Along with AC and DC we also learn the features and characteristics of energy waves. Be it an active or passive circuit through which a waves passes, the qualities of the wave changes in some aspects at least. When a full cycle of 360o passes through an amplifier, considering how much percentage of it comes out, Amplifiers are divided into five. See Fig. C-16/1.
Here it is shown in solid lines the part of a signal cycles that comes out in the output. Those portions marked with dotted lines will be lost in each class. In Class A Operation the output signal is intact without any damage to any portion of the full sine wave cycle fed to the input. This will reduce signal distortion but the stage efficiency reduced to just 20%. Class B is more or less works like a rectifier letting through only one half of the cycle. In Class B Stage efficiency is near 60%. The biasing of the active stage here will be such that according to the active stage biasing parameters there will be no signal current at either one half of the signal.
Sage gain is determined by considering the 'quiescent current' or resting current that a valve or transistor develops in it, in proportion to the input signal it gets. In Class C only one fourth of the input signal is amplified. Here, the quiescent current will be maximum low and the efficiency very high up to 90%. Because of the 'fly wheel effect' of the output tank circuits, adding the missing portion of the signal is possible in Class C. This does not mean that the missing potions of the signal in the other classes are never added or impossible to add. With the intention of utilising maximum stage gain almost all RF output amplifiers are designed for Class C. It is to be noted that Distortion is maximum in Class C.
In Classes AB1 and AB2 only one fourth portion of the signal is lost. In the case of Stage gain these Classes are in between A and B. The minor difference between AB1 and AB2 is the slight increase in the resting current in Class AB2. Even though the efficiency in AB1 and AB 2 are only near 40% these Classes are best for signal quality. This is why more designs fall in these Classes. Modulation is anther important factor that might change the quality of the ongoing signal. Modulation is the process of mixing a carrier with intelligence. It is good to understand a few basic facts about modulation too. It will help us develop a better idea on signal stages. Earlier there was an exclusive portion on common modulation styles like Amplitude Modulation, Frequency Modulation and CW. Though only these three are common, more complicated and acute modulation techniques are used along with modern communication strategies.
Even if the Audio (modulating signal) chosen for mixing with the Radio Frequency is subject to continuous or frequent pitch and amplitude variations, the changes it makes on the carrier RF is limited to proportional amplitude changes only. Since the modulated intelligence is 'Audio' this modulation is Voice modulation. Human voice actually is a combination of many sounds. Most of the elements of this complexity is not needed for a successful voice communication. The sense organ of hearing in humans have the capacity to sort out and identify sounds in a frequency range of approximately 20 to 20 Khz. The fun is that only at a frequency range of 200 Hz to 2.7 Khz that any sound remain in quality and in most cases sounds in that range only are used in Voice communication. Considering reproduction requirement of quality sensitive sounds like that in music, the sounds range is doubled in BC Transmitters. This range aspect is observed from sound recording to modulation.
For CW, as told before, Carrier signal is transmitted in baud rate and not in bit-rate which is used to state serial data speed. Unless with a BFO or VFO CW signals cannot be heard. Here, if the carrier wave is modulated with a tone of around 1Khz before transmission, it becomes tone modulation and this asks no extra fittings in the receiver for the listener to hear the transmission. Another way to transmit CW tone is keeping the carrier steady and cutting the tone frequently. In dual tone modulation, like in a telegraph, the sounds for 'dit' and 'dah' are different.
Every time we work transmitters in VHF (Very High Frequency) and SHF (Super High Frequency), stress is given to more output with minimum electrical power. A giant leap in this angle is possible with pulse modulation. Carrier frequency generated here is like a row of pulses at the same amplitude. It is the inactive portions in between pulses that helps save energy. When the amplitude of pulses are cut according to the modulating intelligence AF, it becomes Pulse Amplitude Modulation (PAM). In C-12/2, diagrammatic representation of the Audio cycle, Pulse carrier and an amplitude modulated pulse carrier (PAM) also is shown.
Frequency modulation also is possible with pulse technology. For this AF signal is first converted into pulses. These pulses proportionally generated in scale with the input audio shifts the pulse carrier frequency also accordingly. This method is known as Frequency Shift Keying (FSK) and Frequency Shift Telegraphy (FST). In C-16/3 A and B how even amplitude pulses modulate the carrier is shown. In C and D see how a modulating signal of varying amplitude signal shift the frequency of the pulse carrier in different intensity. Since a lot of rectangular pulses of different amplitudes are used here, it is natural that the number of sidebands also will be proportionally high.
Every time we work transmitters in VHF (Very High Frequency) and SHF (Super High Frequency), stress is given to more output with minimum electrical power. A giant leap in this angle is possible with pulse modulation. Carrier frequency generated here is like a row of pulses at the same amplitude. It is the inactive portions in between pulses that helps save energy. When the amplitude of pulses are cut according to the modulating intelligence AF, it becomes Pulse Amplitude Modulation (PAM). In C-12/2, diagrammatic representation of the Audio cycle, Pulse carrier and an amplitude modulated pulse carrier (PAM) also is shown.
Frequency modulation also is possible with pulse technology. For this AF signal is first converted into pulses. These pulses proportionally generated in scale with the input audio shifts the pulse carrier frequency also accordingly. This method is known as Frequency Shift Keying (FSK) and Frequency Shift Telegraphy (FST). In C-16/3 A and B how even amplitude pulses modulate the carrier is shown. In C and D see how a modulating signal of varying amplitude signal shift the frequency of the pulse carrier in different intensity. Since a lot of rectangular pulses of different amplitudes are used here, it is natural that the number of sidebands also will be proportionally high.
One common problem of all frequency modulation activities are the drift in the carrier central frequency. One method to escape from this this is Phase Modulation. The theory that any change in the phase of a sinusoidal wave makes corresponding changes in the frequency is the working principle behind Phase Modulation. The word Sinusoid ascribes to smooth repetitive oscillations while sine waves refer to wave continuity without interruptions. Usually a carrier frequency is affected only according to the amplitude changes of the modulating signal. But in Phase Modulation both the amplitude and frequency changes in the modulating signal will ask for frequency changes in the carrier. In PM the proportional frequency change that happens in a carrier frequency is very low when compared to FM, this intelligence is to be multiplied many times before an effective demodulation.
Pulse Width Modulation (PWM) method in which the amplitude of the carrier pulse row is modulated also is in use. Another method is shifting the position of the pulses according to the modulating signal. This is called Pulse Position Modulation (PPM). Whatever be the method used to modulate a carrier, until the modulating signal (intelligence) is demodulated the modulated carrier may have to pass through a series of stages which certainly add on distortion factors to the carrier without fail. This cannot be eliminated completely. What we can do here is to keep the distance between signal voltage and noise voltage (signal to noise ratio) as high as possible. But in Pulse Code Modulation (PCM) this difficulty is manageable. In PCM, modulating signal is sampled frequently and pulse groups are formed in proportion to the modulating intelligence, in a way that each group falls in a different pattern. The PCM coding is in such a way that each pattern represents a particular letter, digit or symbol. The binary code which is a coding system utilising the binary digits 0 and 1 to represent all letters, digits and other characters belongs to PCM.
Earlier it was mentioned that there are no pulse spaces in Pulse Modulated signals. This gaps can be used to fill another signal of a different amplitude arrangement. If the pulses added are modulated with anther signal, in effect the carrier becomes a signal with two modulations - carrier with two different audio packages. Similar modulations fall under Time Division Multiplexing (TDM). According to this, a single carrier is enough for a number of modulating signals. Such complex transmission methods also are in wide use these days.
PWM and PPM are some times called Pulse Length Modulation (PLM) and Pulse Time Modulation (PTM) also respectively. More or less similar is the functioning of Frequency Division Multiplexing. Here even if two signals of the same frequency modulate the carrier, mutual interference do not happen. The secret is that with every modulating signal sub carriers are generated and it is the sub-carrier that gets modulated. In the final stage all these sub carriers join together to modulate the main carrier. Similar complex composite signals are used in satellite data communication. Exchanging too many phone calls at a time without mutual interference using only one carrier uses FDM technology. But in TV transmissions where there are both audio and video, instead of multiple modulation two separate carriers are used for modulation.
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