AMPLIFIER CLASSES OF OPERATION

This newsletter is a little advanced but I think it is a good topic because there is much talk about biasing classes and very few people seem to really understand it. 

The class of operation of an amplifier is determined by the amount of time (in relation to the input signal) that current flows in the output circuit. This is a function of the operating point of the amplifying device. The operating point of the amplifying device is determined by the bias applied to the device. There are four basic classes of operation for an amplifier. These are: A, AB, B and C. Each class of operation has certain uses and characteristics. No one class of operation is “better” than any other class. The selection of the “best” class of operation is determined by the use. The best class of operation for AM is not the best class for SSB. Class AB and Class C will be the only 2 classes that are used in CB radio but I am including all four types in the discussion.  Many times I say amplifier in this discussion, be aware that the output finals in your CB are amplifiers too so this discussion applies to how the radio output is biased as well as external amplifiers. 

Class A Operation

A simple transistor amplifier that is operated class A has an output signal that is a 100% (or 360º) copy of the input signal, current in the output circuit must flow for 100% of the input signal time. This is the definition of a class A amplifier. Amplifier current flows for 100% of the input signal. The class A amplifier has the characteristics of good FIDELITY and low EFFICIENCY. Fidelity means that the output signal is just like the input signal in all respects except amplitude. It has the same shape and frequency. In some cases, there may be a phase difference between the input and output signal (usually 180º), but the signals are still considered to be “good copies.” If the output signal is not like the input signal in shape or frequency, the signal is said to be DISTORTED. DISTORTION is any undesired change in a signal from input to output. The efficiency of an amplifier refers to the amount of power delivered to the output compared to the power supplied to the circuit. Since every device takes power to operate, if the amplifier operates for 360º of input signal, it uses more power than if it only operates for 180º of input signal. If the amplifier uses more power, less power is available for the output signal and efficiency is lower. Since class A amplifiers operate (have current flow) for 360º of input signal, they are low in efficiency. This low efficiency is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity. Most commonly, Stereo amplifiers use class A.

Class AB Operation

If the amplifying device is biased in such a way that current flows in the device for 51% – 99% of then input signal, the amplifier is operating class AB. A simple class AB amplifier has an output signal that is distorted. The output signal no longer has the same shape as the input signal. The portion of the output signal that appears to be cut off is caused by the lack of current through the transistor. Class AB amplifiers have better efficiency and poorer fidelity than class A amplifiers. They are used when the output signal need not be a complete reproduction of the input signal, but both positive and negative portions of the input signal must be available at the output. Class AB amplifiers are usually defined as amplifiers operating between class A and class B because class A amplifiers operate on 100% of input signal and class B amplifiers (discussed next) operate on 50% of the input signal. Any amplifier operating between these two limits is operating class AB. This is the most common biasing type used in radio communication. This is suitable for AM/FM and SSB type communication. Texas Star amplifiers all use class AB (except the Mod V).

Class B Operation

As was stated above, a class B amplifier operates for 50% of the input signal. In a class B amplifier only one half of the waveform is reproduced. You may wonder why a class B amplifier would be used instead of a simple rectifier if only half the input signal is desired in the output. The answer to this is that the rectifier does not amplify. The output signal of a rectifier cannot be higher in amplitude than the input signal. The class B amplifier not only reproduces half the input signal, but amplifies it as well. Class B amplifiers are twice as efficient as class A amplifiers since the amplifying device only conducts (and uses power) for half of the input signal. A class B amplifier is used in cases where exactly 50% of the input signal must be amplified.

Class C Operation

If less than 50% of the input signal is needed, a class C amplifier is used. Notice that only a small portion of the input signal is present in the output signal. Since the transistor does not conduct except during a small portion of the input signal, this is the most efficient amplifier. It also has the worst fidelity. The output signal bears very little resemblance to the input signal. Class C amplifiers are used where the output signal need only be present during part of one-half of the input signal. Any amplifier that operates on less than 50% of the input signal is operated class C. In Communication amplifiers the transistors are in a push-pull configuration and create what they call the “Fly Wheel Effect” and even though the Class c signal was only a small portion of the positive, the rest of it and the negative side are reformed at the output of the amplifier by the “Fly Wheel Effect”. This method creates very little discernable distortion on AM and FM transmitted signals.  It is very common for AM and FM type amplifiers to use class C. Most Dave Made, Xforce and Fatboy amplifiers use class C and are unsuitable for SSB communication. Many amplifiers put a delay switch on the front for the antenna relay and call it SSB mode without actually changing to class AB thus they are too distorted for good SSB regardless of the label on the front. Another item to note is that the amplifiers with Class C biasing can withstand higher voltages. Competition high output amplifiers like the ones previously listed can be operated at 18V input and output considerable extra power. If you apply 18V to a Class AB amplifier the transistors will be destroyed.  Many people convert their class AB amplifiers to Class C so they can use them in competition service at 16-18 VDC. Converting the amplifiers to Class C is relatively easy and inexpensive. Converting to Class AB on the other hand, is a little more pricey and requires many extra parts and time.

REVIEW

• What determines the class of operation of an amplifier?

• What are the four classes of operation of a transistor amplifier?

• If the output of a circuit needs to be a complete representation of one-half of the input signal, what class of operation is indicated?

• Why is class C operation more efficient than class A operation?

• What class of operation has the highest fidelity?