Push-Pull Amplifier Bias Circuit for Crossover Distortion Control
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Solution Overview
Problem
Existing signal amplifiers, particularly audio amplifiers, face issues with transfer distortion due to the temperature-dependent bias voltage, which leads to fluctuations in quiescent current and increased distortion, especially when optimizing for speed.
Innovation Solution
The use of biasing diodes supplied by an independent voltage source, which is galvanically isolated from the amplifier voltage source, ensures a precise and temperature-independent bias current, reducing distortion by decoupling the bias voltage from the amplifier's voltage stage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the biasing diodes are fed by the amplifier voltage source, then the amplifier circuit can be simplified, but the bias voltage fluctuates with temperature causing distortion
Solution Approach 1:
The power supply system is segmented into two independent parts: the amplifier voltage source for the main amplifier circuit and a separate battery voltage source for the biasing diodes. This segmentation isolates the bias voltage from temperature-dependent fluctuations in the amplifier power supply, resolving the contradiction between circuit simplicity and voltage stability.
Solution Approach 2:
A battery serves as an intermediary power source that provides a stable, temperature-independent voltage for the biasing diodes. This intermediary element decouples the bias voltage generation from the amplifier's main power supply, eliminating distortion caused by temperature-induced voltage fluctuations while maintaining circuit functionality.
2Reliability
If the quiescent current is increased to reduce transfer distortion, then the gain at the takeover point increases, but distortion increases due to higher gain at the takeover point
Solution Approach 1:
The bias voltage parameters are precisely controlled through independent battery power supply, allowing optimization of quiescent current to reduce transfer distortion without the penalty of increased takeover distortion. The stable voltage enables precise parameter setting that balances both distortion types.
3Speed
If the amplifier is optimized for speed, then the quiescent current becomes highly temperature-dependent, but this causes unwanted fluctuation in bias voltage leading to distortion
Solution Approach 1:
The bias voltage generation is extracted from the temperature-sensitive amplifier power supply and placed in a separate battery-powered circuit. This extraction removes the coupling between speed-optimized quiescent current fluctuations and bias voltage stability, allowing the amplifier to be optimized for speed without compromising bias stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach minimizes distortion by maintaining a consistent bias voltage, independent of temperature changes, thereby improving the amplifier's performance and reducing unwanted fluctuations in the output signal.
Implementation Method 1
The temperature coefficient of the base-emitter threshold voltage of the transistor or transistors is negative. The bias voltage must therefore decrease with the identical temperature coefficient so that the distortion optimum can be maintained as the heat sink temperature changes.
Implementation Method 2
This is usually achieved by means of diodes, hereinafter referred to as biasing diodes or sensor diodes, whose threshold voltage also has a negative temperature coefficient which is as similar as possible to the temperature coefficient of the base-emitter threshold voltage of the associated transistors.
Implementation Method 3
Each transistor is assigned its own biasing diode and is thermally coupled to it.
Data Source
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AI summary
The invention relates to a signal amplifier circuit for amplifying a signal, particularly an audio amplifier circuit, comprising: at least one first amplifier transistor (Q1) and at least one second amplifier transistor (Q2), wherein said first amplifier transistor (Q1) and second amplifier transistor (Q2) are interconnected in a push-pull circuit and fed by an amplifier voltage source (V+, V-); and one or more biased diodes (D1, D2), each of which is thermally coupled to an associated amplifier transistor (Q1, Q2), said biased diodes (D1, D2) being arranged in a parallel circuit to the amplifier transistors (Q1, Q2) in order to reduce or avoid crossover distortion, and said biased diodes (D1, D2) being at least partly fed by a voltage source (UA) that is independent of the amplifier voltage source (V+, V-). The invention also relates to a system and a voltage converter for providing an outlet-side DC voltage, comprising a a first transformer (T1) and a second transformer (T2) which is connected to said first transformer (T1).