Branched Cascode Amplifier Turn-Off Protection for Thin-Oxide Transistors
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Solution Overview
Problem
Multi-branch cascode amplifiers face performance degradation due to turned OFF branches, as they can subject transistors to voltage levels beyond their tolerable ranges, affecting linearity, gain, and noise figure.
Innovation Solution
A protection circuit is introduced that selectively couples a reference voltage to the source node of the output transistor of a turned OFF branch, maintaining a gate-to-source voltage less than 0 Volts and using a same biasing voltage during both ON and OFF states to prevent voltage overload.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a branch is turned OFF by setting gate biasing voltage to reference ground, then power consumption is reduced, but transistors are subjected to voltage levels beyond their tolerable ranges
Solution Approach 1:
A protection circuit is introduced as an intermediary between the turned-OFF branch and the reference ground. This protection circuit includes a switching element that selectively couples the source node of the output transistor to a reference voltage, preventing direct voltage overload while maintaining the OFF state for power reduction.
Solution Approach 2:
The protection circuit applies preliminary protective action by preemptively coupling a reference voltage to the source node before excessive voltage can develop. This prevents the harmful voltage overload condition from occurring in the first place, while still allowing the branch to remain in a low-power OFF state.
2Object-affected harmful factors
If thick-oxide transistors are used to tolerate higher voltages, then voltage handling capability is improved, but manufacturing complexity and cost increase
Solution Approach 1:
Instead of using thick-oxide transistors throughout the entire amplifier, the invention applies thin-oxide transistors in the turned-OFF branches with local protection circuits. This allows the use of thinner oxide (simpler manufacturing) where protection is provided, while maintaining voltage handling capability through the protective reference voltage coupling.
3Reliability
If protection circuits are added to protect transistors from excessive voltages, then transistor reliability is improved, but device complexity increases
Solution Approach 1:
The protection function is segmented and applied only to specific nodes (source nodes of output transistors in turned-OFF branches) rather than protecting the entire amplifier uniformly. This targeted approach provides necessary reliability while minimizing the addition of protective circuitry and associated complexity.
Data Source
AI summary
Various methods and circuital arrangements for complete turn OFF of branches of a multi-branch cascode amplifier are presented. According to one aspect, a protection circuit coupled to a source node of an output transistor of a branch couples a reference voltage to the source node of the output transistor when the branch is turned OFF, and decouples the reference voltage from the source node when the branch is turned ON. According to another aspect, the protection circuit includes a switch whose off capacitance is sufficiently low so as not to affect performance of the branch when the branch is ON, and whose on resistance is sufficiently low to sufficiently reduce an RF amplitude at the source node of the output transistor when the branch is OFF and other branches are ON, and therefore allow use of low-voltage thin-oxide transistors in the branch. Further aspects include a second switch and use of transistor switches.