Differential Output Circuit Voltage Divider Biasing
Solution Overview
Problem
Conventional bias supply technologies for transistors in differential output circuits, particularly in HDMI and open-drain CML circuits, fail to provide appropriate bias to cascode transistors, leading to unreliable operation due to low withstand voltage and excess voltage sensitivity.
Innovation Solution
A differential output circuit design that includes a voltage divider circuit to supply an intermediate potential as bias to the gates of cascode transistors, ensuring appropriate biasing and enhancing reliability, even with lower withstand voltage transistors, and incorporates additional features like buffer circuits, current supplies, and isolation wells to manage power and voltage effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If low voltage transistors are used to increase transmission speed and reduce power consumption, then speed and power efficiency are improved, but durability against excess voltage deteriorates
Solution Approach 1:
The invention introduces protection transistors (fifth and sixth transistors) that are activated beforehand to prevent excess voltage from reaching the low voltage transistors. These protection transistors have higher withstand voltage capabilities and are configured to clamp or limit the voltage before it can damage the sensitive low voltage transistors, thus cushioning them against future voltage spikes or excess conditions.
Solution Approach 2:
The protection transistors act as intermediary elements between the power supply and the low voltage transistors. They mediate the voltage transmission by selectively conducting or blocking based on voltage levels, allowing normal operation to pass through while intercepting and blocking excess voltage conditions before they reach the vulnerable low voltage transistors.
2Device complexity
If conventional bias supply technology is used in open-drain CML circuits, then circuit simplicity is maintained, but appropriate bias cannot be provided to cascode transistors due to missing power supply
Solution Approach 1:
The invention enables the circuit to self-generate the necessary bias voltages using existing circuit elements. The voltage divider circuit uses the existing power supply voltage and resistor components to automatically generate the required intermediate bias potentials for the cascode transistors, eliminating the need for external bias power supplies while maintaining circuit self-sufficiency.
Solution Approach 2:
The invention changes the voltage parameter by creating intermediate bias voltages through voltage division. Instead of using the full power supply voltage directly, the circuit generates scaled-down intermediate voltage levels (e.g., VDDM/2, VDDL/2) that are appropriate for biasing the cascode transistors, thus adapting the voltage parameters to match the specific requirements of different transistor stages.
Data Source
AI summary
A semiconductor device and a highly reliable circuit are realized using the transistors having a lower withstand voltage. There are provided a differential pair including a first and a second transistor which respectively receive input signals having mutually reversed phases; a third and a fourth transistor respectively cascode-coupled to the first and the second transistor, and having the same conductivity type as the first and the second transistor; a first and a second output terminal coupled to respective drains of the third and the fourth transistor; and a voltage divider circuit which divides an intermediate potential between respective potentials of the first and the second output terminal and supplies the divided potential to gates of the third and the fourth transistor.