Dynamic Bias CMOS VCO for High Amplitude at Low Supply Voltage
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Solution Overview
Problem
Voltage headroom requirements for startup and operation of CMOS complementary voltage-controlled oscillators (VCOs) are challenging with low supply voltages, as they require higher voltages to start up the gain stage, which can reduce oscillation amplitude and increase phase noise when using static bias voltage solutions.
Innovation Solution
A dynamic bias circuit that adjusts the gain stage bias voltage in response to current source feedback voltage changes after the tank circuit has started up, allowing the use of low supply voltages while maintaining high oscillation amplitude and reducing phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a static bias voltage is applied to gates of output transistors in the gain stage, then sufficient voltage headroom is provided for startup of the tank circuit, but the amplitude of oscillations is reduced and phase noise increases
Solution Approach 1:
The patent applies a dynamic bias voltage to the gates of output transistors instead of a static bias voltage. The dynamic bias circuit adjusts the bias voltage in real-time based on the oscillation state, providing higher bias voltage during startup to ensure reliable tank circuit oscillation, and reducing the bias voltage during steady-state operation to maintain high oscillation amplitude and low phase noise.
Solution Approach 2:
The dynamic bias circuit provides preliminary high bias voltage to the gain stage before the tank circuit reaches steady-state oscillation. This preliminary action ensures that the tank circuit has sufficient voltage headroom to start up reliably, and then automatically reduces the bias voltage once oscillation is established, preventing the harmful effects of static bias voltage on oscillation amplitude and phase noise.
2Strength
If additional output transistors are added to the gain stage in a CMOS complementary VCO, then the oscillation amplitude is improved, but the voltage headroom requirement increases
Solution Approach 1:
The patent uses a dynamic bias circuit that adjusts the bias voltage supplied to the additional output transistors in the gain stage. During startup, the dynamic bias provides higher voltage to ensure the tank circuit achieves sufficient oscillation amplitude with the additional transistors. During steady-state operation, the bias voltage is reduced to lower the voltage headroom requirement, enabling operation with low supply voltages while maintaining good supply rejection.
Data Source
AI summary
According to an exemplary embodiment, a high amplitude oscillation generator includes an LC tank circuit, a gain stage, a dynamic bias circuit, a bias current source, and a dynamic bias circuit receiving a current source feedback voltage and outputting a gain stage bias voltage. The dynamic bias circuit adjusts the gain stage bias voltage in response to a change in the current source feedback voltage after a start up of the LC tank circuit. The dynamic bias circuit thereby increases an amplitude of oscillations produced by the oscillation generator. The dynamic bias circuit can include an error amplifier, the error amplifier generating the gain stage bias voltage responsive to the current source feedback voltage. The current source feedback voltage can change with a voltage drop across the bias current source. The current source feedback voltage can also be received from an output of the oscillation generator.