Variable Feedback VGA for Stable Impedance and Low Noise
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Solution Overview
Problem
Conventional high performance variable gain amplifiers (VGAs) face challenges in maintaining minimal input and output impedance variations while achieving low noise figure and high signal-to-noise ratio, as they often degrade noise figure or output third order intercept point with gain variations, requiring additional circuits or stages.
Innovation Solution
The use of variable feedback architecture and coordinated control of multiple variable components to concurrently manage gain, input, and output impedance, maintaining low noise figure and high output third order intercept point across a wide range of gain settings, with noise figure changing by 0.4 dB or less for a 1 dB change in gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If dominant fixed resistors are added to minimize impedance variations, then impedance stability is improved, but noise performance and linearity are dramatically degraded
Solution Approach 1:
The patent applies dynamics by replacing fixed resistors with variable resistors that can be adjusted based on gain settings. The variable feedback network dynamically adapts the feedback factor to compensate for impedance variations caused by gain changes, thereby maintaining impedance stability without requiring dominant fixed resistors that would degrade noise performance.
Solution Approach 2:
The patent changes the feedback factor parameter dynamically based on gain settings. By adjusting the feedback network parameters to match different gain conditions, the system maintains optimal impedance matching and noise performance across the full gain range, avoiding the need for fixed resistors that would compromise noise figure.
2Adaptability or versatility
If a variable attenuator followed by a fixed gain amplifier is used, then gain control is achieved, but noise figure degrades by 1 dB for every 1 dB decrease in gain
Solution Approach 1:
The patent employs feedback by using a variable feedback network that adjusts the feedback factor based on gain settings. This feedback mechanism allows the amplifier to maintain low noise figure across different gain levels by optimally matching the feedback network to the amplifier's operating conditions, rather than using a lossy variable attenuator at the input.
Solution Approach 2:
Instead of placing the variable attenuation element at the input (which degrades noise figure), the patent inverts the approach by placing the variable element in the feedback network. This inversion allows gain control to be achieved while preserving noise performance, as the feedback network's variable elements do not directly add noise to the signal path.
3Stability of the object's composition
If lossy resistive networks are placed at VGA input to control impedance, then input impedance stability is improved, but noise figure is degraded
Solution Approach 1:
The patent replaces static lossy resistive networks with dynamic variable resistive networks in the feedback path. These variable resistors are controlled to provide impedance matching without the continuous power loss and noise degradation associated with fixed lossy resistors at the input.
Solution Approach 2:
The patent introduces a variable feedback network as an intermediary between the amplifier and the source/load. This intermediary provides impedance transformation and matching functionality without requiring lossy resistors directly at the amplifier input, thereby protecting the noise figure while achieving impedance stability.
4Stability of the object's composition
If lossy resistive networks are placed at VGA output to control impedance, then output impedance stability is improved, but output IP3 is degraded
Solution Approach 1:
The patent uses variable resistive networks in the feedback path rather than fixed lossy resistors at the output. The variable nature allows impedance matching to be achieved dynamically based on gain settings, maintaining output IP3 performance while providing impedance stability.
Solution Approach 2:
The patent changes the feedback network parameters to achieve impedance matching at the output without using fixed lossy resistors. By adjusting the feedback factor and network configuration based on operating conditions, the system maintains high output IP3 while achieving stable output impedance.
Data Source
AI summary
Variable feedback architecture and control techniques for variable gain amplifiers (VGAs) concurrently maintain, across a wide range of VGA gain settings, minimal input and output impedance variations, a low noise figure, low rates of change in noise figure, high signal-to-noise ratio (SNR), high quality of service (QoS), low distortion, high and relatively constant output third order intercept point (i.e., IP3 or TOI). Variable feedback counteracts impedance variations caused by gain variations. Compared to conventional high performance VGAs, noise figure is lower (e.g. 3 dB lower at maximum gain and 12 dB lower at minimum gain) and relatively constant, IP3 is higher and relatively constant, small signal third order intermodulation signal (IM3) tone slope is relatively constant and input and output impedances are relatively constant. As gain decreases, the noise figure advantage is nearly dB per dB compared to conventional high performance VGAs.