Chapter 8 electronics

All amplifiers are technically power amplifiers. Only those that handle large signals are called power amplifiers.

The power amplifier is usually the last stage in the signal processing system

power amplifiers should be efficient. Efficiency is a comparison of the signal power output to the dc power input

Power efficiency in a power amplifier means the power supply will have to be larger and more expensive. It also means that the amplifier will convert too much electrical energy to heat

Class A amplifiers operate at the center of the load line. They have low distortion and a conduction angle of 360º

Class B operates at cutoff. The conduction angle is 180º

Class B amplifiers do not present a fixed drain on the power supply. The drain is zero with no input signal. 

Class B is more efficient than class A.

Bias controls the operating point and the class of operation in analog amplifiers.

The maximum theoretical efficiency for class A operation is 25 percent. With transformer coupling, it is 50 percent. 

In a transformer-coupled amplifier, the impedance ratio is equal to the square of the turns ratio.

The fixed drain on the power supply is a major drawback with class A circuits. Efficiency is very poor when signals are small.

 A single class B transistor will amplify half the input signal

Two class B transistors can be operated in push pull

The maximum theoretical efficiency for class B is 78.5 percent

A class B amplifier draws less current from the supply for smaller signals.

For a given output power, class B transistors will require only one-fifth of the power rating needed for class A

The biggest drawback to class B push pull is crossover distortion. Crossover distortion can be eliminated by providing some forward bias for the base-emitter junctions of the transistors.

Crossover distortion can be eliminated by providing some forward bias for the base emitter junctions of the transistors

Class AB amplifiers are forward biased slightly above cutoff.

class AB operation is the most popular for high power audio work

Push pull operation can be obtained without center tapped transformers by using a PNP-NPN pair

An amplifier that uses a PNP NPN pair for push pull operation is called a complementary symmetry amplifier

complementary pairs have symmetrical characteristic curves

diodes may be used to stabilize the operating point in power amplifiers

bridged amplifiers quadruple the maximum output power for any given supply voltage and load resistance and they eliminate the output coupling capacitor

class C amplifiers are biased beyond cutoff

the conduction angle for class C is around 90º