Differential Power Amplifier Bias Circuit for Harmonic Rejection
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Solution Overview
Problem
The nonlinearity of power amplifiers, particularly the 3rd order inter-modulation, affects the adjacent channel leakage ratio (ACLR) performance, necessitating a solution to enhance linearity and ACLR performance.
Innovation Solution
A power amplifier system with a bias circuit that includes a source follower and a harmonic rejection circuit, providing a stable bias voltage with low output impedance to reject kick-back harmonics, thereby improving the symmetry of 3rd order inter-modulations and ACLR performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a bias circuit is used to stabilize the bias voltage, then the bias stability is improved, but the output impedance increases making the bias circuit vulnerable to kick-back harmonics
Solution Approach 1:
A capacitor is introduced as an intermediary component between the bias circuit output and the power amplifier input. This capacitor acts as a frequency-selective mediator that blocks high-frequency kick-back harmonics from reaching the bias circuit while allowing the DC bias voltage to pass through, thus protecting the bias stability without requiring high output impedance
Solution Approach 2:
The harmful kick-back harmonics are extracted and isolated from the bias voltage path using a capacitor. The capacitor separates the DC bias component (which needs to be stable) from the AC harmonic component (which is harmful), allowing the bias circuit to maintain stability without being affected by the harmonics
2Stability of the object's composition
If the bias circuit output impedance is increased to improve bias stability, then the bias voltage stability is improved, but the circuit becomes more susceptible to power amplifier kick-back harmonics
Solution Approach 1:
The capacitor serves as a frequency-selective intermediary that provides low impedance path for DC bias voltage while presenting high impedance to AC kick-back harmonics. This resolves the contradiction by achieving both bias stability and harmonic rejection without requiring high output impedance
Solution Approach 2:
The bias circuit is designed with different impedance characteristics at different frequencies: low output impedance for DC bias voltage to ensure stability, and high impedance at harmonic frequencies through the capacitor to reject kick-back harmonics. This local differentiation of impedance properties resolves the contradiction
Data Source
Figure 1
Figure 2(a)~2(d)
Figure 3
AI summary
A power amplifier system (100) includes a differential power amplifier (1 10) and a bias circuit (120). The differential power amplifier (110) is arranged for receiving a differential input pair to generate an output signal. The bias circuit (120) is arranged for generating a bias voltage to bias the differential power amplifier (110), and the bias circuit (120) comprises a source follower (122) for receiving a reference voltage to generate the bias voltage.