WHAT IS A LINEAR AMPLIFIER?

From just listening to QSO’s it seem that some hams don’t quite understand the real meaning of the term “linear amplifier.”  It sounds like they think any amplifier that is added between the transmitter and the antenna is a linear amplifier.  It is true that most of these amplifiers are linear but not all.

 

This article will be very elementary to some but not everybody is at the same level.

 

The word “linear” comes from the Latin word linearis, which means created by lines.  A linear amplifier could be defined as an active electronic circuit that creates an output wave form that is an accurate reproduction of the input wave form that increases and decreases in direct proportions.  The input lines and the output lines look the same though the output lines are larger and most of the times are 180° out of phase with the input lines.  Thus if a linear amplifier has a 1 KHz sign wave input at .1 watt with an amplification factor of 100 then the output  would be a 1 KHz sign wave at 10 watt times the efficiency factor (the percentage of  output power to DC input power) of the amplifier.  If the input is increased then the output will increase in direct proportion to the input (.12 watt in equals 12 watt out and .15 input equals 15 watts and so on with the efficiency factor not changing.)

 

The difference between a linear and a non linear amplifier is determined mostly by the class of amplifier being used.

 

There are 3 basic classes of amplifiers, the A, the B, and the C with a hybrid class called AB and the AB class is divided into two sub categories the AB1 and the AB2 amplifiers.

 

A class A amplifier is biased so it conducts 100% of the time.  If an amp meter is placed in line with a class A amplifier’s output DC source it will remain constant no matter what the input, that is assuming it is not over driven.  This constant is true even if there is no input signal.   The class A is the most linear but the least efficient of the amplifiers.

 

A class B amplifier is biased at cutoff so it is conducting 50% of the time.  Thus with no signal the class B amplifier will draw no current.  It can be used as a linear amplifier if used in push pull configuration for audio frequencies and single ended if used for RF.  There is a significant increase in efficiency over a class A amplifier.

 

A class C amplifier is biased beyond cutoff.  The class C amplifier like the class B amplifier will draw no current with no signal input but a class B will start to draw current as soon as a signal is applied and a class C will only start to draw current when the signal strength is sufficient to go beyond the bias level.  The class C amplifier is not linear but is the highest in efficiency.

 

Linear amplifiers which are added after a transmitter are usually class AB which means they are biased so they will draw current without a signal but when a signal is applied the current will increase.  This class of amplifier will cutoff during a portion of the signal cycle.   The class AB amplifiers have good linearity at RF frequencies. When amplifying an AM or SSB signal a linear amplifier must be used or the audio will be distorted.  When amplifying FM, CW, RTTY, or digitally modulated RF signals a class C amplifier can be used and will give the greatest efficiency.   Thus many amplifiers sold to use on VHF FM radios are not linear and can not be used for SSB VHF transmission.

 

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