Doherty Amplifier Adaptive Biasing for Low-Voltage Linearity
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Solution Overview
Problem
Doherty power amplifiers exhibit suboptimal performance at low average output power and low power supply voltage due to insufficient self-biasing of the peaking amplifier, leading to significant amplitude modulation-amplitude modulation distortion and power loss, which is critical for cell phone operations.
Innovation Solution
A peaking power supply adaptive bias generator senses the supply voltage and adjusts bias currents to the peaking output transistors, ensuring sufficient load modulation across a wide range of supply voltages, including the low to mid voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the Doherty power amplifier is configured to achieve best efficiency and linearity at maximum average power, then efficiency and linearity are improved at peak power, but performance deteriorates at low average output power and low supply voltage due to insufficient self-biasing
Solution Approach 1:
The bias current of the peaking amplifier is made dynamic rather than fixed. The adaptive bias generator continuously adjusts the bias current based on the instantaneous supply voltage level, allowing the amplifier to adapt its operating point in real-time. This resolves the contradiction by enabling the amplifier to maintain optimal performance across varying supply voltages while preserving the original peak power optimization.
Solution Approach 2:
The invention implements a feedback mechanism where the supply voltage is sensed and used to control the bias current of the peaking amplifier. The adaptive bias generator monitors the supply voltage and adjusts the bias accordingly, creating a closed-loop control system. This feedback approach allows the amplifier to automatically compensate for supply voltage variations and maintain reliable performance across the full voltage range.
2Device complexity
If the peaking amplifier uses fixed bias current, then the amplifier structure is simple, but significant AMAM distortion and P1 dB power loss occur at low to mid supply voltage range
Solution Approach 1:
An adaptive bias generator circuit is introduced as an intermediary component between the power supply and the peaking amplifier. This intermediate stage processes the supply voltage and generates the appropriate bias current dynamically. The bias generator acts as a mediator that translates supply voltage variations into corresponding bias current adjustments, thereby improving linearity and power output without significantly complicating the overall amplifier structure.
Solution Approach 2:
The invention changes the bias current parameter dynamically based on supply voltage conditions. Instead of using a fixed bias current, the system adjusts the bias current magnitude according to the instantaneous supply voltage level. This parameter change enables the peaking amplifier to maintain proper operating conditions across different voltage ranges, reducing AMAM distortion and preserving P1 dB power while adding minimal complexity through the adaptive bias generator.
3Adaptability or versatility
If the peaking amplifier self-biasing is insufficient at low supply voltage, then the amplifier can operate at low voltage, but efficiency and power output are compromised in the medium to low power range
Solution Approach 1:
The adaptive bias generator performs preliminary action by pre-adjusting the bias current before the peaking amplifier processes the signal. By anticipating the supply voltage level and setting the appropriate bias current in advance, the system ensures the peaking amplifier is properly conditioned for efficient operation at any given voltage level. This preliminary bias adjustment prevents efficiency losses that would otherwise occur due to insufficient self-biasing at low voltages.
Data Source
AI summary
An amplifier is disclosed having a carrier amplifier and a peaking amplifier coupled in parallel with the carrier amplifier, wherein the peaking amplifier has peaking output transistors. A peaking power supply adaptive bias generator is coupled to bias control terminals of the peaking output transistors. The peaking power supply adaptive bias generator is configured to sense supply voltage to the peaking amplifier and increase bias currents to the peaking output transistors as the supply voltage decreases.