Linear amplifier

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

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, a transmitter driving the input with 100 watts 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. However, AM radio broadcast transmitters of up to 50 kW are now solid state. Large vacuum valves or klystrons are still used for international long, medium, and shortwave broadcast transmitters between 500 kW up to 2 MW. Usually, several 500 kW transmitters are used in parallel.

The legal power limit for licensed amateur operators vary from country to country but in the United States it is legal to transmit up to 1.5 kW peak envelope power, while in the UK the limit is 400 watts PEP.

As most amateur radio transceivers can output between 100 and 150 watts, an amplifier is needed to reach higher power levels. Large vacuum tube linear amplifiers 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 operate with class A or class AB biasing, which makes them relatively inefficient. While class C has far higher efficiency, a class C amplifier is not linear, and is only suitable for the amplification of constant envelope signals. Such signals include FM, FSK, and CW (morse code).

Amplifier classes

There are a number of amplifier classes providing various trade-offs between implementation cost, efficiency, and signal accuracy. Their use in RF applications are listed briefly below:

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

• Class B can be 60 to 65% efficient. The semiconductor or vacuum tube conducts through half the RF cycle but require large drive power.

• 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.

• 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 vacuum tube 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

The first large amplifier used in the United States for public domestic radio broadcasting was in operation between 1934 and 1939 at WLW in Cincinnati. It was an experimental amplifier and was driven by the radio station's regular 50 kW transmitter. Not a linear amplifier, it operated in Class C with high-level plate modulation. The amplifier required a dedicated 33kV electrical substation and a large pond complete with fountains for cooling. It operated with a power input of about 750 kW (plus another 400 kW of audio for the modulator) and its output was 500 kW.

See also

• Amplifiers

v • d • e Transistor amplifiers Bipolar junction transistor: Common emitter • Common collector • Common base Field-effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington transistor • Sziklai pair • Cascode • Long-tailed pair