Variable-Gain Amplifier Bias Segmentation for Constant Phase
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Solution Overview
Problem
Existing variable gain amplifiers (VGAs) face challenges in maintaining constant output phase and linearity across different gain states, particularly with the advent of 5G technology, where phase stability and high linearity are crucial, and previous solutions like phase compensation networks lead to linearity deterioration as gain is switched, especially at higher frequencies.
Innovation Solution
A variable gain amplifier design featuring a first and second transconductance stage circuit with differential input pairs and independently biased parallel transistors, where each amplifying circuit is split into small-sized parallel transistors to improve linearity and maintain constant phase, using degeneration circuits to adjust bias currents and ensure linear gain adjustment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a current steering variable gain amplifier uses different quantities of cascode transistors in different gain states, then the gain control function is achieved, but the output capacitance changes with gain and the output phase cannot be kept constant
Solution Approach 1:
The patent divides the amplifier into multiple gain stages, where each stage contributes a portion of the total gain. By segmenting the gain control across stages rather than using a single large transistor, the output capacitance variation is reduced and phase constancy is maintained across different gain states.
Solution Approach 2:
The patent applies different transistor sizes and configurations to different parts of the amplifier circuit. Specifically, the input transistors are designed with specific width-to-length ratios that are optimized for each gain state, allowing local optimization of phase characteristics while maintaining overall gain control.
2Ease of operation
If a bias-controlled variable gain amplifier changes the bias voltage to control transconductance, then the gain control function is achieved, but the junction capacitor and parasitic capacitor values change with bias current and the output phase cannot be kept constant
Solution Approach 1:
The patent carefully selects and maintains bias conditions such that the transistors operate in a region where the variation of junction and parasitic capacitances with bias current is minimized. This parameter optimization allows gain control while maintaining relatively constant phase characteristics.
3Stability of the object's composition
If a phase compensation network is added to the input stage of a variable gain amplifier, then the output phase is kept constant in different gain states, but the linearity deteriorates as gain is switched and the problem becomes more serious at higher frequencies
Solution Approach 1:
The patent removes the phase compensation network from the input stage that was causing linearity deterioration. Instead, phase constancy is achieved through the intrinsic properties of the segmented amplifier stages and careful transistor sizing, eliminating the need for the problematic compensation network.
Data Source
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AI summary
This specification discloses a variable gain amplifier and an electronic apparatus. The variable gain amplifier includes a first transconductance stage circuit and a second transconductance stage circuit. The first transconductance stage circuit includes a first amplifying circuit and a second amplifying circuit, and the second transconductance stage circuit includes a third amplifying circuit and a fourth amplifying circuit. The first amplifying circuit and the fourth amplifying circuit form a differential input pair, and the second amplifying circuit and the third amplifying circuit form a differential input pair. Each amplifying circuit of the first amplifying circuit, the second amplifying circuit, the third amplifying circuit, and the fourth amplifying circuit includes a plurality of parallel transistors, and bias control of the plurality of transistors is independent of each other. Linearity of the variable gain amplifier can be improved by separately controlling a bias voltage of the plurality of transistors.