Operational Amplifier Output Stage With Parallel Assist Transistor
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Solution Overview
Problem
Operational amplifiers with large maximum output current require significant bias current, leading to high current consumption, which increases circuit size and power consumption.
Innovation Solution
Incorporating an assist circuit with transistors connected in parallel to the output transistor, driven by a separate drive circuit, allowing for increased output current without the need for large steady bias currents, thereby reducing overall current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the size of the output transistor is increased to achieve high maximum output current, then the output current capability is improved, but the bias current and current consumption increase significantly
Solution Approach 1:
The output stage is segmented into a main output transistor and a parallel assist transistor. The main transistor handles the bulk of the current while the assist transistor provides additional current capability only when needed, allowing the main transistor to be smaller and consume less bias current while still achieving high maximum output current capability.
Solution Approach 2:
The assist transistor is dynamically controlled by a drive circuit that activates it based on the operating conditions. This dynamic control allows the circuit to optimize between low bias current consumption during normal operation and high current capability when maximum output is required, resolving the contradiction between continuous bias current and peak current capability.
2Power
If a large bias current is used to enable high output current in class AB output stage, then the output current capability is improved, but the current consumption increases
Solution Approach 1:
The bias current path is segmented such that the main output transistor operates with a reduced bias current, while the assist transistor provides additional current capability when activated. This segmentation allows the circuit to achieve high output current capability without requiring the main transistor to be oversized with high continuous bias current.
Solution Approach 2:
The drive circuit acts as an intermediary that controls the assist transistor based on the operating conditions. This intermediary circuit enables the system to achieve high current capability on demand without requiring high continuous bias current, effectively mediating between the conflicting requirements of low bias current and high output capability.
Data Source
AI summary
An operational amplifier includes a differential input stage that amplifies a differential input signal to generate an intermediate signal; an amplification stage including an output transistor that is connected between an output terminal and a fixed voltage line, and is driven according to the intermediate signal; and an assist circuit, wherein the assist circuit includes: a first transistor connected in parallel with the output transistor; and a drive circuit that drives the first transistor according to a gate voltage of the output transistor.