Closed-Loop Bias Control in Push-Pull Amplifiers for Crossover Distortion
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Solution Overview
Problem
Existing Class AB push-pull amplifiers face challenges in accurately controlling quiescent bias due to variations in turn-on voltage across devices and temperature, leading to inefficiencies and crossover distortion, particularly in low-cost, high-power devices like MOS switching FETs.
Innovation Solution
A closed-loop control system is implemented in the Class AB push-pull amplification stage, which senses the current flowing through transistors, applies non-linear processing to determine quiescent current, and adjusts the DC bias voltage to minimize variations, using a combiner and control circuit to maintain optimal biasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If factory calibration and open loop temperature compensation techniques are used to control quiescent bias, then the amplifier can operate with reduced crossover distortion, but the control accuracy is insufficient for device classes with large turn-on voltage variations
Solution Approach 1:
The patent implements closed-loop feedback control by sensing the quiescent current through sensing resistors and using the sensed signal to automatically adjust the bias voltage via a control circuit. This feedback mechanism continuously monitors and corrects quiescent bias drift, achieving accurate control despite device parameter variations and temperature changes.
Solution Approach 2:
The amplifier system performs self-calibration by using its own output to generate the adjustment signal. The control circuit automatically regulates the bias voltage based on sensed quiescent current, eliminating the need for external calibration equipment or manual adjustment, thereby achieving consistent performance across device variations.
2Device complexity
If the turn-on voltage of output stage devices is allowed to vary with temperature and device characteristics, then the amplifier design remains simple, but crossover distortion increases and efficiency decreases
Solution Approach 1:
The patent employs feedback control where the quiescent current is sensed and fed back to the bias control circuit, which automatically adjusts the bias voltage to compensate for temperature-induced turn-on voltage variations. This maintains optimal operating point without requiring complex external compensation circuits.
Solution Approach 2:
The patent replaces complex mechanical or external compensation mechanisms with an electronic control system that uses sensing resistors and active control circuitry to dynamically adjust bias voltage, achieving temperature compensation through electrical feedback rather than physical compensation networks.
3Adaptability or versatility
If the turn-on voltage of output stage devices is allowed to vary with device characteristics, then the amplifier can accommodate different device classes, but quiescent bias control accuracy deteriorates
Solution Approach 1:
The feedback control system senses the actual quiescent current regardless of device characteristics and automatically adjusts the bias voltage to achieve the desired operating point. This universal feedback approach works with different device classes (MOSFETs, BJTs, etc.) without requiring device-specific calibration, thereby maintaining both adaptability and accuracy.
Solution Approach 2:
The patent dynamically changes the bias voltage parameter based on sensed quiescent current to compensate for variations in device turn-on voltage. By adjusting this key parameter in real-time, the system adapts to different device characteristics while maintaining consistent performance and control accuracy across device classes.
Data Source
AI summary
An amplification stage comprising: a combiner to generate a sum input signal by combining a voltage signal with a DC bias voltage; a subtractor to generate a difference input signal by subtracting the voltage signal from the DC bias voltage; a first transistor for generating a first part of an amplifier output signal from the sum input signal; a second transistor for generating a second part of an amplifier output signal from the difference input signal; a combiner for combining the first and second parts of the amplifier output signal; a sensing circuit arranged to sense a current flowing in each of the first and second transistors; a control circuit arranged to determine the quiescent current of the first and second transistors in dependence on the sensed currents; and an adjustment circuit arranged to adjust the DC bias voltage in order to minimize variation in the quiescent current.