Class ab rf power amplifier design

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. The different amplifier classes provide different performance characteristics. These make the different types of amplifier class suitable for different situations.

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Content:

• Adaptive Feedforward Linearization For RF Power Amplifiers
• Design of radio frequency high power linear and harmonically tuned amplifiers
• RF Power Amplifiers, 2nd Edition
• Engineering Notes
• Advanced Techniques in RF Power Amplifier Design
• How to Design an RF Power Amplifier: Class A, AB and B
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• Go to School on RF Power Amplifier Classes

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Adaptive Feedforward Linearization For RF Power Amplifiers

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. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current.

The angle of flow is closely related to the amplifier power efficiency. In the illustrations below, a bipolar junction transistor is shown as the amplifying device.

The active element remains conducting [2] all of the time. Amplifying devices operating in class A conduct over the entire range of the input cycle. A class-A amplifier is distinguished by the output stage devices being biased for class A operation. Subclass A2 is sometimes used to refer to vacuum-tube class-A stages that drive the grid slightly positive on signal peaks for slightly more power than normal class A A1; where the grid is always negative [3] [4].

This, however, incurs higher signal distortion [ citation needed ]. Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists. There is a market for expensive high fidelity class-A amps considered a "cult item" among audiophiles [6] mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion.

Class A power amplifiers are also used in some "boutique" guitar amplifiers due to their unique tonal quality and for reproducing vintage tones. Some hobbyists who prefer class-A amplifiers also prefer the use of thermionic valve tube designs instead of transistors, for several reasons:. Transistors are much less expensive than tubes so more elaborate designs that use more parts are still less expensive to manufacture than tube designs.

A classic application for a pair of class-A devices is the long-tailed pair , which is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps. Class-A amplifiers may be used in output stages of op-amps [9] although the accuracy of the bias in low cost op-amps such as the may result in class A or class AB or class B performance, varying from device to device or with temperature.

They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers. The power consumption is unrelated to the output power. At idle no input , the power consumption is essentially the same as at high output volume.

The result is low efficiency and high heat dissipation. In a class-B amplifier, the active device conducts for degrees of the cycle. This would cause intolerable distortion if there were only one device, so two devices are usually used, especially at audio frequencies.

At radio frequency , if the coupling to the load is via a tuned circuit , a single device operating in class B can be used because the stored energy in the tuned circuit supplies the "missing" half of the waveform. Devices operating in Class B are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage.

This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through the amplifier. When Class-B amplifiers amplify the signal with two active devices, each operates over one half of the cycle.

Efficiency is much improved over class-A amplifiers. A practical circuit using class-B elements is the push—pull stage , such as the very simplified complementary pair arrangement shown at right.

Complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This arrangement gives good efficiency, but usually suffers from the drawback that there is a small mismatch in the cross-over region — at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they are not in use.

This approach is called class AB operation. In a class-AB amplifier, the conduction angle is intermediate between class A and B; each one of the two active elements conducts more than half of the time. Class AB is widely considered a good compromise for amplifiers, since much of the time the music signal is quiet enough that the signal stays in the "class-A" region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small.

The crossover distortion can be reduced further by using negative feedback. In class-AB operation, each device operates the same way as in class B over half the waveform, but also conducts a small amount on the other half.

The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether. The exact choice of quiescent current the standing current through both devices when there is no signal makes a large difference to the level of distortion and to the risk of thermal runaway , which may damage the devices.

Often, bias voltage applied to set this quiescent current must be adjusted with the temperature of the output transistors. For example, in the circuit shown at right, the diodes would be mounted physically close to the output transistors, and specified to have a matched temperature coefficient. Another approach often used with thermally tracking bias voltages is to include small value resistors in series with the emitters.

Class AB sacrifices some efficiency over class B in favor of linearity, thus is less efficient below It is typically much more efficient than class A. A vacuum tube amplifier design will sometimes have an additional suffix number for the class, for example, class B1.

A suffix 1 indicates that grid current does not flow during any part of the input waveform, where a suffix 2 indicates grid current flows for part of the input waveform. This distinction affects the design of the driver stages for the amplifier. Suffix numbers are not used for semiconductor amplifiers. Distortion is high and practical use requires a tuned circuit as load.

The usual application for class-C amplifiers is in RF transmitters operating at a single fixed carrier frequency , where the distortion is controlled by a tuned load on the amplifier. The input signal is used to switch the active device, causing pulses of current to flow through a tuned circuit forming part of the load.

The class-C amplifier has two modes of operation: tuned and untuned. This is called untuned operation, and the analysis of the waveforms shows the massive distortion that appears in the signal. When the proper load e. The first is that the output's bias level is clamped with the average output voltage equal to the supply voltage.

This is why tuned operation is sometimes called a clamper. This restores the waveform to its proper shape, despite the amplifier having only a one-polarity supply.

This is directly related to the second phenomenon: the waveform on the center frequency becomes less distorted. The residual distortion is dependent upon the bandwidth of the tuned load, with the center frequency seeing very little distortion, but greater attenuation the farther from the tuned frequency that the signal gets. The tuned circuit resonates at one frequency, the fixed carrier frequency, and so the unwanted frequencies are suppressed, and the wanted full signal sine wave is extracted by the tuned load.

The signal bandwidth of the amplifier is limited by the Q-factor of the tuned circuit but this is not a serious limitation. Any residual harmonics can be removed using a further filter. In practical class-C amplifiers a tuned load is invariably used. In one common arrangement the resistor shown in the circuit above is replaced with a parallel-tuned circuit consisting of an inductor and capacitor in parallel, whose components are chosen to resonate at the frequency of the input signal.

Power can be coupled to a load by transformer action with a secondary coil wound on the inductor. The average voltage at the collector is then equal to the supply voltage, and the signal voltage appearing across the tuned circuit varies from near zero to near twice the supply voltage during the RF cycle.

The input circuit is biased so that the active element e. The active element conducts only while the collector voltage is passing through its minimum. By this means, power dissipation in the active device is minimised, and efficiency increased. Class-D amplifiers use some form of pulse-width modulation to control the output devices.

The conduction angle of each device is no longer related directly to the input signal but instead varies in pulse width. In the class-D amplifier the active devices transistors function as electronic switches instead of linear gain devices; they are either on or off. The analog signal is converted to a stream of pulses that represents the signal by pulse-width modulation , pulse-density modulation , delta-sigma modulation or a related modulation technique before being applied to the amplifier.

The time average power value of the pulses is directly proportional to the analog signal, so after amplification the signal can be converted back to an analog signal by a passive low-pass filter. The purpose of the output filter is to smooth the pulse stream to an analog signal, removing the high frequency spectral components of the pulses. The frequency of the output pulses is typically ten or more times the highest frequency in the input signal to amplify, so that the filter can adequately reduce the unwanted harmonics and accurately reproduce the input.

The main advantage of a class-D amplifier is power efficiency. Because the output pulses have a fixed amplitude, the switching elements usually MOSFETs , but vacuum tubes, and at one time bipolar transistors , were used are switched either completely on or completely off, rather than operated in linear mode. A MOSFET operates with the lowest resistance when fully on and thus excluding when fully off has the lowest power dissipation when in that condition.

Compared to an equivalent class-AB device, a class-D amplifier's lower losses permit the use of a smaller heat sink for the MOSFETs while also reducing the amount of input power required, allowing for a lower-capacity power supply design. Therefore, class-D amplifiers are typically smaller than an equivalent class-AB amplifier.

Another advantage of the class-D amplifier is that it can operate from a digital signal source without requiring a digital-to-analog converter DAC to convert the signal to analog form first.

If the signal source is in digital form, such as in a digital media player or computer sound card , the digital circuitry can convert the binary digital signal directly to a pulse-width modulation signal that is applied to the amplifier, simplifying the circuitry considerably.

A class-D amplifier with moderate output power can be constructed using regular CMOS logic process, making it suitable for integration with other types of digital circuitry. Thus it is commonly found in System-on-Chips with integrated audio when the amplifier shares a die with the main processor or DSP.

Class-D amplifiers are widely used to control motors —but are now also used as power amplifiers, with extra circuitry that converts analogue to a much higher frequency pulse width modulated signal. Switching power supplies have even been modified into crude class-D amplifiers though typically these only reproduce low-frequencies with acceptable accuracy.

High quality class-D audio power amplifiers have now appeared on the market. These designs have been said to rival traditional AB amplifiers in terms of quality. An early use of class-D amplifiers was high-power subwoofer amplifiers in cars. Because subwoofers are generally limited to a bandwidth of no higher than Hz, switching speed for the amplifier does not have to be as high as for a full range amplifier, allowing simpler designs.

Class-D amplifiers for driving subwoofers are relatively inexpensive in comparison to class-AB amplifiers. The letter D used to designate this amplifier class is simply the next letter after C and, although occasionally used as such, does not stand for digital. Class-D and class-E amplifiers are sometimes mistakenly described as "digital" because the output waveform superficially resembles a pulse-train of digital symbols, but a class-D amplifier merely converts an input waveform into a continuously pulse-width modulated analog signal.

Design of radio frequency high power linear and harmonically tuned amplifiers

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RF Power Amplifiers, 2nd Edition

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Engineering Notes

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Advanced Techniques in RF Power Amplifier Design

This page describes RF power amplifier design basics including RF amplifier stages, power amplifier classes, specifications, applications and manufacturers. The solid state rf power amplifier design example is also mentioned. Amplifier is the device or module which boost i. Mainly it is referred as RF power amplifier owing to its use to amplify radio frequency signal or increase power at the input to give more power at the output. Following are the RF power amplifier stages or modules.

How to Design an RF Power Amplifier: Class A, AB and B

Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. The main aim of the design is making the power amplifier linear at the center frequency 1. Also, efficiency was tried to be kept as high as… Expand. Save to Library Save. Create Alert Alert. Share This Paper. Figures and Tables from this paper.

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Control and monitoring features are available. Class A power amplifier is also called Class A power amplifier Class A , which is a completely linear amplifier. When the pure Class A power amplifier is working, the positive and negative channels of the transistor are always open regardless of whether there is a signal or not, which means that more power is consumed as heat.

Go to School on RF Power Amplifier Classes

This multi--stage structure is rated. Joined: Nov 23, It's a bias control that should be used to control class of operation and not the gain of the amplifier since doing so, will introduce distortion. The power being measured at the same level for peak and "RMS" is also

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.

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