Replica Amplifier Circuit for DC Offset Cancellation Without Feedback Noise
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Solution Overview
Problem
In GSM type direct conversion receivers, existing DC offset cancellation methods using feedback structures often deteriorate the noise figure characteristic as they increase feedback gain to improve performance, making it difficult to completely cancel input DC offsets during repeated switching operations.
Innovation Solution
A DC offset canceling circuit employing a replica amplifier with a storage unit, where a replica amplifier with the same structure as the main amplifier receives input signals of the same sign but with opposite output signs, and a switch maintains a DC value for a predetermined period, effectively canceling DC offsets without feedback-induced noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If feedback gain is increased to improve DC offset cancellation performance, then DC offset cancellation performance is improved, but noise figure characteristic deteriorates
Solution Approach 1:
The patent divides the DC offset cancellation function into two separate units: a sensing unit that detects DC offset voltage without affecting the main signal path, and a storing unit that holds the sensed voltage. This segmentation allows DC offset cancellation to be achieved without introducing feedback noise into the main amplifier path, thereby improving noise figure while maintaining cancellation performance.
Solution Approach 2:
The patent extracts the DC offset sensing function from the main amplifier output and places it at the antenna input through a separate sensing path. By taking out the DC offset detection function and implementing it independently with a storing unit, the system achieves DC offset cancellation without requiring feedback gain adjustment that would deteriorate noise figure.
2Measurement precision
If feedback loop is used for DC offset cancellation, then DC offset can be canceled, but the feedback circuit becomes a noise source
Solution Approach 1:
The patent introduces a storing unit as an intermediary between the sensing unit and the main amplifier input. This storing unit holds the sensed DC offset voltage and provides it to the replica amplifier without requiring a direct feedback loop. The intermediary storing unit eliminates the noise-generating feedback circuit while maintaining the DC offset cancellation function.
Solution Approach 2:
The patent uses a replica amplifier that copies the main amplifier's DC offset characteristics. By sensing the DC offset at the antenna input and copying it through the storing unit to the replica amplifier, the system achieves DC offset cancellation without using a noisy feedback loop from the main amplifier output.
3Measurement precision
If DC offset cancellation is performed during repeated switching operations, then DC offset can be removed, but complete cancellation becomes difficult
Solution Approach 1:
The patent performs DC offset sensing and storage in advance during idle periods between time slots. The storing unit maintains the stored DC offset voltage throughout the switching operations, so that DC offset cancellation is already prepared and can be applied continuously without requiring additional cancellation time during active signal transmission.
Solution Approach 2:
The storing unit automatically maintains the stored DC offset voltage without requiring continuous sensing or adjustment operations. This self-service capability allows the system to maintain DC offset cancellation effectiveness throughout repeated switching operations without additional time overhead, as the stored voltage naturally persists until the next sensing cycle.
Data Source
AI summary
A DC offset canceling apparatus includes a main amplifier, a replica amplifier, a switch, and a storage unit. The replica amplifier has substantially the same structure as the main amplifier, receives an input signal having the same sign as that of a signal input to the main amplifier, and has an output connected to main amplifier with an opposite sign. The switch switches a signal input to the replica amplifier in response to a control signal. The storage unit is connected to an input terminal of the replica amplifier to maintain a DC value for a predetermined period of time when the switch is open.