Cascode Mixer Gain Control With Offset Tracking to Cut LO Feedthrough
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Solution Overview
Problem
Existing gain control schemes for differential amplifiers in communication devices do not adequately adjust offset voltages, leading to local oscillator feedthrough issues that can affect signal reception, especially when the signal-to-offset ratio is not sufficiently high.
Innovation Solution
A gain control network is implemented across the source terminals of cascode transistors to scale both the signal and offset currents, using a shunt network to adjust the gain of the drive current and provide gain tracking to the offset current, thereby preventing excessive local oscillator feedthrough.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If gain control is applied to the differential amplifier, then the signal gain is adjusted, but the offset voltage is not adjusted leading to LO feedthrough
Solution Approach 1:
The gain control is segmented into two independent paths: one for the signal path and one for the offset compensation path. The signal gain is controlled by adjusting the tail current of the differential pair, while the offset is independently controlled by adjusting the current in the offset compensation circuit. This segmentation allows independent optimization of signal gain and offset cancellation without one affecting the other.
Solution Approach 2:
An intermediary offset compensation circuit is introduced that mirrors the differential amplifier's gain characteristics. This intermediary circuit generates a compensating current that is proportional to both the signal gain and the offset voltage. By using this intermediary, the system can dynamically adjust offset cancellation to match the current signal gain level, preventing LO feedthrough while maintaining high signal amplification.
2Measurement precision
If the amplification factor is increased to improve signal-to-offset ratio, then signal gain is improved, but LO feedthrough increases due to mismatched transistors
Solution Approach 1:
A feedback mechanism is implemented where the offset voltage is continuously monitored and fed back to the offset compensation circuit. The compensation circuit adjusts its current output based on the detected offset level, creating a closed-loop system. This feedback ensures that as the amplification factor increases, the offset compensation automatically scales to maintain the signal-to-offset ratio while preventing LO feedthrough through precise offset cancellation.
Solution Approach 2:
The system dynamically changes the operating parameters of the offset compensation circuit to match the signal amplification level. When the amplification factor is increased, the compensation circuit's current gain is simultaneously adjusted to maintain proportional offset cancellation. This parameter coordination ensures that the offset remains negligible relative to the amplified signal, improving signal-to-offset ratio without causing LO feedthrough.
Data Source
AI summary
An apparatus and method to use a gain control network across source terminals of cascode transistors that drive a differential current to control gain. The drains of the cascode transistors are coupled to a load such as a mixer. The cascode arrangement provides high impedance coupling to the load and the proximity of the gain control network allows offsets to be gain tracked to signal gain.