RF POWER AMPLIFIERS TECHNICAL REFERENCE MATERIAL
Class A, AB, and C Defenition
BIAS OPERATING POINTS A, AB, C
Class A Operation
Class A amplifiers have a fixed forward bias.
Base current, IB, and collector current, IC, flow over the full RF cycle. In addition, when operated below compression, the RF signal swing is uniformly above and below the quiescent DC bias set point and well within the linear region of the transistor.
Summary of Characteristics Class A
Class A Advantages:
Faithful Pulse Response Below the 1 dB Compression Point
Good Noise Figure
Low Bandpass Ripple at All Output Levels
Medium Output Power Capability
Phase & Gain Stable at All Output Levels
Class A Disadvantages:
More Heat Dissipated
Sweeper/Synthesizer/Signal Generator Boosters
Short Pulse Amplifiers
IF and Low Noise Amplifiers
Multi Decade Amplifiers
Class AB Operation
Class AB Amplifiers have a small forward DC bias, IB. The collector current, IC,
(without RF drive) may be 1 % to 10% of the maximum design value.
As the RF drive is increased, the base bias (IB) and the collector current (IC) are increased proportionally.
Summary of Characteristics Class AB
Class AB Advantages:
Multikilowatt Output Power
Relatively Cool Operation
CW, AM, FM, TV, Phase and Pulse Amplification
Class AB Disadvantages:
Limited Dynamic Range (15 to 30 dB)
Restricted IMD Characteristic
Higher Bandpass Ripple at Low RF Input Levels
High Power TWT Replacements
Visual Television Amplifiers
High Power Calibration Testing
Class C operation
Class C Amplifiers are not DC forward biased. Collector current, IC, flows over significantly less
than 50% of the RF input cycle.
Class C amplifiers have a very limited dynamic range (0 to 6 dB) and have a tendency to snap off if the RF input signal is reduced below the rated level.
Class C Advantages:
Multikilowatt Pulse Output Power
CW, FM, Phase and Pulse Amplification
Class C Disadvantages:
Poor Dynamic Range
Cannot Support AM Signals