Doherty Implementations – The Zeroth Embodiment

Arguably, the most common (and often quickest) starting point for Doherty amplifier realization comprises:

• A fixed RF input power splitter
• A Main and Auxiliary amplifier, differently biased (e.g. using “Class AB” and “Class C”)
• A “Doherty combiner”, made from a quarter-wavelength transmission line.
• In most applications, this architecture does not provide sufficient power gain, at least not from a single, final stage. Therefore, additional gain stages are cascaded ahead of the input.

Criticisms of this most commonly used technique might include:

• No method for compensation gain-phase variations (in any domain), after design freeze.
• Both efficiency and output power are traded-off because of the bias class (in effect, an open loop analog circuit, the class C bias, is driving this)
• Efficiency enhancement of only a single stage, in a multistage cascade, limits the performance improvement, especially as gain diminishes at higher frequencies