If you say linear amplifier, you are referring to a type of electronic circuit. With this circuit, it has a proportional output and input yet delivers more power straight into a load. This is also the term that is used to refer to a power amplifier that is powered by radio frequency. You find this particular circuit in amateur radios where some of the power in terms of output can be measured in kilowatts. These linear amplifiers are also being used in laboratory and audio equipment.
Linearity is the concept that enables these amplifiers to produce signals that are more or less the same as the input. Power levels should be at increased numbers for the signals to be produced. A few factors, however, have some effect on how linear amplifiers are able to function. These include input base current, load impedance, power output, and supply voltage capabilities.
There are different classes of amplifiers. You have the so-called Class A amplifiers that can showcase good linearity for both push-pull and single-ended variants. For Class AB1, AB2, and B amplifiers, though, they require a tuned tank circuit before they can be linear. Moreover, for these classes of amplifiers with push-pull topology, there must be two active elements for the positive and negative parts. Transistors and tubes are these two active elements. And last, you have the so-called Class C amplifiers that are not linear at all.
There are different uses when it comes to these amplifier classes. You can expect their efficiency, implementation cost, and signal accuracy to be different. They have different uses in radio frequencies.
Class A linear amplifiers are not that efficient. When it comes to their efficiency rating, it does not go beyond 50%. In terms of conduction for the whole radio frequency cycle, either a vacuum tube or semiconductor helps in this regard. The curve of the anode current of the vacuum tube should have a mean anode current that is set to the middle portion of the linear section.
Meanwhile, Class B linear amplifiers are 60 to 65% efficient. Its vacuum tube or semiconductor can conduct within half the cycle, but to do so, a large drive power is required.
Compared with Class A linear amplifiers, Class AB1 has a negatively biased grid. Meanwhile, Class AB2 linear amplifiers have a grid that is more negatively biased than that in Class AB1 linear amplifiers. Furthermore, the input signal in these linear amplifiers is oftentimes larger. You can expect the grid current to increase as well when the drive can turn its grid positive.
Even so, Class C linear amplifiers are the most efficient. With a conduction range of 120 degrees, they can go as much as 75% efficient. 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.