Current Mode Logic Bias Circuit for Stable Output Swing
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Solution Overview
Problem
Current current mode logic circuits face challenges with variations in tail current and resistor resistance due to temperature, leading to output swing inconsistencies and bandwidth limitations, especially in low supply voltage and high-speed applications like USB 3.0 and PCIe.
Innovation Solution
A current mode logic circuit design that maintains a constant voltage drop between the supply voltage and resistor, independent of current mismatch and resistance variations, using a reference current generator and negative feedback loops to stabilize currents and resistances, ensuring consistent output swings across different temperatures and locations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If core transistors are used to reduce die space and power consumption, then area and energy usage are improved, but output swing consistency deteriorates due to temperature variations affecting tail current and resistor resistance
Solution Approach 1:
The patent implements a feedback mechanism using an operational amplifier that continuously monitors the voltage at the tail node and adjusts the tail current to maintain a constant voltage drop across the tail transistor. This feedback loop compensates for temperature-induced variations in resistor resistance and tail current, thereby maintaining consistent output swing despite using compact core transistors
Solution Approach 2:
The patent dynamically adjusts the tail current parameter based on temperature and operating conditions to maintain constant voltage drop. By changing the tail current parameter in response to environmental variations, the circuit maintains reliable output swing consistency while using space-efficient core transistors
2Stability of the object's composition
If higher supply voltage is used to increase output swing, then output swing is improved, but power consumption increases and headroom is reduced
Solution Approach 1:
The operational amplifier monitors the tail node voltage and dynamically adjusts the tail current to maintain a constant voltage drop (e.g., 0.7V) across the tail transistor. This feedback control allows the circuit to achieve stable output swing at lower supply voltages without increasing power consumption, as the system adapts the current rather than relying on fixed high-voltage operation
Solution Approach 2:
The patent transforms the static tail current into a dynamic parameter that automatically adjusts with supply voltage and temperature variations. The tail current is no longer fixed but is continuously regulated to maintain constant voltage drop, enabling the circuit to operate efficiently across varying supply voltages while maintaining consistent output swing and minimizing power consumption
3Measurement precision
If precision current mirror circuit with four MOS transistors is used to reduce current mismatch, then current accuracy is improved, but headroom is reduced due to low supply voltage under 1.0V
Solution Approach 1:
The patent extracts the voltage regulation function from the current mirror circuit itself and places it in a separate operational amplifier-based feedback loop. By taking out the voltage monitoring and adjustment function, the current mirror can use fewer transistors (reducing headroom requirements) while the external feedback loop ensures current accuracy is maintained through active regulation of the tail node voltage
4Stability of the object's composition
If resistor size is increased to obtain larger output swing at FF corner, then output swing is improved, but bandwidth is limited at SS corner
Solution Approach 1:
The patent makes the tail current dynamic and adaptive, allowing it to adjust automatically with temperature and operating corner variations. At the FF corner, the regulated tail current maintains appropriate voltage drop for sufficient output swing, while at the SS corner, the same feedback mechanism adjusts the current to prevent bandwidth degradation, eliminating the need for fixed large-resistor designs that limit speed
Data Source
AI summary
A circuit includes a bias generating circuit, an operational amplifier, and a current mode logic circuit. The operational amplifier has a first input terminal, a second input terminal, and an output terminal. The bias generating circuit is configured to provide a first bias voltage to the first terminal. The second terminal is configured to receive a second bias voltage. The second terminal and the output terminal are configured to form a negative feedback loop. The output terminal is coupled with the current mode logic circuit.