Ab biased amplifier classifications
The Class A amplifier uses a single switching transistor in the standard common emitter circuit configuration to produce an inverted output. In other words, a class B amplifier is biased to conduct over half the waveform. By using two transistors that conduct each half of a cycle, the complete signal is recovered. This results in very high levels of distortion, but also achieves very high efficiency levels. Class C amplifiers are more efficient than class A, class B or class AB, which means that more output power can be obtained from class C operation.
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Electronic devices: POWER AMPLIFIERS [part 1]
Transistor amplifiers use several different biasing strategies. The strategies are identified as classes of amplifiers ranging from Class A to Class G. Class A—C amplifiers have the same impedance presented to the output of the amplifier at the operating frequency and at.
Amplifier design consists of both design for low-power linear operation, requiring maximum power transfer at the input and output of the amplifier, and a trade-off of acceptable distortion and efficiency. In practice, a certain level of distortion must be tolerated, and what is acceptable is embedded in the specifications of the various wireless systems. For low distortion, the peaks of the RF signal must be amplified linearly, however, the DC power consumed depends on the amplifier class.
The situation is similar for Class AB amplifiers, with the difference being that the intent is to accept some distortion of the peak signal so that the relationship between peak power and DC power still exists, but the direct proportionality no longer holds. For Class C and higher classes, the back-off required comes from experience and experimentation. The characteristics of the signal also determine how much distortion can be tolerated.
Putting this another way, the DC bias must be set so that there is minimum distortion when the signal is at its peak, but.
This is especially true for Class A amplifiers. The Class A amplifier has limited efficiency because there is always substantial quiescent current flowing whether or not RF current is flowing. Higher-order classes of amplifiers achieve higher efficiency, but distort the RF signal.
The current and voltage loci of Class A, B, AB, and C amplifiers have a similar trajectory on the output current-voltage characteristics of a transistor. The loadline is the locus of the DC current and voltage as the DC input voltage is varied. With the Class A amplifier, the transistor is biased in the middle of the transistor characteristics, where the response has the highest linearity.
That is, when the gate voltage varies due to an applied signal, the output voltage and current variations are nearly linearly proportional to the applied input. The drawback is that there is always considerable DC current flowing, even when the input signal is very small. That is, there is DC power consumption whether or not RF power is being generated at the output of the transistor. This is not of concern if small RF signals are to be amplified, as then a small transistor can be chosen so that the DC current levels are small.
It is a problem if an amplifier must handle both large and small signals. The Class A amplifier is defined by its ability to amplify small to medium and even large signals with minimal distortion. The loadline is the locus of the output current and voltage.
For the Class A amplifiers in Figure 2. These are called single-ended amplifiers, as the input and output voltages are referred to ground. The notable difference between these characteristics and those of the bipolar transistor is that the curves are less abrupt at low output voltage i.
The bipolar and FET amplifiers of Figure 2. As well, the bias resistor is also the load resistor. Various alternative topologies have been developed yielding a range of output voltage swings. The quiescent drain-source voltage is halfway between these extremes. The AC loadline is also called the dynamic loadline. Since the Class A amplifier is always drawing DC current, its efficiency is near zero when the input signal is very small.
Reducing the bias results in signal distortion for large RF signals. Class A amplifiers have the highest linearity and Class B and C amplifiers result in considerable distortion. As a compromise, Class AB amplifiers are used in many cellular applications, although Class C amplifiers are used with constant envelope modulation schemes, as in GSM.
Nearly all small-signal amplifiers are Class A. The effect of parasitic capacitances and delay effects such as those due to the time it takes carriers to move across a base for a BJT or under the gate for a FET result in the current-voltage locus for RF signals differing from the DC situation.
The Class A amplifier is a low-efficiency, but highly linear class. The output of the Class B amplifier contains an amplified version of only half of the input signal but draws just a small leakage current when no signal is applied.
With the Class C amplifier there must be some positive RF input signal before there is an output: there is more distortion but no current flows, not even leakage current, when there is no RF input signal. Less DC current flows than with Class A when there is negligible input signal, and the distortion is less than with Class B.
Filtering, often provided by matching networks, eliminates harmonics from the output of the amplifier, but in-band distortion of finite bandwidth signals remains. This distortion is important only if there is information in the amplitude of the signal.
Therefore errors introduced into the amplitude of a signal are of lesser significance and efficient saturating mode amplifiers such as a Class C amplifier can be used.
For these modulation techniques, reasonably linear amplifiers are required. The Class A amplifier presents input and output impedances that are almost independent of the level of the signal. Thus design requires more care, as the chances of instability are higher and it is more likely that an oscillation condition will be met.
Class A amplifiers are then the preferred solution as design is simpler and the amplifier is more tolerant of parasitic effects and variations. For maximum output voltage swing, the quiescent point should be halfway between the maximum and minimum drain source voltages,. This is the maximum efficiency of a resistively biased Class A amplifier.
Class AB and Class C Power Amplifiers
The meters on our amplifiers are different. This is because the electrical current consumption of our circuits has a fairly high value at all times, a property called the bias. For several years Pass Labs has specified the nominal wattages at which our amplifiers leave push-pull Class A operation into an eight ohm load. We get a lot of questions about this. For starters, there is no special Class A circuit that kicks in and out, and for that matter, there certainly is no Klunk. There is just a push-pull amplifier output stage which is operated at a constant idle current known as the bias. In this regard, our power amplifiers are like other amplifiers on the market.
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The Application Activity in this is a public address system. Recall that the complete system includes the preamplifier, a power amplifier, and a dc power supply. You will focus on the power amplifier in this section and complete the total system by combining the three component parts. Power amplifiers are large-signal amplifiers. This generally means that a much larger portion of the load line is used during signal operation than in a small-signal amplifier. In this section, we will cover four classes of power amplifiers: class A, class B, class AB, and class C. These amplifier classifications are based on the percentage of the input cycle for which the amplifier operates in its linear region. Each class has a unique circuit configuration because of the way it must be operated.
Module 5.5
In earlier days, before the invention of electronic amplifiers , the coupled carbon microphones are used as crude amplifiers in telephone repeaters. The first electronic device that practically amplifies was the Audion vacuum tube, invented by the Lee De Forest in the year The term amplifier and amplification are from Latin word amplificare to expand or enlarge. The vacuum tube is the only simplifying device for 40 years and dominated electronics up to When the first BJT was in the market it has created another revolution in the electronics and it is a first portable electronic device like transistor radio developed in the year
Class AB Amplifiers
Title: Large Signal Amplifier Objectives: - Understand the importance of amplifier efficiency - Know the most common classes of power amplifier and their basic characteristics Figure Large signal amplifier The early stages of amplifier systems are dealing with small signals. These stages are designed to give good voltage gain. Small signal transistors have a power rating of less than half a watt and power transistors have a power rating of more than half a watt. The later stages of an amplifier system have much larger collector currents, because the load impedances are much smaller i. Efficiency is most important when large amounts of signal power are required: Classes This refers to how the amplifying device is biased. Amplifier can be biased for class A, B or AB operation.
Power amplifier classes
In electronics , power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier 's characteristics and performance. The classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period. A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse width modulation output is obtained from the stage. Additional letter classes are defined for special purpose amplifiers, with additional active elements or particular power supply improvements; sometimes a new letter symbol is used by a manufacturer to promote its proprietary design.
Difference between Class A and Class AB amplifier
The class A and class B amplifier so far discussed has got few limitations. Let us now try to combine these two to get a new circuit which would have all the advantages of both class A and class B amplifier without their inefficiencies. Before that, let us also go through another important problem, called as Cross over distortion , the output of class B encounters with.
Amplifier Classification A vs. AB
RELATED VIDEO: Setting Bias on a Class AB Power AmpThe Web This site. The class AB push-pull output circuit is slightly less efficient than class B because it uses a small quiescent current flowing, to bias the transistors just above cut off as shown in Fig. In class AB each of the push-pull transistors is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A. As each cycle of the waveform crosses zero volts, both transistors are conducting momentarily and the bend in the characteristic of each one cancels out.
On the other hand, class B amplifiers present a very high efficiency up to Indeed, a crossover distortion appears in a push-pull configuration and the use of a negative feedback is required to limit this effect. In order to combine both the excellent linearity of class A and the high efficiency of class B, the class AB has been developed. In the very first section, we will present the general functioning and characteristics of class AB amplifiers. Thereafter, we will discuss the efficiency of class AB configurations. The following section will present some possible biasing methods and we will highlight that the diode biasing is the most appropriate.
Amplifier Design Concepts Includes: Basic concepts Amplifier classes Amplifiers are given a classification according to the way in which they are biassed and they operate. The class of an amplifier is selected to meet the overall requirements. Different amplifier classes provide different characteristics, enabling the amplifier to perform in a particular way and also with a level of efficiency.
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