Class AB Amplifier Input Feed-Forward for Wide-Band Stability
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Solution Overview
Problem
Class AB amplifiers become unstable and power hungry when applied to wide-band applications due to parasitic poles from pads, making design difficult and inefficient.
Innovation Solution
Incorporating an input feed-forward path into a cascoded circuit and using a class AB control circuit to avoid positive feedback in the common mode loop, which reduces the poles of the overall transfer function and improves stability by using high-pass filters and short circuits between cascoded transistors to enhance common mode rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a class AB amplifier is applied to wide-band applications, then the bandwidth is increased, but the amplifier becomes unstable due to parasitic poles from pads
Solution Approach 1:
The amplifier is divided into separate functional blocks: an amplifying stage and a cascoded circuit with feed-forward path. This segmentation allows independent optimization of each block's stability characteristics while achieving wide bandwidth through their combined operation, preventing the parasitic poles from causing overall instability.
Solution Approach 2:
An input feed-forward circuit is introduced as an intermediary element that feeds the differential input pair forward to the cascoded circuit. This intermediary path compensates for the destabilizing effect of parasitic poles by providing an alternative signal path that maintains stability across wide bandwidth.
2Speed
If a class AB amplifier is applied to wide-band applications, then the bandwidth is increased, but the design becomes difficult and power consumption increases
Solution Approach 1:
The amplifier employs dynamic class AB control that adjusts the operating point based on signal conditions. This dynamic operation allows the amplifier to achieve wide bandwidth when needed while reducing power consumption during low-signal conditions, avoiding the need for continuous high-power operation.
Solution Approach 2:
The amplifier utilizes parameter changes in the cascoded circuit transistors through the feed-forward path to extend bandwidth without proportionally increasing power consumption. By changing the effective operating parameters of the cascoded transistors via the feed-forward input pair, wide bandwidth is achieved with moderate power overhead.
3Stability of the object's composition
If an input feed-forward path is added to the cascoded circuit, then the poles of the transfer function are reduced and stability is improved, but the circuit complexity increases
Solution Approach 1:
The input feed-forward circuit is merged with the existing cascoded circuit structure by sharing the differential input pair and integrating the feed-forward path into the cascoded transistor gates. This merging approach reduces overall circuit complexity compared to having separate independent circuits, while still achieving the pole reduction and stability improvement.
Data Source
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AI summary
An amplifier (100) includes an amplifying stage (110), a cascoded circuit (102), an input feed-forward circuit (120) and an output stage (150). The amplifying stage (110) is arranged receiving a differential input pair to generate an amplified differential input pair. The input feed-forward circuit (120) is coupled to the cascoded circuit (102), and is arranged for feeding the differential input pair forward to the cascoded circuit (102). The output stage (150) is coupled to the amplifying stage (110) and the cascoded circuit (102), and is arranged for generating a differential output pair according to the amplified differential input pair and an output of the cascoded circuit (102).