Digitally Tunable Gain Amplifier for Wideband Equalization
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Solution Overview
Problem
Traditional voltage gain amplifiers (VGAs) and continuous-time linear equalizers (CTLEs) suffer from reduced bandwidth due to parasitic capacitor effects causing overpeaking in high-frequency ranges, and modifications to reduce overpeaking often degrade overall bandwidth.
Innovation Solution
Implementing a VGA or combination VGA/CTLE with tunable gain by digitally adjusting current in a differential amplifier instead of using a source degeneration variable resistor, allowing configuration as either a buffer or equalizer with flexible gain control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If source degeneration variable resistors are used for gain control in traditional VGAs, then gain control is achieved, but parasitic capacitor effect produces unwanted high amplitude overpeaking in high-frequency ranges
Solution Approach 1:
The patent extracts and removes the source degeneration variable resistors from the VGA circuit. By eliminating these resistors that cause parasitic capacitor effects, the harmful overpeaking in high-frequency ranges is removed while maintaining gain control functionality through alternative means (adjusting tail current or load impedance).
Solution Approach 2:
The patent changes the method of gain control from using variable resistance to using variable current or variable impedance. By adjusting the tail current magnitude or load impedance, gain is controlled without introducing parasitic capacitors, thus avoiding overpeaking while maintaining ease of gain adjustment.
2Object-generated harmful factors
If modifications are made to reduce overpeaking in traditional VGAs, then overpeaking is reduced, but overall bandwidth is degraded
Solution Approach 1:
The patent removes the source degeneration resistors that cause overpeaking, and crucially, does not add compensating elements that would limit bandwidth. This extraction approach reduces overpeaking while preserving the full bandwidth of the amplifier circuit.
Solution Approach 2:
The patent changes the gain control mechanism to use current or impedance variation instead of resistance variation. This parameter change allows overpeaking reduction without introducing bandwidth-limiting modifications, as the control is achieved through active device parameters rather than passive reactive elements.
3Reliability
If CTLE follows VGA to minimize distortion, then distortion is reduced, but overall bandwidth is diminished
Solution Approach 1:
The patent merges the VGA and CTLE functions into a single unified circuit. By integrating the equalization function within the VGA itself, the need for a separate CTLE stage is eliminated, thus preserving overall bandwidth while still achieving distortion reduction through the combined functionality.
Solution Approach 2:
The patent designs the VGA to perform multiple functions simultaneously - both voltage amplification and equalization. This multi-functionality allows the circuit to reduce distortion like a CTLE would while maintaining the full bandwidth of a VGA, eliminating the bandwidth penalty of cascaded stages.
Data Source
AI summary
A amplifier circuit in some embodiment includes a differential amplifier have a pair of current sources. Each of the current sources includes two or more current-generating transistors and respective switching transistors, which can be turned on and off by a gain input code to tune the gain of the amplifier. A common-mode controller includes a similar pair of current sources as the differential amplifier. The common mode controller receives a common-mode signal of the input signal and a common-mode gain input code, and maintains the common-mode gain of the amplifier circuit when the differential gain changes. The amplifier circuit is switchable between a buffer mode and an equalizer mode.