Bulk Capacitance Driver Circuit for Symmetrical Amplifier Slew Rates
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Solution Overview
Problem
Bulk electrode capacitance in high-speed operational amplifiers leads to unsymmetrical slew rates and slower signal settling times due to its direct loading effect on the signal path, degrading the common mode rejection ratio and increasing power consumption when conventional buffer circuits are used to drive the bulk electrodes.
Innovation Solution
A bulk electrode driver circuit is introduced, which includes a transconductance stage and a bulk capacitance driver circuit with cascode transistors cross-coupled to input voltage signals, preventing tail current from flowing into the ground and minimizing power consumption by ensuring all tail current is used to charge the bulk electrode capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If bulk electrodes are connected directly to sources to improve CMRR, then common mode rejection ratio is improved, but slew rate becomes unsymmetrical and settling time increases
Solution Approach 1:
The bulk electrode connection is segmented into two independent paths: one path connects bulk electrodes to sources through cascode transistors (11A, 11B) for DC biasing to improve CMRR, while another path provides AC grounding through capacitors (14A, 14B) to prevent bulk capacitance from loading the signal path, thereby maintaining symmetrical slew rates and fast settling times
Solution Approach 2:
Capacitors (14A, 14B) are introduced as intermediary elements between the bulk electrodes and ground. These capacitors provide AC grounding for the bulk electrodes, preventing the bulk capacitance (Cb) from directly loading the signal path at the sources. This intermediary approach allows DC connection for CMRR improvement while blocking AC signal degradation
2Reliability
If conventional buffer circuits are used to drive bulk electrodes, then bulk capacitance is driven, but power consumption increases
Solution Approach 1:
The bulk electrode driver circuit uses the existing tail current (I1) from the differential input stage to drive the bulk capacitance, eliminating the need for separate buffer circuits. The cascode transistors (11A, 11B) configured with cross-coupled gates automatically regulate the current distribution, ensuring all tail current is used efficiently to charge the bulk capacitance without flowing to ground, thereby minimizing power consumption while maintaining reliable bulk capacitance driving capability
Data Source
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AI summary
Amplifier circuitry includes an input stage having a transconductance stage including first and second input transistors (60A) and a first tail current source, gates of the first and second input transistors being coupled to first and second input signals, respectively. A bulk electrode capacitance driver (15) includes third and fourth input transistors and first and second associated cascode transistors (111A) and a second tail current source coupled to the sources and bulk electrodes of the third and fourth input transistors and to the bulk electrodes of the first and second input transistors. The gates of the third and fourth input transistors are coupled to the first and second input voltage signals, respectively, and the gates of the first and second cascode transistors are coupled to the second and first input voltage signals, respectively.