Selectable Gain Differential Amplifier With Uniform Common-Mode Output
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Solution Overview
Problem
Conventional variable gain amplifiers face performance issues due to different common mode voltages associated with variable gain outputs, leading to noise degradation and compatibility problems with downstream devices, and AC coupling with capacitors limits low-frequency response.
Innovation Solution
A selectable gain differential amplifier design featuring multiple cascode leg pairs with a variable voltage generator to set uniform common mode voltages and a selector circuit to activate only one cascode leg pair for each gain setting, eliminating the need for AC coupling capacitors and reducing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional variable gain amplifiers use different load resistor configurations to provide variable gain levels, then gain variability is achieved, but different common mode voltage levels are produced causing performance problems with downstream devices
Solution Approach 1:
The patent applies equipotentiality by maintaining a constant common mode voltage at the output nodes regardless of gain setting. This is achieved through the switched cascode architecture where the active cascode device ensures that both output nodes remain at the same voltage level (VOUTP and VOUTN both equal to VCM), eliminating the common mode voltage variations that occur in conventional designs when load resistor configurations are changed.
Solution Approach 2:
The patent uses dynamic switching of cascode devices based on the desired gain setting. The control circuit dynamically activates or deactivates specific cascode devices (Q16-Q19) depending on the gain selection, allowing the amplifier to adapt its gain while maintaining stable output characteristics. This dynamic control enables gain variation without the need to physically change load resistor configurations.
2Adaptability or versatility
If AC coupling capacitors are used to connect stages to allow independent DC common mode voltages, then stage independence is achieved, but low frequency response is limited and capacitor size becomes large
Solution Approach 1:
The patent extracts and eliminates the need for AC coupling capacitors by implementing a DC-coupled switched cascode architecture. The invention removes the capacitor element entirely from the signal path between stages, replacing the traditional AC coupling approach with a direct DC-coupled switched cascode stage that can handle both AC and DC signals without requiring capacitive isolation.
Solution Approach 2:
The patent substitutes the mechanical/electrical capacitor-based AC coupling mechanism with an active switched cascode transistor system. Instead of using passive capacitors to block DC and pass AC, the invention uses actively controlled transistors (Q16-Q19) that can dynamically switch signal paths while maintaining proper DC operating points, thereby eliminating the need for large coupling capacitors and their associated low-frequency limitations.
3Adaptability or versatility
If the Gilbert cell uses cascode pairs to deliver current to load resistors for variable gain, then gain control is achieved, but output noise increases undesirably
Solution Approach 1:
The patent applies dynamic switching of cascode devices to control gain while minimizing noise. By dynamically activating or deactivating specific cascode devices (Q16-Q19) based on the selected gain setting, the circuit maintains optimal noise performance for each gain level. The dynamic control ensures that only the necessary cascode devices are active, preventing the noise accumulation that occurs in Gilbert cell designs where multiple cascode pairs operate simultaneously.
Solution Approach 2:
The patent applies local quality by optimizing the noise characteristics of individual cascode device pairs for specific gain settings. Each cascode device pair (Q16-Q19) is configured to operate optimally at particular gain levels, and the control circuit selectively activates only the appropriate pair for the desired gain setting. This localized optimization ensures that each active cascode pair operates in its noise-optimal region, rather than having all cascode pairs active as in conventional Gilbert cell designs.
Data Source
AI summary
The selectable gain differential amplifier includes a differential amplifier, a plurality of cascade leg pairs connected to the differential amplifier, each leg of each cascode pair including a cascode device and a load resistor configured to provide a selectable gain. A variable voltage generator is connected to each leg configured to set gain resistor voltage of any active cascode leg pair to a uniform predetermined common mode voltage and the output node voltage of any inactive cascode leg pair to a voltage different from the predetermined common mode voltage. A selector circuit is configured to select the output of any said cascode leg pair.