RF Assignments

ASSIGNMENT 7

List of Amplifier Classes of Operation and Biasing Networks. Describing their advantages and disadvantages

Class A
Class A amplifying devices operate over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. They are not very efficient; a theoretical maximum of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling.
In a Class A circuit, the amplifying element is biased so the device is always conducting to some extent, and is operated over the most linear portion of its characteristic curve (known as its transfer characteristic or transconductance curve). Because the device is always conducting, even if there is no input at all, power is drawn for the power supply. This is the chief reason for its inefficiency.

Class A Amplifier

If high output powers are needed from a Class A circuit, the power waste (and the accompanying heat) will become significant. For every watt delivered to the load, the amplifier itself will, at best, dissipate another watt. For large powers. this means very large and expensive power supplies and heat sinking. Class A designs have largely been superseded for audio power amplifiers, though some audiophiles believe that Class A gives the best sound quality, due to it being operated in as linear a manner as possible which provides a small market for expensive high fidelity Class A amps. In addition, some aficionados prefer thermionic valve (or "tube") designs instead of transistors, for several claimed reasons:
tubes are more commonly used in class A designs, which have an asymmetrical transfer function. This means that distortion of a sine wave creates both odd- and even-numbered harmonics. The claim is that this sounds more "musical" than the higher level of odd harmonics produced by a symmetrical push-pull amplifier.[1][2] Though good amplifier design can reduce harmonic distortion patterns to almost nothing, the increase in distortion in some amplifier designs is essential to the sound of intentional electric guitar distortion.
Another is that valves use many more electrons at once than a transistor, and so statistical effects lead to a "smoother" approximation of the true waveform — see shot noise for more on this. Junction field-effect transistors (JFETs) have similar characteristics to valves, so these are found more often in high quality amplifiers than bipolar transistors. Historically, valve amplifiers often used a Class A power amplifier simply because valves are large and expensive; many Class A designs uses only a single device.
Transistors are much cheaper, and so more elaborate designs that give greater efficiency but use more parts are still cost effective. 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 are not often used in output stages of op-amps; they are sometimes used as medium-power, low-efficiency, and high-cost audio 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.

Class B and AB

Class B amplifiers only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A. Class B has a maximum theoretical efficiency of 78.5%. This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single Class B element is rarely found in practice, though it can be used in RF power amplifiers where the distortion levels are less important. However Class C is more commonly used for this.

Class B Amplifier

A practical circuit using Class B elements is the complementary pair or "push-pull" arrangement. Here, complementary or quasi-complementary devices are used to each amplify the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch at the "joins" between the two halves of the signal. This is called crossover distortion. A solution to this is to bias the devices to be just on, rather than completely off when they're not in use. This is called Class AB operation.
Each device is operated in a non-linear region which is only linear over half the waveform, but still conducts a small amount on the other half. Such a circuit behaves as a class A amplifier in the region where both devices are in the linear region, however the circuit cannot strictly be called class A if the signal passes outside this region, since beyond that point only one of the devices will remain in its linear region and the transients typical of class B operation will occur. The result is that when the two halves are combined, the crossover is greatly minimised or eliminated altogether.
Class AB sacrifices some efficiency over class B in favor of linearity, so will always be less efficient. (below 78.5%) It is typically much more efficient than class A.

Class B Push-Pull Amplifier
Class B or AB push-pull circuits are the most common design type found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music 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. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are also favored in battery-operated devices, such as transistor radios.

Digital Class B
A limited power output Class-B amplifier with a single-ended supply rail of 5V +/- 10%.

Class C
Class C amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies (up to 90%) are possible. Some applications (for example, megaphones) can tolerate the distortion. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to roughly switch the amplifying device on and off, which causes pulses of current to flow through a tuned circuit.
The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be abstracted by the tuned load. Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter.

Class C Amplifier

Class D
Class D amplifiers are much more efficient than Class AB power amplifiers. As such, Class D amplifiers do not need large transformers and heavy heatsinks, which means that they are smaller and lighter in weight than an equivalent Class AB amplifier. All power devices in a Class D amplifier are operated in on/off mode. Output stages such as those used in pulse generators are examples of class D amplifiers. The term usually applies to devices intended to reproduce signals with a bandwidth well below the switching frequency.
These amplifiers use pulse width modulation, pulse density modulation (sometimes referred to as pulse frequency modulation) or more advanced form of modulation such as Delta-sigma modulation (for example, in the Analog Devices AD1990 Class-D audio power amplifier).
The input signal is converted to a sequence of pulses whose averaged value is directly proportional to the instantaneous amplitude of the signal. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The output of such an amplifier contains unwanted spectral components (that is, the pulse frequency and its harmonics) which must be removed by a passive filter. The resulting filtered signal is then an amplified replica of 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 valves and bipolar transistors were once used) are switched either on or off, rather than operated in linear mode. This means that very little power is dissipated by the transistors, except during the very short interval between the on and off states. The wasted power is low because the instantaneous power dissipated in the transistor is the product of voltage and current, and one or the other is almost always close to zero. The lower losses permit the use of a smaller heat sink while the power supply requirements are lessened too.
Class D amplifiers can be controlled by either analog or digital circuits. The digital control introduces additional distortion called quantization error caused by its conversion of the input signal to a digital value.
Class D amplifiers have been widely used to control motors, and almost exclusively for small DC motors, but they are now also used as audio amplifiers, with some extra circuitry to allow analogue to be converted to a much higher frequency pulse width modulated signal. The relative difficulty of achieving good audio quality means that nearly all are used in applications where quality is not a factor, such as modestly-priced bookshelf audio systems and "DVD-receivers" in mid-price home theater systems.
High quality Class D audio amplifiers are now, however, starting to appear in the market. Tripath have called their revised Class D designs Class T. Perhaps more famously, Bang and Olufsen's ICEPower Class D system has been used in the Alpine PDX range and some Pioneer's PRS range and for other manufacturers' equipment. These revised designs have been said to rival good traditional AB amplifiers in terms of quality.
Before these higher quality designs existed an earlier use of Class D amplifiers and prolific area of application was high-powered, subwoofer amplifiers in cars. Because subwoofers are generally limited to a bandwidth of no higher than 150 Hz, the switching speed for the amplifier does not have to be as high as for a full range amplifier. The drawback with Class D designs being used to power subwoofers is that their output filters (typically inductors that convert the pulse width signal back into an analogue waveform) lower the damping factor of the amplifier.
This means that the amplifier cannot prevent the subwoofer's reactive nature from lessening the impact of low bass sounds (as explained in the feedback part of the Class AB section). Class D amplifiers for driving subwoofers are relatively inexpensive, in comparison to Class AB amplifiers. A 1000 watt Class D subwoofer amplifier that can operate at about 80% to 95% efficiency costs about $250 USD, much less than a Class AB amplifier of this power, which would cost several thousand dollars.
The letter D used to designate this amplifier class is simply the next letter after C, and 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 (square wave) analog signal. (A digital waveform would be pulse-code modulated.)