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Engineering Notes


RF POWER AMPLIFIERS TECHNICAL REFERENCE MATERIAL


Class A, AB, and C Defenition


BIAS OPERATING POINTS A, AB, C

rf amplifiers classes

Class A Operation


Class A amplifiers have a fixed forward bias.

rf amplifiers class a

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.

rf amplifiers class a

Summary of Characteristics Class A

Class A Advantages:

Excellent Linearity
Low Distortion
Faithful Pulse Response Below the 1 dB Compression Point
Broad Bandwidth
Good Noise Figure
Low Bandpass Ripple at All Output Levels
Medium Output Power Capability
Phase & Gain Stable at All Output Levels

Class A Disadvantages:

Poor Efficiency
More Heat Dissipated
Larger Size

Applications:

TWT Replacements
Sweeper/Synthesizer/Signal Generator Boosters
TV Amplifiers
Short Pulse Amplifiers
Multicarrier Amplification
Multicouplers
Baseband Amplifiers
IF and Low Noise Amplifiers
Multi Decade Amplifiers
AM Amplifiers
Laboratory Drivers
EW/ECM Jammers
RFI Testing

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.

rf amplifiers class ab

Summary of Characteristics Class AB

Class AB Advantages:

Multikilowatt Output Power
Good Efficiency
Compact Size
Good Linearity
Low Distortion
Broad Bandwidth
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

Applications:

ECM/EW Jammers
Booster Amplifiers
Transmitters
RFI/EMI Testing
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.

rf amplifiers class c

Class C Advantages:

Excellent Efficiency
Compact Size

Multikilowatt Pulse Output Power
Cool Operation
CW, FM, Phase and Pulse Amplification

Class C Disadvantages:

Poor Dynamic Range
Cannot Support AM Signals

Applications:

Troposcatter Amplifiers
FM Amplifiers
EW/ECM
Booster Amplifiers
Communications Systems
TWT Replacements
TACAN Systems
Radar Systems