A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio. Other types of linear amplifier are used in audio and laboratory equipment.

Explanation

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An RF linear amplifier can be based on either solid state or vacuum tube technology. Most commercially manufactured one to two kilowatt linear amplifiers used in amateur radio still use vacuum tubes (valves) and can provide between 10 to 20 times RF power amplification. For example 100 watts input from a transmitter will be amplified to 2000 watts (2 kW) output to the antenna. Solid state linear amplifiers are more commonly in the 500 watt range and can be driven by as little as 25 watts, although AM radio broadcast transmitters of up to 50 kW are now solid state. For International long, medium and Shortwave broadcasting between 500 kW up to 2MW large vacuum valves or klystrons are still used, usually several 500 kW transmitters in parallel.

The legal power limit for licenced 'ham' operators vary from country to country but in the United States it is legal to transmit up to 1.5 kW(PEP). While in the UK the limit is 400 watts peak envelope power.

As most amateur radio transceivers for output between 100 and 150 watts an amplifier is needed to reach 2000 or 400 watts. Large valved linear amplifies are based on old radio broadcast techniques and generally rely on a pair of large vacuum tubes supplied by a very high voltage power supply to convert large amounts of electrical energy into Radio Frequency energy. Linear amplifiers need to be in class A, class AB. A class C amplifier is not linear, the class C amplifier is suitable for the amplification of simple sine waves only, such signals include FM and A1A morse (keyed carrier).

Classes of amplifier

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• The class A amplifiers are very inefficient, they can never have an efficiency better than 50%. The semiconductor or valve conducts throughout the entire RF cycle. The mean anode current for a valve should be set to the middle of the linear section of the curve of the anode current vs grid bias potential.

• Class B amplifiers are more efficient, they can be 60 to 65% efficient. The semiconductor or vacuum tube conducts through half the RF cycle.

• Class AB1 is where the grid is more negatively biased than it is in class A.

• Class AB2 is where the grid is often more negatively biased than in AB1, also the size of the input signal is often larger. When the drive is able to make the grid become positive the grid current will increase.

• In a class B amplifier the grid current drawn will be large, and a large drive power will be required.

• Class C amplifiers are still more efficient, they can be about 75% efficient with a conduction range of about 120^{o but they are very non linear. They can only be used for FM or CW use only. The semiconductor or valve conducts through less than half the RF cycle. The increase in efficiency can allow a given valve to deliver more RF power than it could do so in class A or AB. For instance two 4CX250B tetrodes operating at 144 MHz can deliver 400 watts in class A, but when biased into class C they can deliver 1000 watts without fear of overheating. Even more grid current will be needed.}

A side effect of improving the efficiency is that the current drawn from the high voltage supply will vary more as a function of the power input into the amplifier, this can result in unwanted effects such as the output of the HT pack being modulated by the audio modulated RF driven into the amplifier. An extreme example of this has been observed during radio contests where a large linear is used to amplify morse (carrier on/off keying), it has been the case under some conditions that the wildly changing load on the petrol-driven 240 volt 50 Hz generator set has been sufficient to cause the petrol motor to change speed (and supply frequency) as it attempts to maintain its AC output voltage at 240 volts. In short any person able to hear the petrol engine will then be able to hear the morse.

A simple cure for this is to always attach a fixed small load such as several light bulbs to the output of the 240 volt AC generator.

Early large amplifiers

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The first large linear amplifier used in the United States for public domestic radio broadcasting was used between 1934 and 1939 at WLW in Cincinnati. It was an experimental amplifier and was driven by the radio stations regular 50 kW transmitter. The amplifier required a dedicated 33Kv electrical substation and a large pond complete with fountains for cooling and output 500 kW.

Reference

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Radio communication handbook (5th Ed), Radio society of Great Britain

Electronic amplifiers