Multistage RF Amplifier Common-Mode Coupling for Oscillation Cancellation
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Solution Overview
Problem
RF amplifiers in communication systems, particularly those used in WLANs, experience undesirable oscillations due to parallel FET configurations, which can reduce stability and output power when cascaded with sensitive digital components, and current methods to mitigate these oscillations often result in reduced signal output.
Innovation Solution
A multistage RF amplifier design where a first amplifier circuit is coupled with a second amplifier circuit through a common mode connection, using parallel FETs and inductors to create a voltage offset that counters oscillations, thereby minimizing interference and maintaining maximum power output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If parallel FETs are used to increase power output, then power rating is improved, but oscillation susceptibility worsens
Solution Approach 1:
A common-mode voltage offset circuit is introduced as an intermediary between parallel FET amplifier stages. This circuit generates a compensating voltage that counteracts the oscillations caused by bondwire inductance, allowing parallel FETs to operate at high power without suffering from oscillation susceptibility.
Solution Approach 2:
The invention applies preliminary anti-action by proactively generating a common-mode voltage offset that opposes oscillations before they can develop. The offset circuit anticipates and counteracts the harmful oscillatory effects of bondwire inductance in parallel FET configurations, enabling stable high-power operation.
2Reliability
If gain is reduced to attenuate oscillation, then oscillation is reduced, but output power decreases
Solution Approach 1:
Rather than reducing gain, the invention introduces a common-mode voltage offset circuit as a mediator that specifically targets oscillations without affecting the signal amplification path. This allows the amplifier to maintain full output power while selectively counteracting oscillatory components through the separate common-mode feedback path.
3Reliability
If matching resistive network is added to attenuate gate currents, then oscillation is reduced, but total output is reduced
Solution Approach 1:
The common-mode voltage offset circuit serves as an intermediary that addresses oscillation without requiring resistive attenuation in the signal path. By operating through a separate common-mode feedback mechanism, it eliminates the need for power-lossy resistive networks while maintaining full output power capability.
4Productivity
If amplifier stages are cascaded for maximum amplification, then amplification is improved, but oscillation interference worsens
Solution Approach 1:
When amplifier stages are cascaded, a common-mode voltage offset circuit is introduced as an intermediary between stages. This circuit generates a compensating offset that counteracts oscillations generated by bondwire inductance in parallel FETs, allowing maximum amplification through cascading without the harmful oscillation interference that would otherwise accumulate.
Data Source
AI summary
A multistage RF amplifier amplifies RF signals used for communication in a WLAN communications system. The multistage RF amplifier comprises a first amplifier circuit coupled to a second amplifier circuit to maximize amplification. A common mode of the first amplifier circuit is coupled to a common mode of the second amplifier circuit to provide a voltage offset. The voltage offset counters voltage changes due to oscillations in the first amplifier circuit, thereby reducing interference from the multistage RF amplifier.