Linear Amplifier Facts Worth Noting
A linear amplifier is basically one kind of electronic circuit. With this circuit, it has a proportional output and input yet delivers more power straight into a load. Usually, a linear amplifier is used to denote a kind of radio frequency power amplifier. Some of them have the power to their output measured in kilowatts that is why they are often used in amateur radios. These linear amplifiers are also being used in laboratory and audio equipment.
The concept of linearity means that the amplifier has the ability to create very accurate input copies for the signals that they produce. Such signals can be produced with high power levels. The efficiency of linear amplifiers, though, is affected by certain factors. These include input base current, load impedance, power output, and supply voltage capabilities.
Amplifiers come in different classes. There are the Class A amplifiers that offer you good linearity whether you get them in push-pull or single-ended variants. For the amplifiers belonging to classes B, AB1, and AB2, they can only be linear when the use of a tuned tank circuit is employed. For these classes of amplifiers, though, with push-pull topology, there must be two active elements that can help amplify both positive and negative parts. These elements are transistors and tubes. Finally, you have the Class C amplifiers that are not linear in any way.
For these different classes of amplifiers, they have a variety of uses. You can expect their efficiency, implementation cost, and signal accuracy to be different. In terms of radio frequency applications, they also differ.
The most inefficient of these different linear amplifiers are the Class A ones. When it comes to their efficiency rating, it does not go beyond 50%. It is either the vacuum tube or semiconductor that will conduct throughout the radio frequency cycle. For a vacuum tube, the mean anode current should be set to the middle linear section of the curve found on the anode current.
Class B linear amplifiers, on the other hand, can be between 60 and 65% efficient. With the use of a large drive power, conduction in half the cycle can be achieved with their vacuum tube or semiconductor.
The grid of Class AB1 linear amplifiers compared with that of Class A linear amplifier is negatively biased. Meanwhile, Class AB2 linear amplifiers have a grid that is more negatively biased than that in Class AB1 linear amplifiers. Moreover, the input signal has been shown to be larger. Once the drive can make the grid positive, the grid current will also increase further.
Nonetheless, the most efficient linear amplifiers are the Class C variant. Getting an efficiency of 75% is expected when a 120-degree conduction range is achieved. But then, they are not so linear. This implies then that non-AM modes are their only use with the likes of FM, RTTY, and CW.