Bias Circuit Impedance Switching for Multi-Mode PA Linearity
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Solution Overview
Problem
Current single-chain, multi-mode linear HBT RF power amplifiers experience large gain expansion when biased at lower quiescent current levels for medium or low power modes, requiring additional circuitry like FET switches to manage gain and linearity, which limits the minimum quiescent bias current achievable.
Innovation Solution
The solution involves dynamically adjusting the output impedance of the bias circuit for each gain stage to minimize gain expansion in low power mode by increasing the output resistance of the bias circuit, allowing for optimal quiescent current levels that meet target gain without the need for series attenuators or additional feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the quiescent bias current is reduced to achieve lower power consumption and gain in low power modes, then power efficiency is improved, but gain expansion increases causing linearity degradation
Solution Approach 1:
The bias circuit dynamically adjusts its output impedance based on the operating mode. In low power mode, the bias circuit transitions from a low impedance state to a high impedance state, automatically adapting to compensate for gain expansion without requiring manual intervention or additional control circuits.
Solution Approach 2:
The invention changes the output impedance parameter of the bias circuit to control the amount of negative feedback. By increasing the output impedance in low power mode, the circuit introduces appropriate negative feedback to linearize the gain response, directly addressing the linearity degradation caused by reduced quiescent bias current.
2Reliability
If additional circuitry such as series attenuators or feedback switches is added to control gain expansion, then linearity is improved, but device complexity increases
Solution Approach 1:
The bias circuit performs multiple functions: it provides the necessary DC bias current to the amplifier stages and simultaneously acts as a variable impedance element that introduces negative feedback to control gain expansion. This eliminates the need for separate feedback circuits or attenuators.
Solution Approach 2:
The bias circuit automatically adjusts its own output impedance based on the operating mode requirements. The circuit self-regulates to provide the appropriate amount of negative feedback without requiring external control signals or additional active components to manage the feedback path.
3Reliability
If the quiescent bias current is increased to reduce gain expansion, then linearity is improved, but power consumption increases
Solution Approach 1:
The bias circuit dynamically changes its output impedance characteristic based on the operating mode. In low power mode, it transitions to a high impedance state that provides negative feedback to linearize the amplifier, allowing the use of lower quiescent bias current without sacrificing linearity.
4Adaptability or versatility
If FET switches are added to manage gain in multi-mode operation, then adaptability is improved, but device complexity increases
Solution Approach 1:
The bias circuit serves as a universal solution that handles both biasing and gain control functions across all power modes. By dynamically adjusting its output impedance, it adapts to different operating conditions without requiring mode-specific control circuits or switching elements.
Data Source
AI summary
In a preferred embodiment, the gain expansion in low power mode of a single chain PA is minimized by dynamically adjusting the output impedance of the bias circuit of each gain stage for each mode of operation. Instead of switching in a series attenuator or switching in additional feedback in the first gain stage of a single-chain PA to limit the gain at the increased quiescent current level, this embodiment achieves linear performance by adjusting the quiescent current in each stage to the minimum level that meets the target gain and then increasing the output resistance of the bias circuit of each gain stage in low power mode (LPM) to provide the appropriate level of negative feedback at the base of each amplifying HBT to linearize the gain versus power response.