Cascode Current Source Layout for Low-Voltage Inverting Amplifiers
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Solution Overview
Problem
Conventional inverting amplifiers, such as CMOS inverting amplifiers, are susceptible to common mode noise and have limited power supply voltage margins due to their single-phase configuration, leading to potential oscillation and reduced output amplitude, especially in miniaturized transistor-based current sources.
Innovation Solution
A semiconductor integrated circuit device employing a cascode current source configuration with additional transistors to increase output impedance, allowing for stable bias current supply and reduced power consumption, while maintaining a sufficient power supply voltage margin by using a cascode current source with transistors configured to operate in a saturated region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional current source using a single transistor is used, then the circuit is simple, but the output impedance decreases and drain current increases with output voltage due to channel length modulation effect
Solution Approach 1:
The current source is divided into two stages: a first transistor that sets the basic current level and a second transistor that provides output impedance enhancement. This segmentation allows the first transistor to handle current setting while the second transistor specifically addresses the output impedance issue, resolving the contradiction between simplicity and reliability.
Solution Approach 2:
The second transistor is nested within the current source structure, with its source connected to the drain of the first transistor. This nesting creates a cascode configuration where the inner transistor (first) sets current and the outer transistor (second) provides impedance multiplication, achieving high reliability while maintaining reasonable complexity.
2Area of moving object
If miniaturized transistors are used to reduce circuit area, then circuit area decreases, but channel length modulation effect increases causing output impedance to decrease
Solution Approach 1:
By segmenting the current source into two transistors, the patent compensates for the increased channel length modulation effect in miniaturized transistors. The second transistor's impedance multiplication effect counteracts the degradation caused by miniaturization, maintaining output impedance stability despite reduced circuit area.
3Device complexity
If a single-phase amplifier configuration is used, then the circuit is simple, but the amplifier is susceptible to common mode noise and output amplitude may become small or oscillation may occur
Solution Approach 1:
The patent introduces asymmetry by adding a feedback path from the output to the gate of the first transistor through a capacitive element. This asymmetric feedback structure creates phase correction that eliminates common mode noise susceptibility and prevents oscillation, improving reliability without requiring a fully differential configuration.
4Reliability
If additional transistors are added to increase output impedance, then current source characteristics improve, but circuit area and power consumption increase
Solution Approach 1:
Instead of using excessive transistors, the patent applies partial action by using just one additional transistor (second transistor) in a cascode configuration. This provides sufficient output impedance enhancement for reliable current source operation while minimizing the increase in circuit area and power consumption.
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 cascode current source enhances current source characteristics, providing improved stability and output impedance without increasing the minimum potential at the amplifier's input-output voltage range, thus reducing circuit area and power consumption while maintaining a sufficient voltage margin.
Implementation Method 1
in a transistor, an output impedance decreases, and a drain current increases with a rise in output voltage, due to channel length modulation effect caused by miniaturization
Data Source
AI summary
A semiconductor integrated circuit device constituting an inverting amplifier employs a cascode current source as a current source. In the semiconductor integrated circuit device, a high-potential-side transistor of the cascode current source and a low-potential-side transistor constituting an amplification portion are shared. The configuration can not only make an output impedance of the cascode current source high and improve current source characteristics but also make a minimum potential at a minimum potential point of the amplification portion low and ensure a sufficient power supply voltage margin.