Variable Gain Amplifier Circuit With Stable Impedance and OIP3
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Solution Overview
Problem
Conventional variable gain amplifiers experience undesirable changes in noise figure and third-order output intercept point (OIP3) as gain varies, affecting input and output impedances.
Innovation Solution
A variable gain amplifier circuit comprising a Darlington transistor pair and a cascode amplifier, with a variable capacitor and shunt feedback resistor configuration, maintains constant input impedance, output impedance, noise figure, and OIP3 across varying gain settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If gain is varied in conventional variable gain amplifiers, then amplification level changes, but noise figure and third-order output intercept point (OIP3) change undesirably
Solution Approach 1:
The amplifier is divided into multiple parallel amplifier paths (e.g., first amplifier path with first amplifier, second amplifier path with second amplifier) where each path has different gain characteristics. By selectively combining outputs from these segmented paths, the overall gain can be varied while maintaining stable noise figure and OIP3 performance.
Solution Approach 2:
The multiple amplifier paths serve multiple functions: they provide different gain levels while simultaneously maintaining consistent noise figure and linearity characteristics. The circuit architecture allows a single system to achieve both gain variation and performance stability through the coordinated operation of parallel amplifier stages.
2Adaptability or versatility
If gain is varied in conventional variable gain amplifiers, then amplification level changes, but input impedance and output impedance change undesirably
Solution Approach 1:
The amplifier is divided into multiple parallel amplifier paths (e.g., first amplifier path with first amplifier, second amplifier path with second amplifier) where each path has different gain characteristics. By selectively combining outputs from these segmented paths, the overall gain can be varied while maintaining stable noise figure and OIP3 performance.
Solution Approach 2:
The circuit employs feedback mechanisms through the parallel amplifier architecture where the interaction between multiple amplifier paths provides implicit feedback that stabilizes input and output impedances. The combined output from multiple paths ensures impedance consistency across different gain settings.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution ensures stable gain variation without altering noise figure or OIP3, enhancing the performance and reliability of the variable gain amplifier.
Implementation Method 1
a variable capacitor connected in series between the first amplifier and the second amplifier... The variable capacitor may include at least one field effect transistor (FET) and a control voltage. The control voltage may vary to change the capacitance of the variable capacitor.
Implementation Method 2
The input impedance of the variable gain amplifier circuit is set by a shunt feedback resistor and an emitter degeneration resistor. In one example, the gain of the variable gain amplifier circuit is set by the shunt feedback resistor and the emitter degeneration resistor.
Data Source
AI summary
A variable gain amplifier circuit including a first amplifier, a second amplifier, and a variable capacitor connected in series between the first amplifier and the second amplifier is disclosed. As a gain of the variable gain amplifier circuit varies, the input impedance, output impedance, noise figure and third-order output intercept point (OIP3) of the variable gain amplifier circuit remain unchanged.