MOSFET Differential Amplifier Gain Control by Gate Width Ratio
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Solution Overview
Problem
Conventional common source differential amplifier circuits face challenges in achieving consistent programmable voltage gain and step control across variations in fabrication process, voltage, and temperature due to the mismatch in performance characteristics between MOSFETs and tunable resistors, leading to reduced voltage gain range and inconsistent step size.
Innovation Solution
A common source differential amplifier design that utilizes a pair of MOSFETs with a common source degeneration MOSFET and a controller to control the gain by adjusting the gate width ratio, eliminating the need for a resistor and ensuring performance consistency across process, voltage, and temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a tunable resistor is used to control voltage gain, then the voltage gain range can be adjusted, but the area consumption increases significantly (ten times larger than input transistors) and parasitic capacitances increase
Solution Approach 1:
The patent changes the control parameter from resistor resistance value to MOSFET gate width ratio. By varying the gate width of the source degeneration MOSFET relative to the input MOSFETs, the voltage gain is controlled through transconductance ratio rather than resistance, eliminating the need for large-area tunable resistors and reducing parasitic capacitances.
Solution Approach 2:
The patent substitutes the mechanical/tunable resistor system with an active MOSFET-based system. Instead of using a physically adjustable resistor with switches and polysilicon elements, the control is achieved through electrical modulation of MOSFET gate widths, replacing a passive component system with an active transistor system that offers superior area efficiency and frequency performance.
2Adaptability or versatility
If a tunable resistor is used to control voltage gain, then the gain can be adjusted, but the high frequency performance deteriorates due to greater parasitic capacitances
Solution Approach 1:
The patent changes the gain control mechanism from resistance-based to transconductance-based by modifying MOSFET gate widths. This parameter change eliminates the parasitic capacitances associated with large-area polysilicon resistors and switch networks, thereby improving the circuit's high-frequency response and speed performance while maintaining gain adjustability.
3Adaptability or versatility
If a tunable resistor is used for gain control, then the resistance can be tuned, but the performance characteristics do not track MOSFETs across PVT variations, leading to inconsistent gain control
Solution Approach 1:
The patent employs homogeneity by using MOSFETs throughout the gain control mechanism - both the input transistors and the source degeneration transistor are MOSFETs with matching fabrication processes. This ensures that all components respond identically to PVT variations, causing their performance characteristics to track together and maintaining consistent gain control across process, voltage, and temperature conditions.
Solution Approach 2:
The patent changes the control parameter from resistor resistance (which has poor PVT tracking) to MOSFET gate width ratio (which tracks well with MOSFET characteristics). This parameter substitution ensures that the gain control mechanism shares the same PVT sensitivity as the input transistors, resulting in consistent and predictable gain adjustment across varying operating conditions.
Data Source
AI summary
An apparatus and method are provided for controlling the gain of a common source differential amplifier. The common source differential amplifier includes a pair of a metal-oxide-semiconductor field effect transistors (MOSFETs) each including a gate, a drain, and a source and at least one common source degeneration MOSFET in electrical communication between the sources of the pair of MOSFETs, the at least one common source degeneration MOSFET including a plurality of gate structures. A controller is in electrical communication with the gate structures and is configured to selectively activate one or more of the gate structures for controlling the gain of the common source differential amplifier.