Differential Amplifier Bias Topology for Low Distortion Gain
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Solution Overview
Problem
Differential amplifiers face challenges in reducing distortion of output signals due to large-amplitude input voltages and temperature-dependent gain errors across a broad range of frequencies, leading to operational instability and saturation issues.
Innovation Solution
The design incorporates a pair of emitter-follower transistors with a specific configuration of amplification transistors, bias current setting circuits, and voltage shift circuits to maintain identical collector-emitter voltages across all transistors, reducing distortion and preventing saturation, while using a simple circuit structure for high-speed operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the emitter-collector voltage VCE is set identical to the base-emitter voltage VBE when no input voltage is applied, then the amplification transistors can be easily saturated in operation when an input voltage is applied, but the circuit constitution becomes simple
Solution Approach 1:
The patent changes the voltage parameter relationship by setting VCE = 2×VBE instead of VCE = VBE. This parameter modification increases the voltage headroom of the amplification transistors, preventing saturation when input voltage is applied, while maintaining a relatively simple circuit structure without adding complex components.
2Reliability
If a power transmission circuit with four current mirror circuits is used to solve operational instability, then the operational stability is improved, but the propagation path becomes long and high-speed operation becomes difficult to achieve
Solution Approach 1:
The patent extracts and removes the complex four-current-mirror power transmission circuit from the design. Instead, it uses a simplified bias current setting circuit that directly provides stable bias currents to the amplification transistors, eliminating the long propagation path while maintaining operational stability.
Solution Approach 2:
The patent segments the bias current setting function into independent circuits that directly supply currents to each amplification transistor, avoiding the cascaded mirror structure. This segmentation reduces the signal propagation path length while maintaining the stability benefits of current mirroring.
3Adaptability or versatility
If transistors operate with different power consumptions, then the circuit can handle varying signal conditions, but temperature differences cause gain errors and nonlinear amplification errors in broad frequency ranges
Solution Approach 1:
The patent creates equipotential conditions for temperature management by ensuring all amplification transistors operate at identical voltage levels (VCE = 2×VBE) and identical current levels. This uniform operating condition minimizes temperature differences between transistors, reducing thermal drift and maintaining accurate gain characteristics across broad frequency ranges.
Data Source
AI summary
A differential amplifier is constituted of first emitter-follower transistors, second emitter-follower transistors, and amplification transistors whose bases are alternately connected to the emitters of the second emitter-follower transistors and whose collectors are connected to the emitters of the first emitter-follower transistors, as well as emitter resistors and constant current sources, whereby it is possible to reduce distortions of output signals in response to large-amplitude input signals, thus ensuring high-speed operation. It is possible to further incorporate base-grounded transistors and diodes, by which substantially the same collector-emitter voltage is applied to the emitter-follower transistors and amplification transistors, thus achieving the same power consumption and the same temperature variations with respect to these transistors. This reduces the nonlinear amplification error due to temperature differences of transistors, thus achieving flat gain characteristics in broad ranges of frequencies.