Multi-Stage Doherty Power Amplifier With SiGe Driver and GaN Output
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Solution Overview
Problem
Gallium nitride (GaN) transistors face challenges in high-power amplifier circuits due to high cost, impracticality for certain amplifier topologies, and limitations in input impedance and bandwidth, which affect the efficiency and linearity of Doherty power amplifiers.
Innovation Solution
A multiple-stage amplifier design using SiGe driver stage IC dies and GaN final stage IC dies in a cascode configuration with variable feedback capacitance to control gain expansion, enabling abrupt turn-on of the peaking amplifier for improved efficiency and linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If GaN transistors are used in Doherty power amplifiers, then power density and unit current gain frequency are improved, but input impedance mismatch and bandwidth are worsened
Solution Approach 1:
A driver stage amplifier is introduced as an intermediary component between the signal source and the GaN power amplifier. This driver stage performs impedance transformation to match the low input impedance of the GaN transistor to the standard 50 Ohm system impedance, enabling proper signal coupling without direct connection issues
Solution Approach 2:
The amplifier is divided into separate functional stages: a driver stage for impedance matching and a power amplification stage for high-power output. This segmentation allows each stage to be optimized independently - the driver stage handles impedance transformation while the GaN stage provides high power density
2Power
If GaN transistors are used in Doherty power amplifiers, then power density is improved, but manufacturing cost is worsened
Solution Approach 1:
GaN transistors are used only in the power amplification stage where their high power density and frequency characteristics are most beneficial, while the driver stage uses more cost-effective conventional transistors. This localized application of expensive materials optimizes performance where needed while controlling overall manufacturing cost
3Adaptability or versatility
If peaking amplifier transitions between off and on states, then amplifier adaptability is improved, but input impedance variation and reflection are worsened
Solution Approach 1:
The driver stage acts as an impedance buffer between the signal source and the peaking amplifier. It maintains a stable 50 Ohm output impedance regardless of the peaking amplifier's on/off state, preventing impedance variations from affecting the input return loss
Solution Approach 2:
The driver stage prepares and conditions the signal before it reaches the peaking amplifier, including impedance transformation and signal level adjustment. This preliminary conditioning ensures that the peaking amplifier receives a properly matched signal regardless of its operational state
4Power
If GaN transistors are used, then power density is improved, but device complexity is worsened due to integration challenges
Solution Approach 1:
The amplifier system is segmented into separate driver stage and power amplifier stage, allowing them to be manufactured using different processes and then integrated. This avoids the complexity of integrating GaN devices throughout the entire signal chain while still achieving high power density at the output stage
Data Source
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AI summary
A device includes an integrated circuit (IC) die. The IC die includes a silicon germanium (SiGe) substrate, a first RF signal input terminal, a first RF signal output terminal, a first amplification path between the first RF signal input terminal and the first RF signal output terminal, a second RF signal input terminal, a second RF signal output terminal, and a second amplification path between the second RF signal input terminal and the second RF signal output terminal. The device includes a first power transistor die including a first input terminal electrically connected to the first RF signal output terminal and a second power transistor die including a second input terminal electrically connected to the second RF signal output terminal. The first amplification path can include two heterojunction bipolar transistors (HBTs) connected in a cascode configuration and the second amplification path can include two HBTs connected in a cascode configuration.