The directional coupler is a very useful device that can be used to sample signals for analysis or control. Typically a directional coupler is used to return part of a RF power amplifier signal back to a control circuit for various functions such as controlling the output power or monitoring the RFPA performance. It is a four port device. Two ports are input port and output port which along with the connection, constitute the main line. The third port is the coupling port where the sampled signal appears. Finally there is the isolation port which is terminated in the characteristic impedance. Ideally no signal should appear there.

The directional coupler is defined by the following specifications:

Some suppliers of directional couplers are : Macom, Mini-circuits, Pulsar, Agilent, Narda Microwave .

Directional couplers can also be designed on – chip and on a board. If you need a custom directional coupler for on chip or on board application please contact us. Please visit the SPG website for more content of interest.

The following is a list of classes of RF Power amplifier and associated efficiencies:

Class A: 25% to 50%,

Class B: 78.5%;

Class AB: 50% to 78.5%.

Class C: Up to 90%.

Class D: Over 90%,

Class E: Over 80% .

Class F: 100% (ideally).

Class G: Depends on input signal power . 60% to 80%.

Class H: 80%

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When designing a bipolar transistor the collector current density has to be chosen ( and design to be) less than or equal to: 1.602E-17 NCO where NCO is the doping of the epitaxial layer in atoms/cm**3. If this current density is exceeded, the transistor will not perform well. Please visit our website for more items of interest.