Doherty Amplifier Output Circuit for Low-Power Impedance Control
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Solution Overview
Problem
The efficiency of Doherty amplifiers deteriorates when the peaking amplifier outputs power lower than the saturated output power due to impedance transformation into a capacitive region, leading to increased capacitive components and reduced efficiency.
Innovation Solution
Incorporating a second output circuit that transforms the impedance seen by the peaking amplifier into an inductive region when amplifying, utilizing non-linear output capacitance to reduce the inductive component of the impedance, thereby preventing efficiency deterioration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the peaking amplifier outputs power lower than saturated output power, then the output power is reduced, but the impedance transforms into capacitive region causing efficiency deterioration
Solution Approach 1:
The patent changes the impedance parameter by introducing a series inductor in the peaking amplifier output circuit. This inductor compensates for the capacitive impedance that occurs at low power levels, transforming the overall impedance back toward the real axis and maintaining efficiency across different power output levels.
Solution Approach 2:
The patent applies asymmetric impedance transformation by using different electrical lengths for the carrier amplifier path (first phase line) and peaking amplifier path (second and third phase lines). Specifically, the electrical length of the first phase line is set to be different from the sum of electrical lengths of the second and third phase lines, creating asymmetric impedance characteristics that prevent simultaneous capacitive transformation in both paths.
2Manufacturing precision
If the electrical lengths of phase lines are set to transform impedance into capacitive region, then impedance matching is achieved at high power, but efficiency deteriorates at low power
Solution Approach 1:
The patent introduces an inductor parameter to change the overall impedance characteristic. By adding this inductive element in series with the peaking amplifier output, the patent counteracts the capacitive impedance transformation, allowing impedance matching to be maintained across both high and low power operating conditions.
3Device complexity
If the peaking amplifier is designed with standard phase line connections, then the structure is simple, but the efficiency deteriorates when output power is below saturated output power
Solution Approach 1:
The patent modifies the peaking amplifier output circuit by adding a series inductor, changing the circuit parameter to compensate for capacitive impedance. This simple parameter addition maintains structural simplicity while effectively preventing efficiency deterioration at low power levels.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration maintains efficiency by transforming impedance into an inductive region, reducing the inductive component, and preventing power degradation when the peaking amplifier operates below saturated output power.
Implementation Method 1
when the peaking amplifier performs the amplification operation, the second output circuit transforms an impedance seen by looking into the combining circuit from the peaking amplifier into an impedance in an inductive region
Implementation Method 2
the peaking amplifier reduces an inductive component of the impedance seen by looking into the combining circuit from the peaking amplifier by the non-linear output capacitance of the peaking amplifier
Data Source
AI summary
This Doherty amplifier includes: a carrier amplifier for amplifying a first signal and outputting the amplified first signal; a peaking amplifier for amplifying a second signal and outputting the amplified second signal, the peaking amplifier having a non-linear output capacitance; a first output circuit for transmitting the first signal output from the carrier amplifier; a second output circuit for functioning as a virtual short stub when the peaking amplifier does not perform an amplification operation, and transmitting the second signal output from the peaking amplifier; and a combining circuit for combining the first signal transmitted by the first output circuit and the second signal transmitted by the second output circuit and outputting a combined signal of the first signal and the second signal, wherein, when the peaking amplifier performs the amplification operation, the second output circuit transforms an impedance seen by looking into the combining circuit from the peaking amplifier into an impedance in an inductive region.