Parallel RF Amplifier Biasing for IM3 Distortion Cancellation
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Solution Overview
Problem
RF bipolar amplifiers suffer from third-order intermodulation distortion (IM3) due to their non-linear behavior, which causes interference with adjacent transmission channels, and existing techniques have limitations in fully addressing this issue for improved linearity.
Innovation Solution
The RF amplifier design incorporates two parallel common-emitter stages with emitter degeneration and biasing to equalize the third-order Taylor coefficients, ensuring the IM3 currents are in anti-phase, allowing for effective cancellation of the K3 coefficient and reduction of indirect mixing terms through proper impedance terminations and resonant circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If bipolar transistors are used in RF amplifiers, then high gain and efficiency are achieved, but third-order intermodulation distortion (IM3) occurs due to exponential nonlinearity
Solution Approach 1:
The amplifier is divided into two parallel common-emitter stages with different biasing conditions. Each stage processes the input signal independently, and their outputs are combined. The first stage is biased to generate IM3 currents with opposite phase to those from the second stage, enabling cancellation of the harmful distortion while maintaining high gain through the parallel configuration.
Solution Approach 2:
The biasing parameters (base voltages, emitter degeneration resistances) are specifically adjusted for each transistor to control the phase and magnitude of IM3 currents. By changing the operating point parameters of the two parallel transistors, the invention achieves IM3 cancellation while preserving the beneficial nonlinear characteristics for gain enhancement.
2Reliability
If out-of-band matching is used to reduce IM3 distortion, then linearity is improved, but the solution is insufficient for further linearity enhancement
Solution Approach 1:
The invention converts the harmful IM3 distortion into a useful cancellation mechanism. By deliberately designing the biasing conditions to generate equal-magnitude IM3 currents with opposite phases from two parallel stages, the harmful nonlinear effect is transformed into a benefit that actively cancels distortion in the output signal, achieving superior linearity beyond conventional out-of-band matching.
3Object-generated harmful factors
If two parallel common-emitter stages with different biasing are used, then IM3 cancellation is achieved, but impedance matching and stability must be maintained
Solution Approach 1:
Each parallel common-emitter stage is given different local characteristics through distinct biasing conditions and emitter degeneration values. The first transistor has specific base voltage and emitter resistance tailored to generate IM3 currents of opposite phase, while the second transistor has different parameters. This local differentiation enables IM3 cancellation while the combined output maintains proper impedance matching and stability.
Data Source
Figure 1~2
Figure 3
Figure 4A~4B
AI summary
An RF amplifier (200) is described including an input (206), an output (210), a parallel arrangement of a first branch (202a) and at least one further branch (202b), each branch comprising a bipolar transistor (T1a',T1b') in a degenerative emitter configuration having a base coupled to the input (206), a collector coupled to a common collector node (208), and an emitter degeneration impedance (Zea', Zeb') arranged between the emitter and a common rail (212). The common collector node (208) is coupled to the output (210), the base of the first branch bipolar transistor (T1a') is biased at a first bias voltage (204a) and the base of the at least one further branch bipolar transistor (T1b') is biased (204b) at a bias voltage offset from the first bias voltage (204a). In operation of the RF amplifier (200) a IM3 distortion current output (210) by the first branch bipolar transistor (T1a') is in antiphase to a IM3 distortion current output by the at least one further branch bipolar transistor (T1b').