Audioclinic (Sept. 1977)

Classes of Amplifiers

Q. What are the various classes of amplifiers? Please give a simple explanation of the differences between them, along with their advantages and disadvantages.

Neall Dralle, Sacramento, Cal.

A. There are a number of classifications into which amplifiers fall. There is class A, B, AB1, AB2, C, and D. The Class A amplifier is one whose output circuit is biased in the center of its operating curve between saturation at one extreme and cut-off at the other. Thus, the current rises and falls symmetrically as the input swings above and below its idling position.

Because of this linear relationship between output and input, this circuit can be run as either a single-ended or a push-pull output.

A Class B amplifier is one which is biased to its cut-off point, and under these conditions only a very small amount of current will flow when no signal is applied to the input, which makes it much more efficient than a Class A stage where a considerable amount of current flows when no signal is applied to the input. Because of the non-linear characteristics of a Class B amplifier, it can be used for audio applications only in a push-pull circuit. While the Class B amplifier is much more efficient than the Class A, it also tends to have more distortion.

Besides Class A and Class B, there are two intermediate classes, known as Class AB1 and AB2. Class AB1 amplifiers are biased more towards cutoff than a true Class A amplifier, but not so close to it as a Class AB2 amplifier. Because these two classes of audio -amplifiers are operated somewhere in their nonlinear region, they must be run in push-pull so that some of the distortion can be cancelled.

The Class C amplifier is biased to a voltage twice that required to cut the stage off. This amplifier can never conduct for anything like a full cycle, even in a push-pull arrangement, therefore these circuits are never used for audio applications.

There is a more modern class of amplifiers known as Class D which operate on a much different principle than merely adjusting the bias point. The Class D amplifier is turned off and on with a very high frequency signal, thus the output of the amplifier contains this switching signal and special circuits are required at the output of the amplifier to reconstitute the signal from the bits and pieces actually presented. Because it is conducting for very brief intervals, it runs very efficiently, but it is a complex piece of equipment to maintain.

Finally, there is a Class G amplifier from Hitachi which is designed to operate more efficiently over more of the operating range. It boasts an efficiency of 90 percent or better. Instead of using two transistors, as is the case with Class B and D amplifiers, the Class G uses four. The smaller pair has relatively low power dissipation and output, but whenever a higher amplitude signal comes along, a sensing diode switches on the larger pair of transistors. Consequently, each pair of transistors operates in close to its most efficient operating area, and the overall circuit can be smaller and lighter.

Television Interference with AM Reception

There is an almost perfect remedy for television interference to AM radio reception. It's very simple, but usually only practical for private dwellings.

The cure is an outdoor antenna with shielded lead-in. A simple dipole, constructed from ordinary insulated wire and placed in my attic eliminated all television "whistling." The antenna requires no insulators.

Simply tack as long a wire as possible along the attic beams, break it at the center, and connect an appropriate length of RG 59/U between this dipole and the receiver input terminals.

One of the leads from the center of the dipole is connected to the "hot" lead, or center conductor of the cable; the other dipole lead is connected to the shield of the RG 59/U.

-Michael Stosich, Bolling Brook, Ill., USA.

(Source: Audio magazine, Sept. 1977; Joseph Giovanelli)

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