Amplifiers

Engr. Aaron Don M. Africa

RF Amplifiers

RF power amplifier is the last active stage before the transmitting antenna. It provides the power amplification necessary for the antenna to radiate RF signals.

Class A – The amount of the output signal flow varies for a full 360 degrees of the cycle

-The Q point is usually set at ½ the supply voltage so that the output signal can have a maximum output swing

-The maximum efficiency is 25%

-50% efficiency can be obtained by transformer coupling

Class B – The amount of the output signal is 180 degrees

-The Q point is set at the cut-off region

-The maximum efficiency is 78.5%

-In most practical amplifiers, the Q-point is set just a little above cut-off to eliminate crossover distortion

Class AB – The amount of the output signal flows for more that 180 degrees but less than 360 degrees

-The Q point is set between class A and class B level

- Efficiency is less than 78.5%

-Is at the active region but near cut-off

Class C – The output signal flows less than 180 degrees

-The Q point is set below cutoff region

- Efficiency is not an important factor for a class C amplifier, for it is not usually designed to deliver large amount of power

-Used in HF oscillators

Class A Class B Class AB Class C

Q-point Active Region

Cuff-off region

A little above cutoff

Below cutoff

Conduction Angle

360° 180 ° 180 ° -359 ° Less than 180°

Distortion low high moderate Very high

Max Efficiency

25% & 50% for transformer coupled

78.5% Between class B and A

More than 90%