Variable Gain Amplifier Current Steering for Fine Gain Steps
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Solution Overview
Problem
Automatic gain control designs in variable gain amplifiers suffer from parasitic peaking at lower gain codes due to parasitic capacitance and discrete gain settings, leading to design trade-offs and silicon area wastage.
Innovation Solution
A variable gain amplifier architecture incorporating a folded-Gilbert stage with a cascode circuit and a digital-to-analog converter (DAC) to control gain by steering currents between differential pairs of transistors, minimizing parasitic peaking and providing fine gain-steps with a wide tuning range and constant output common mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If more programmable branches are used to create finer gain-steps, then gain resolution is improved, but parasitic capacitance increases causing more undesired peaking
Solution Approach 1:
A new node is introduced between the tail current source and the differential pairs to provide an additional degree of freedom. This intermediary node allows independent control of the tail current while the degeneration resistors control the gain, separating the functions and allowing fine gain-steps without increasing parasitic capacitance at the tail node.
Solution Approach 2:
The patent changes the control parameter for gain from directly controlling the tail current through multiple branches to controlling the degeneration resistors while maintaining a fixed tail current. This parameter change allows continuous or fine-discrete gain adjustment without the parasitic capacitance issues associated with multiple tail current branches.
2Stability of the object's composition
If degeneration resistor RS is increased to reduce gain sensitivity to resistor corners, then gain stability is improved, but parasitic capacitance at the tail node increases causing peaking
Solution Approach 1:
The patent introduces an intermediary node that decouples the tail current control from the gain control. This allows the use of smaller degeneration resistors for gain adjustment while the tail current is independently controlled, reducing parasitic capacitance effects while maintaining gain stability through proper biasing.
Solution Approach 2:
The patent segments the gain control function into two independent parts: tail current control for overall gain level and degeneration resistor control for fine gain adjustment. This segmentation allows each component to be optimized independently, reducing the need for large degeneration resistors and their associated parasitic capacitance.
3Object-generated harmful factors
If load capacitors are added to reduce parasitic peaking at lower gain codes, then parasitic peaking is reduced, but silicon area is wasted
Solution Approach 1:
Instead of adding more capacitors to compensate for parasitic effects, the patent converts the harmful parasitic capacitance into a beneficial element by using it for pole cancellation. The existing parasitic capacitance at the tail node is utilized to cancel the unwanted peaking, eliminating the need for additional load capacitors and saving silicon area.
Solution Approach 2:
The patent makes the system self-correcting by using its own parasitic capacitance to cancel the harmful peaking effects. The tail node parasitic capacitance, which was previously harmful, now serves the dual purpose of maintaining DC bias while providing pole cancellation to reduce peaking, making the system self-sufficient without external compensation components.
Data Source
AI summary
An integrated circuit and method for providing a variable gain amplifier are disclosed. One embodiment of the a variable gain amplifier comprises at least one load, a cascode circuit coupled to the load, a folded-gilbert stage, coupled to the cascode circuit, the folded-gilbert stage comprising a main differential pair of transistors and an internal pair of transistors, and a digital to analog converter, coupled to the folded-gilbert stage, for steering currents between the main differential pair of transistors and the internal pair of transistors to change a gain of the variable gain amplifier.