Multistage Low-Noise Amplifier With Filtered Stability Control
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Solution Overview
Problem
High-frequency, low-noise amplifiers suffer from worsened noise factor and gain attenuation due to the inclusion of band-rejection filters, which cause circuit loss at frequencies near the operation band, affecting performance in satellite and terrestrial communication systems.
Innovation Solution
A high-frequency, multistage amplifier design incorporating bandpass and band-rejection filters strategically positioned to minimize circuit loss by adjusting the resonance frequencies relative to the operation band, ensuring low noise factor and flat gain characteristics across the frequency range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a band-rejection filter is disposed to eliminate a frequency band in the vicinity of the operation band, then the distortion characteristic is improved and unwanted waves are eliminated, but circuit loss occurs at the resonance frequency causing gain attenuation and worsened noise factor in the operation band
Solution Approach 1:
A bandpass filter is introduced as an intermediary component between the band-rejection filter and the amplifier. This bandpass filter selectively transmits frequencies within the operation band while blocking frequencies outside the operation band, thereby preventing the circuit loss from the band-rejection filter from affecting the operation band. The bandpass filter acts as a mediator that protects the operation band from the harmful effects of the band-rejection filter's circuit loss.
2Object-affected harmful factors
If the band-rejection filter is positioned closer to the amplifier stage, then the elimination of unwanted waves is more effective, but the circuit loss has a greater impact on the noise factor and gain in the operation band
Solution Approach 1:
The bandpass filter serves as a protective intermediary that decouples the positioning relationship between the band-rejection filter and the amplifier. By placing the bandpass filter between them, the system can position the band-rejection filter close to the amplifier for effective unwanted wave elimination, while the bandpass filter prevents the circuit loss from degrading the noise factor and gain in the operation band.
3Loss of energy
If multiple transistors are directly connected in series to form a multistage amplifier, then the passage loss is reduced and gain flatness is improved, but stability outside the operation band deteriorates
Solution Approach 1:
Stability circuits are extracted and added to each amplifier stage to address the stability issue. These stability circuits selectively increase circuit loss only at frequencies outside the operation band without affecting the passage loss within the operation band. This allows the multistage amplifier to maintain low passage loss and good gain flatness while achieving stability outside the operation band.
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 amplifier achieves a low noise factor and flat gain characteristic from the low to high frequency ends of the operation band by optimizing the placement and resonance frequencies of bandpass and band-rejection filters, thereby improving signal quality in communication systems.
Implementation Method 1
a stabilizing circuit provided in at least two of the amplifiers of the multistage circuit and including a bandpass filter and a resistor connected in parallel to each other
Implementation Method 2
a band-rejection filter provided between the at least two of amplifiers and eliminating a frequency lower than an operation frequency of the amplifier
Implementation Method 3
when a band-rejection filter is disposed, a circuit loss occurs at the resonance frequency of the filter
Data Source
AI summary
An amplifier includes: a high-frequency input terminal; a high-frequency output terminal; a multistage circuit provided between the high-frequency input terminal and the high-frequency output terminal and including two or more amplifiers and connected in series, each amplifier including an input matching circuit, a transistor, and an output matching circuit; a stabilizing circuit provided in at least two of the amplifiers and including a bandpass filter and a resistor connected in parallel; and a band-rejection filter provided between the at least two of amplifiers and eliminating a frequency lower than an operation frequency of the amplifier. The stabilizing circuit and the band-rejection filter are provided between an output terminal of the transistor of the amplifier and the output matching circuit or provided in the output matching circuit. The closer to the high-frequency input terminal the bandpass filter is, the lower a resonance frequency of the bandpass filter is.