Amplifier Output Stage With Selective Drive Boost Near Supply Rails
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Solution Overview
Problem
Amplifier circuits face limitations in achieving wide output signal ranges while maintaining satisfactory performance characteristics, as the output voltage approaches power supply limits, leading to increased quiescent current and potential signal distortion.
Innovation Solution
The solution involves selectively boosting the drive signal to output transistors in the amplifier circuit, using a separate drive boost circuit for each transistor with adjustable thresholds, optimizing base current to maintain reduced quiescent current and extend output range by compensating for varying loading conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the output voltage approaches the power supply limits to achieve wide output signal range, then the output signal range is improved, but the quiescent current increases and signal distortion occurs
Solution Approach 1:
The patent implements dynamic adjustment of base drive current to output transistors based on operating conditions. The circuit selectively boosts base current when the amplifier operates near power supply rails, allowing the output stage to maintain low quiescent current during normal operation while dynamically increasing drive current when needed to extend output swing range without excessive distortion.
Solution Approach 2:
The invention changes the base current parameter dynamically based on output voltage level and loading conditions. By monitoring the operating point and adjusting base drive current accordingly, the circuit optimizes the trade-off between output range and quiescent current, enabling extended swing without proportionally increasing power consumption.
2Ease of operation
If the base drive current is increased to extend output range, then the output signal range is improved, but the quiescent current increases
Solution Approach 1:
The patent implements dynamic adjustment of base drive current to output transistors based on operating conditions. The circuit selectively boosts base current when the amplifier operates near power supply rails, allowing the output stage to maintain low quiescent current during normal operation while dynamically increasing drive current when needed to extend output swing range without excessive distortion.
Solution Approach 2:
The invention applies base current boosting only partially - specifically when and where needed near the power supply rails - rather than continuously increasing all bias currents. This selective application of excessive base current during critical operating regions extends output range without the penalty of continuously elevated quiescent current throughout all operating conditions.
3Ease of operation
If the amplifier operates near power supply limits to maximize output swing, then the output range is improved, but signal distortion increases
Solution Approach 1:
The patent applies preliminary action by proactively boosting base drive current before the output transistor enters saturation or cutoff regions. The circuit detects when the output voltage approaches power supply limits and preemptively increases base current to maintain adequate transistor drive, preventing distortion before it occurs rather than reacting after distortion has manifested.
Solution Approach 2:
The invention employs feedback mechanisms to monitor output voltage level and loading conditions, then adjusts base drive current accordingly. This closed-loop control ensures that sufficient base current is supplied to maintain linear operation of output transistors even when operating near power supply rails, thereby extending output range while maintaining signal quality.
Data Source
AI summary
An output stage of an amplifier circuit includes one or more output transistors that are selectively driven by a boosted drive circuit, where the boosted drive circuit is arranged such that the output range of the amplifier circuit is increased while maintaining reduced quiescent current. The drive signal to each output transistor is selectively increased only when demanded by the output load conditions. The threshold for boosting the drive signal can be adjusted for optimized performance. In one example, a class AB output stage includes a separate drive boost circuit for each output transistor. For this example, each drive boost circuit has a separate threshold for boosting each of the drive signals to the output transistors. The boosting can also be adjusted to optimize the differential input stage and current mirror maximum current requirement while maintaining minimum required bias currents.