VCSEL Driver Circuit Folded Cascode Output Impedance
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Solution Overview
Problem
Vertical-cavity surface-emitting laser (VCSEL) driver circuits face challenges in achieving high output impedance to efficiently modulate current and sustain voltage variations during ON and OFF states, which affects the modulation amplitude of optical output.
Innovation Solution
Incorporating a folded cascode transistor in the VCSEL driver circuit to enhance output impedance, isolating the VCSEL from the bias current source and using transistors with thick gate oxide for I/O functions to sustain large voltage swings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional VCSEL driver circuit is used, then the circuit structure is simple, but the output impedance is insufficient to efficiently modulate current and sustain voltage variations
Solution Approach 1:
The driver circuit is segmented into distinct functional blocks: a first current source for bias current, a second current source for modulation current, and a folded cascode stage. This segmentation allows each block to be optimized independently, with the folded cascode providing high output impedance while current sources handle current delivery, resolving the contradiction between simplicity and performance.
Solution Approach 2:
The folded cascode transistor acts as an intermediary element between the current sources and the VCSEL. It provides the necessary high output impedance to sustain voltage variations while allowing the current sources to efficiently modulate the current, thus mediating between the conflicting requirements of simple structure and high reliability.
2Speed
If transistors with thin gate oxide are used, then the device can operate at higher speeds, but the transistor cannot sustain large voltage swings required for efficient VCSEL modulation
Solution Approach 1:
Different transistor regions are assigned different gate oxide thicknesses based on their specific functional requirements. The folded cascode transistor uses thick gate oxide to sustain large voltage swings, while other transistors in the circuit can use thin gate oxide for high-speed operation. This local differentiation resolves the contradiction between speed and voltage swing capability.
Data Source
AI summary
In one embodiment, a driver circuit of a vertical-cavity surface-emitting laser (VCSEL) includes bias current sources, modulation current sources, and a switch component connected to the bias current sources at a first node and to the modulation current sources at second nodes; the switch component is configured to modulate a current from the bias and modulation current sources based on an input signal to the switch component; and the switch component is also configured to provide the modulated current to the VCSEL through a folded cascode transistor.