Doherty amplifier Background and Challenges
The primary motivation for using a Doherty amplifier is to improve energy efficiency over a wider dynamic range, whilst maintaining quasi-linear operation.
One of the two engines for this improved efficiency is the “difference” between the output current magnitude of the Main and Auxiliary signal paths. This engine drives efficiency enhancement by “Load Modulation”.
The challenge is to enable that engine to approximate Doherty operation requirements, within the appropriate cost-performance paradigm.
Amplitude and phase matching
Amplitude and phase matching of the signals incident to the combining node, especially over frequency. Mismatching can cause degraded efficiency and output power. Potentially, it can be more destructive.
Dog-leg or hockey stick characteristic
The Doherty engine demands that the output current of the Auxiliary path exhibits a dog-leg or hockey-stick characteristic. Failing to achieve sufficient compliance here is often the primary reason for not realizing the famous efficiency peak.
Differential biasing
Using the measurement database as a look-up-table (LUT), values for efficiency and saturated output power were extracted. These can now be parametrically plotted, to demonstrate the spread in those parameters at the low- and high- frequencies.
