Cascode Amplifier Biasing for Stable Current Mirror Ratio
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Solution Overview
Problem
Conventional cascode amplifiers face challenges in maintaining a stable current mirror ratio due to transistors operating in different regions and the channel-length modulation effect, especially in deep sub-micro processes where short channel lengths lead to disparity in voltage levels and current mismatch.
Innovation Solution
A cascode amplifier design with a bias circuit that couples the gate electrodes of transistors M3 and M4 to the gate electrodes of transistors M1 and M2, and biases the voltage level at the first electrode of transistor M4 to match the voltage level at transistor M2, ensuring both transistors operate in the same region, thereby maintaining a precise current mirror ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If short channel devices are used to achieve good amplifier performance, then gain and noise figure are improved, but channel-length modulation effect increases causing current mirror ratio instability
Solution Approach 1:
The patent changes the voltage parameter (VDS) by introducing a bias circuit that applies a fixed voltage to the drain of transistor M4, ensuring it matches the drain voltage of transistor M2. This parameter adjustment compensates for channel-length modulation effects in short channel devices, stabilizing the current mirror ratio while maintaining the benefits of short channel length for gain and noise performance.
2Adaptability or versatility
If transistors M2 and M4 operate in different regions, then device design flexibility is improved, but voltage level tracking between nodes N2 and N4 deteriorates
Solution Approach 1:
The patent applies equipotentiality by using a bias circuit to force the drain voltage of transistor M4 to equal the drain voltage of transistor M2. This creates equal potential conditions at nodes N2 and N4, ensuring accurate voltage level tracking and enabling precise current mirroring between the transistor pairs.
3Measurement precision
If channel length is reduced in deep sub-micro process, then amplifier performance is improved, but voltage disparity between nodes N2 and N4 increases
Solution Approach 1:
The patent introduces a bias circuit as an intermediary component that mediates between the drain of transistor M4 and ground. This intermediary actively adjusts the voltage at node N4 to match node N2, compensating for the voltage disparity caused by reduced channel length in deep sub-micro processes and maintaining precise voltage level matching.
Data Source
AI summary
A cascode amplifier includes a first transistor, a second transistor, a third transistor, a fourth transistor and a bias circuit. Each of the transistors has a gate electrode, a first electrode and a second electrode, a first electrode of the first transistor is coupled to a second electrode of the second transistor, a first electrode of the third transistor is coupled to a second electrode of the fourth transistor, and gate electrodes of the third and the fourth transistors are coupled to gate electrodes of the first and the second transistors, respectively. The bias circuit is coupled to the first electrode of the fourth transistor, and is used for biasing a voltage level at the first electrode of the fourth transistor to make the second and the fourth transistors operate in a same region.