Feedforward Transimpedance Amplifier for Gain-Bandwidth Tradeoffs
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Solution Overview
Problem
Transimpedance amplifiers face design complexity and performance trade-offs when amplifying large output currents, leading to issues with power dissipation, speed, output swing, and linearity, particularly due to large output transistors with high parasitic capacitances that impact frequency performance and stability.
Innovation Solution
Incorporating a controllable current source that provides a feedforward current, which changes in relation to the input current, to enhance gain and speed performance by relaxing output drive characteristics and allowing for smaller output transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If large output transistors are used to amplify large output currents, then the amplifier can handle larger currents, but parasitic capacitances increase which degrades frequency performance and stability
Solution Approach 1:
The output current is segmented into two components: a feedforward current component (Iff) that handles the majority of the current delivery, and a residual current component (Ires) that is amplified by the transimpedance amplifier. This segmentation allows the use of smaller output transistors for the amplified portion while maintaining overall current handling capability through the combined approach.
Solution Approach 2:
The feedforward current is generated in advance based on the input current magnitude, before the amplification stage processes the residual current. This preliminary action of generating Iff based on |Iin| allows the main current delivery to occur outside the feedback loop, improving stability and frequency response.
2Power
If large output transistors are used to provide large output currents, then the amplifier can deliver higher power, but device complexity increases due to design constraints and trade-offs
Solution Approach 1:
The output stage is segmented into a feedforward current source path and a feedback amplification path. This segmentation simplifies the design constraints by allowing independent optimization of each path, reducing the overall device complexity while maintaining high output power capability.
Solution Approach 2:
The feedforward current source dynamically adjusts the feedforward current magnitude based on the input current magnitude, allowing the system to adapt its operating point. This dynamic behavior simplifies design by eliminating the need for oversized transistors to handle peak currents, as the system adapts to actual signal levels.
3Power
If the amplifier operates with high gain, then signal amplification is improved, but bandwidth is reduced impacting transient signal amplification
Solution Approach 1:
The amplification function is segmented between the feedforward path (handling broadband current delivery) and the feedback path (providing high gain for residual current). This segmentation allows the system to achieve both high gain and wide bandwidth, as each path can be optimized for its specific function without compromising the other.
Solution Approach 2:
The residual current (Ires) acts as an intermediary that carries the high-frequency and transient signal components to the feedback amplifier. By separating the current components in this manner, the system achieves high gain for the residual signal while the feedforward path maintains wide bandwidth for overall signal delivery.
Data Source
AI summary
Transimpedance amplifiers with feedforward current are provided herein. In certain embodiments, an amplifier system includes a transimpedance amplifier that amplifies an input current received at an input to generate an output voltage at an output. The amplifier system further includes a controllable current source that is coupled to the output of the transimpedance amplifier, and operable to provide a feedforward current that changes in relation to the input current of the transimpedance amplifier. By providing a feedforward current in this manner, gain and speed performance of the transimpedance amplifier is enhanced.