Bypass-Biased Mixer Circuit for Low-Frequency Flicker Noise
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Solution Overview
Problem
Direct conversion and Low-IF systems in radio communication receivers suffer from deteriorated reception sensitivity due to low-frequency noise characteristics of mixer circuits, particularly from MOS devices, which conventional techniques fail to effectively address without compromising gain or frequency characteristics.
Innovation Solution
A mixer circuit design that includes a bypass current supply portion to reduce bias current in the LO signal processing portion without affecting the RF signal processing portion, thereby reducing flicker noises and improving noise characteristics at lower frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If bias current is reduced in the LO signal processing portion to reduce flicker noises, then noise characteristics at lower frequencies are improved, but gain of the mixer is reduced
Solution Approach 1:
The bias current supply is segmented into separate paths: a first bias current supply for the RF signal processing portion and a second bias current supply for the LO signal processing portion. This allows independent control of bias currents in each portion, enabling reduction of flicker noise from the LO portion without compromising the gain contribution from the RF portion.
Solution Approach 2:
Different bias current characteristics are applied to different portions of the mixer. The RF signal processing portion receives a bias current optimized for gain, while the LO signal processing portion receives a bias current optimized for low flicker noise. This local optimization resolves the contradiction by allowing each portion to have tailored current characteristics.
2Object-affected harmful factors
If bias current in the LO signal processing portion is reduced to improve noise characteristics, then reception sensitivity is improved, but the overall performance of the mixer deteriorates
Solution Approach 1:
The bias current supply is segmented into separate paths: a first bias current supply for the RF signal processing portion and a second bias current supply for the LO signal processing portion. This allows independent control of bias currents in each portion, enabling reduction of flicker noise from the LO portion without compromising the gain contribution from the RF portion.
Solution Approach 2:
Different bias current characteristics are applied to different portions of the mixer. The RF signal processing portion receives a bias current optimized for gain, while the LO signal processing portion receives a bias current optimized for low flicker noise. This local optimization resolves the contradiction by allowing each portion to have tailored current characteristics.
Data Source
AI summary
A mixer circuit of a receiving system, requiring low noise characteristics at lower frequency, includes a bypass current source (41) which is connected in parallel with an LO transistor (21) between an IF output terminal (33) and a drain terminal of an RF transistor (11), and a bypass current source (42) which is connected in parallel with an LO transistor (22) between an IF output terminal (34) and the drain terminal of the RF transistor (11), thereby decreasing a current flowing through the LO transistors (21, 22) without decreasing a bias current flowing through the RF transistor (11). Accordingly, it is possible to suppress flicker noises occurring from the LO transistors (21, 22) without lowering the gain of the mixer, thereby realizing a mixer circuit with excellent low frequency noise characteristics, which can improve the NF characteristics at lower frequency.