Operational Amplifier Auxiliary Current Boost for Capacitive Loads
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Solution Overview
Problem
Conventional operational amplifiers have a low slew rate and high power consumption, making them unsuitable for battery-driven devices like portable phones when driving capacitive loads, such as liquid crystal panels, due to the limitations in improving the discharging/charging slew rate of phase compensation and load capacitors.
Innovation Solution
An operational amplifier with an auxiliary circuit that disconnects from the differential amplifier circuit when the voltage difference between input voltages is small and accelerates voltage shift by increasing current when the voltage difference exceeds a predetermined threshold, using comparators and current mirrors to enhance the slew rate without significantly increasing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the bias current ID2 of the differential amplifier circuit is increased to improve the slew rate (SR1) of the phase compensation capacitor, then the slew rate improves, but the power consumption increases
Solution Approach 1:
The patent applies dynamics by making the bias current ID2 variable rather than constant. The current is dynamically adjusted based on the operating conditions: it remains at a low level during normal operation to minimize power consumption, and is increased only when a large voltage change is detected at the input terminals, thereby improving slew rate only when necessary. This is achieved through detection circuits that monitor input voltage changes and control circuits that adjust the bias current accordingly.
Solution Approach 2:
The patent changes the parameter of bias current ID2 from a fixed value to a variable value that adapts to different operating conditions. By detecting the voltage difference between input terminals and adjusting the bias current parameter dynamically, the system achieves high slew rate when needed while maintaining low power consumption during normal operation.
2Speed
If the current ID2 is increased to improve the discharging/charging slew rate of the phase compensation capacitor, then the slew rate improves, but it causes increase of power consumption in normal state
Solution Approach 1:
The patent makes the bias current ID2 dynamic by implementing detection circuits that monitor the voltage difference between input terminals. When the voltage difference is small (normal operation), the current remains low. When the voltage difference exceeds a threshold (slew rate needed), the current is increased. This dynamic adjustment resolves the contradiction between maintaining high slew rate capability and keeping normal-state power consumption low.
Solution Approach 2:
The patent employs periodic detection and adjustment of the bias current based on input conditions. The detection circuits continuously monitor the input voltage difference, and the control circuit periodically adjusts the bias current level accordingly, enabling the system to switch between low-power and high-performance modes as needed.
Data Source
AI summary
An operational amplifier includes: a differential amplifier circuit configured to receive an inverting input voltage (VIN−) and a non-inverting input voltage (VIN+); and an auxiliary circuit for improving a slew rate of an output voltage of the differential amplifier circuit, wherein when a voltage difference between the inverting input voltage (VIN−) and the non-inverting input voltage (VIN+) is less than a predetermined small voltage difference, an output terminal of the auxiliary circuit is disconnected from an output terminal of the differential amplifier circuit, and when the voltage difference exceeds the predetermined small voltage difference so that a voltage waveform is shifted to at least one direction, the voltage shift is accelerated by receiving/transferring a current from/to the output terminal of the differential amplifier circuit toward a shifting direction of an output voltage of the differential amplifier circuit.