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  Power Amplifiers, Physics tutorial

Introduction:

An amplifier is the device for increasing power of the signal and in this context describes the electronic amplifier with the input signal that might be the voltage or current.

Amplifiers may be categorized according to input source which they are developed to amplify, device they are intended to drive or by frequency range which they amplify. You might thus refer to the amplifier as audio amplifier, video amplifier, guitar amplifier, headphone amplifier, Intermediate Frequency, Radio Frequency and Very High Frequency amplifiers among several others. Classes A, B, AB and C are best suited for analogue applications while classes D and E/F are best for switching designs

Categorization of power amplifiers:

Categorization of power amplifiers is generally by their output stage circuit configurations and method of operation. Therefore they are either class A, B, C, and F. And class an amplifier is categorized in is closely associated to amplifier's efficiency and linearity.

Class-A: These amplifiers have highest linearity as they work in the linear portion of their characteristic. To get high linearity and gain, amplifier's active device biasing point must be cautiously chosen so that amplifier operates in linear region.

1155_Class A Amplifiers.jpg

Class B: These amplifiers operate preferably at zero quiescent current, so that dc power is small and thus, their efficiency is higher than that of class-A amplifier. Linearity is sacrificed at expense of efficiency.

2088_Class B and Class AB Configuration.jpg

Class-AB: These amplifiers are the compromise between class A and class B in terms of effectiveness and linearity. Active device is biased as close to pinch-off as possible that means it will conduct for slightly more than half a cycle, but less than full cycle of input signal.

A Class C amplifier is biased so that output current is zero for more than one half of the input sinusoidal signal cycle. The tuned circuit or filter is essential part of class-C amplifier. It is more proficient than preceding classes.

1356_Class C Configuration.jpg

Power amplifier specifications:

Some significant power amplifier specifications that are the measure of amplifier performance are given below.

Power Gain: Power gain is a ratio of output to input power estimated in decibels.

Bandwidth: A power amplifier's bandwidth is range of frequencies for which amplifier provides satisfactory performance and that are determined by half power points on output vs. frequency curve.

Efficiency: This is the measure in percentage of ratio of beneficially power applied to amplifier's output of the amplifier and power delivered by amplifier's power source.

Linearity: Power amplifier linearity is the indication of response of the power amplifier to increasing input signal until clipping and subsequent distortion commences.

Noise Figure: This is the comparison between output signal to noise ratio and thermal noise of input signal of power amplifier.

Output Dynamic Range: Given in decibels, Output Dynamic Range is a ratio between smallest and largest useful output levels.

Slew Rate: Slew rate is maximum rate of change of output in volts per second

Rise Time: Time taken for output to change from 10% to 90% of its final level when driven by the step input is known as Rise Time.

Setting Time: Setting time is time taken for output to settle to within the given percentage of final value.

Ringing: Ringing refers to the output variation, which cycles above and below the amplifier's final value.

Overshoot: This is the amount by which output exceeds the steady state value.

Stability: The main concern in power amplifiers is stability. It is directly associated to feedback and can be quantified by stability factor.

Practical limitations in power amplifiers:

Practical power amplifiers use active components like transistors that suffer the several limitations which in turn affect power amplifier operation and eventually decrease their efficiency and output power.

Practical active devices have four fundamental effects which force their operation to deviate from ideal case. These are their output resistance, Maximum output current, output breakdown voltage and avalanche breakdown voltage.

Other restrictions are high power dissipation and need for forced cooling, signal distortion; mainly at high output power levels and dynamic impedance matching of output and load.

Class A power amplifiers are plagued by their ineffectiveness because of high bias requirements that results in the high thermal dissipation requirement whereas class B suffers from high level of distortion. Class AB power amplifiers are restricted both by inefficiency when output level is low compared with maximum voltage swing. They also have thermal dissipation problems and have to be given heat sink.

Additionally class AB power amplifiers need optimum characteristic curve matching of output power transistors and identical biasing of two.

Class C power amplifiers have insufferably high level of distortion that produces unwanted harmonics of amplified frequencies. Such harmonics can be coupled through power cables and cause interference and are only decreased at expense of extra noise suppression filters.

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