Traveling Wave Amplifier Biasing to Suppress Output Jitter
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Solution Overview
Problem
Travelling wave amplifiers (TWAs) with cascade connections often introduce substantial jitter in output signals due to variations in base-emitter resistance and emitter current, especially when switching transistors fully turn on or off, affecting high-speed optical communication systems.
Innovation Solution
The TWA design incorporates a differential circuit with a pair of switching transistors and cascade transistors connected in series, along with current sources that provide idle currents to the cascade transistors, even when switching transistors are fully turned off, to stabilize the base-emitter resistance and reduce jitter. This configuration ensures consistent operation and minimizes the impact of variations in device characteristics and operating conditions across the amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If cascade connection of HBTs is used in TWA, then amplification capability is improved, but jitter in output signal increases substantially
Solution Approach 1:
The amplifier is divided into multiple parallel unit amplifiers instead of using a single cascade connection. Each unit amplifier processes a portion of the signal, and their outputs are combined. This segmentation eliminates the cumulative jitter problem of cascade connections while maintaining amplification capability through parallel processing.
Solution Approach 2:
Instead of connecting amplifiers in series (cascade), the invention inverts the approach by connecting them in parallel. This fundamental topological inversion allows the signal to be amplified by multiple independent paths simultaneously, preventing the jitter accumulation that occurs in series connections.
2Speed
If switching transistors are fully turned on or off, then switching speed is improved, but base-emitter resistance variation increases causing jitter
Solution Approach 1:
The circuit maintains a predetermined minimum emitter current even when switching transistors are fully turned off. This preliminary action of pre-establishing a current floor prevents the base-emitter resistance from varying excessively, thereby reducing jitter while allowing the switching transistors to operate at full speed.
Solution Approach 2:
The invention changes the operating parameter of the cascade transistors by maintaining a minimum emitter current rather than allowing it to drop to zero. This parameter change stabilizes the base-emitter resistance during switching transitions, reducing jitter without compromising switching speed.
Data Source
AI summary
A traveling wave amplifier (TWA) with suppressed jitter is disclosed. The TWA includes a plurality of unit amplifiers with the differential arrangement comprised of a pair of transistors and a cascade transistors connected in series to the switching transistors. The unit amplifiers further includes current sources to provide idle currents to the cascade transistors. Even when the switching transistors fully turn off, the idle currents are provided to the cascade transistors, which set the operating point of the cascade transistor in a region where an increase of the base-emitter resistance is suppressed.