Cascode Amplifier Feedback Path for Oscillation Suppression
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Solution Overview
Problem
The cascode-connection amplification circuit suffers from instability due to parasitic inductance caused by ground wiring, leading to potential oscillation when feedback occurs via parasitic capacitance, which reduces the stability factor and amplification performance.
Innovation Solution
Incorporating a resistive element in series with the output and control terminals of the second transistor, along with capacitance elements to isolate the control terminal potential from the power supply, forms a feedback loop with reduced loop gain, thereby attenuating high-frequency feedback signals and preventing oscillation. Additionally, inductance and capacitance elements are used to form a matching network outside the transistors to maintain gain and suppress off-leakage currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the second transistor's control terminal is directly grounded, then the grounding is simple, but parasitic inductance from the ground wiring line causes unstable grounding characteristic and reduces stability factor
Solution Approach 1:
A resistive element is introduced as an intermediary between the second control terminal and ground. This resistor acts as a mediator that blocks high-frequency feedback signals while allowing DC grounding, thereby eliminating the harmful effect of parasitic inductance without requiring complex grounding structures. The resistive element transforms the simple direct grounding into a filtered grounding path that maintains stability.
2Reliability
If a resistive element is added to form a feedback loop with reduced gain, then oscillation is prevented, but the device complexity increases
Solution Approach 1:
A feedback path is intentionally created by connecting the fourth terminal (output) to the second control terminal through the resistive element. This feedback mechanism deliberately introduces a controlled signal path that reduces the loop gain below 1, preventing oscillation. The feedback principle transforms potential instability into controlled stability by using the output signal to modulate the control terminal in a stabilizing manner.
Solution Approach 2:
The resistive element serves as an intermediary component that enables the feedback connection while simultaneously providing signal attenuation. By placing this resistor in the feedback path, the circuit achieves stability without requiring multiple additional components, as the single resistive element performs both the feedback function and the gain reduction function.
3Reliability
If the fourth terminal and second control terminal are connected via resistive element, then high-frequency feedback signal is attenuated, but the loop gain may be reduced below acceptable levels
Solution Approach 1:
The resistive element is strategically placed only in the feedback path between the fourth terminal and second control terminal, rather than in the main signal path. This local application of resistance provides high-frequency attenuation where needed for stability while leaving the main amplification path unaffected, thereby maintaining power gain. The local quality principle ensures that the damping effect is applied precisely where oscillation occurs without degrading overall amplification performance.
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 provides a stable cascode-connection amplification circuit that prevents oscillation by ensuring the feedback loop does not exceed a gain of 1, maintaining high-frequency signal amplification performance and reducing off-leakage currents, even with high-performance transistors operating at several tens of GHz.
Implementation Method 1
a first capacitance element that is serially arranged on a second path that connects the second control terminal and the power supply terminal
Implementation Method 2
a first resistive element that is serially arranged on a first path that connects the fourth terminal and the power supply terminal or is serially arranged on the second path; the fourth terminal and the second control terminal being connected to each other via the first resistive element and the first capacitance element
Implementation Method 3
a parasitic inductance component is generated at the second control terminal due to for example a wiring line that is for grounding the second control terminal
Data Source
AI summary
An amplification circuit includes: a power supply terminal that is connected to a power supply; a first transistor that has a first source terminal, a first drain terminal, and a first gate terminal to which a high-frequency signal is inputted; a second transistor that has a second source terminal that is connected to the first drain terminal, a second drain terminal that outputs a high frequency signal, and a second gate terminal that is grounded; a capacitor that is serially arranged on a second path that connects the second gate terminal and the power supply terminal; and a switch that is serially arranged on a first path, which connects the second drain terminal and the power supply terminal, or the second path. The second drain terminal and the second gate terminal are connected to each other via the switch and the capacitor.