a Voltage controlled Device, but Transistor is a Current controlled Device.)