Symmetrical Differential Amplifier With Ratioed Input Currents
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Solution Overview
Problem
Conventional differential amplifiers provide asymmetrical output signals and poor power efficiency due to limited current gain, requiring two copies connected in opposite polarities to achieve symmetry, which increases complexity and reduces efficiency.
Innovation Solution
A differential amplifier design with a bias circuit and load devices that allow symmetrical output signals by sizing input devices such that total currents in pairs bear a predetermined ratio, enabling both output signals to range freely between supply voltage and ground, with a reduced size current source for improved power efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a conventional current mirror plus source-coupled pair differential amplifier is used, then the circuit provides amplification function, but the output signal is asymmetrical and power efficiency is poor
Solution Approach 1:
The patent applies asymmetry principle by intentionally creating asymmetrical current distribution through differently sized input devices (first and second input devices have different sizes) to achieve symmetrical output signals. The bias circuit provides different bias currents to the first and second load devices, creating an asymmetrical internal current distribution that results in symmetrical external output behavior, resolving the contradiction between output symmetry and power efficiency.
2Ease of operation
If two copies of differential amplifier are connected in opposite polarities to provide symmetrical output, then output symmetry is achieved, but device complexity increases
Solution Approach 1:
The patent merges the functions of two separate differential amplifiers into a single integrated circuit. The bias circuit simultaneously controls both load devices, and the shared current source integrates the functionality of two independent amplifiers into one unified structure, reducing device complexity while maintaining symmetrical output capability.
Solution Approach 2:
The bias circuit serves multiple functions: it provides bias voltages to both load devices, generates the reference current, and enables independent control of the two output signals. This multi-functional design eliminates the need for separate bias circuits in each amplifier copy, reducing overall circuit complexity while achieving symmetrical output.
3Stability of the object's composition
If the current source is set to half current in conventional design, then circuit stability is maintained, but current gain is limited and power efficiency deteriorates
Solution Approach 1:
The patent applies local quality principle by allowing different regions of the circuit to have different current characteristics. The first and second load devices operate with different current levels optimized for their respective functions, while the bias circuit locally adjusts current distribution to maximize overall current gain. This localized optimization enables higher current gain without compromising circuit stability.
Data Source
AI summary
A differential amplifier has improved power efficiency, reduced offset penalty and a symmetrical output differential signal. Such a differential amplifier may include: (a) a bias circuit that has a first input device and a second input device; (b) a first load device and a second load device, each biased by a bias voltage from the bias circuit; and (c) a third input device and a fourth input device that are connected in series with the first load device and the second load device, respectively. In that differential amplifier, the differential input signal is applied across the first and second input devices, as well as across the third and the fourth input devices. The first, second, third and fourth input devices are sized such that a total current in the first and second input devices bears a predetermined ratio to a total current in the third and fourth input devices.