Folded-Cascode Class AB Amplifier With Embedded Cascode Compensation
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Solution Overview
Problem
Class AB folded-cascode amplifiers face degradation in frequency response due to feedforward signals, and existing compensation methods, such as simple Miller compensation and cascode compensation, are not effective in improving gain-bandwidth performance.
Innovation Solution
The implementation of embedded cascode compensation, which involves feeding back complementary compensation signals from the output stage to the differential input circuit without signal inversion, and using a current mirror for signal inversion to stabilize the amplifier, thereby preventing destabilizing positive feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If simple Miller compensation is used to stabilize the amplifier, then stability is improved, but gain-bandwidth performance deteriorates due to feedforward signals
Solution Approach 1:
The harmful feedforward signal path is extracted and removed from the compensation network by using the cascode transistor's inherent blocking capability, allowing Miller compensation to work without the detrimental feedforward effect while maintaining stability
Solution Approach 2:
The cascode transistor M13 acts as an intermediary element that blocks the feedforward signal from reaching the output node, thereby eliminating the harmful direct path while allowing the Miller compensation capacitor to provide stabilizing feedback
2Object-affected harmful factors
If nulling resistors are increased in size to reduce feedforward signal magnitude, then feedforward degradation is mitigated, but gain margin deteriorates as the LHP zero moves closer to crossover frequency
Solution Approach 1:
The feedforward signal is completely removed from the system by using the cascode transistor's natural blocking property, eliminating the need for nulling resistors and their associated gain margin degradation
Solution Approach 2:
Instead of trying to reduce feedforward signal magnitude with resistors, the approach inverts the strategy by completely blocking the feedforward path using the cascode transistor's high impedance, achieving better results without the trade-off
3Productivity
If explicit current buffer is added to block feedforward signal for cascode compensation, then gain-bandwidth product is improved, but device complexity increases
Solution Approach 1:
The cascode transistor M13 is made to serve multiple functions: it acts as both the input stage transistor and the feedforward blocking element, eliminating the need for separate explicit current buffers and reducing overall device complexity
Solution Approach 2:
The compensation function is merged with the existing cascode structure by utilizing the inherent blocking capability of transistor M13, combining the input stage and compensation functions in a single integrated approach
Data Source
AI summary
A class AB folded-cascode amplifier having improved gain-bandwidth product, comprises a differential input circuit including a differential transistor pair coupled to a source of tail current and responsive to a differential input signal for conducting a first current, a cascode circuit coupled to the differential input circuit for supplying a second current thereto, and a class AB output stage. A compensation circuit is configured for feeding back mutually complementary compensation signals from an output node to the differential input circuit.