Variable Gain Amplifier Biasing for Output Phase Invariance
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Solution Overview
Problem
Conventional variable gain amplifiers (VGAs) suffer from output phase variability as gain settings change, leading to input impedance and transconductance variations, which can result in undesirable loading of other RF circuitry and limited gain range, making them unsuitable for applications requiring precise phase and amplitude control in RF systems.
Innovation Solution
The implementation of a VGA with degeneration resistors that provide phase invariance by introducing a zero in the transfer function, canceling the pole arising from cascode transistors, and maintaining a constant bias current across gain settings, ensuring independent input impedance and phase delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional VGA gain control methods are used, then gain range is achieved, but output phase varies and input impedance changes
Solution Approach 1:
The patent changes the electrical parameters of the amplifier circuit by introducing degeneration resistors in the cascode configuration. This modifies the transfer function to include a zero that cancels the pole, thereby maintaining constant phase and input impedance across different gain settings while achieving wide gain range through bias current adjustment.
Solution Approach 2:
The degeneration resistors act as intermediary elements that mediate between the gain control mechanism and the output stage. These resistors introduce a zero in the transfer function that compensates for the phase variation caused by the cascode transistors, enabling independent control of gain and phase characteristics.
2Adaptability or versatility
If gain control is implemented in conventional VGAs, then variable gain is achieved, but input impedance and transconductance vary
Solution Approach 1:
The patent modifies the circuit parameters by adding degeneration resistors that stabilize the input impedance. The resistors change the overall transfer function characteristics, making the real part of the input impedance constant across gain settings while allowing the gain to be varied through bias current control.
3Adaptability or versatility
If cascode transistors are used for gain control, then gain adjustment is achieved, but phase variation occurs due to pole in transfer function
Solution Approach 1:
The degeneration resistors serve as intermediary components that introduce a zero in the transfer function to cancel the pole created by the cascode transistors. This compensation mechanism simplifies the phase control by making the phase independent of gain settings, eliminating the need for complex phase adjustment circuits.
Data Source
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AI summary
Variable gain amplifiers (VGA) with output phase invariance are provided herein. In certain embodiments, a VGA is operable in a selected gain setting chosen from multiple gain settings that provide different amounts of amplification to a radio frequency (RF) input signal. The VGA includes a gain transistor that has a substantially constant bias current across the gain settings, such that the VGA's output phase, input impedance matching, and/or input return loss are substantially constant. The gain setting of the VGA is selected by controlling relative biasing of a pair of cascode transistors each connected to the gain transistor by a corresponding degeneration resistor. The degeneration resistors provide compensation that reduces or eliminates a difference in output phase of the VGA across gain settings, for instance, by introducing a zero in a transfer function of the VGA that cancels a pole arising from the cascode transistors.