Shared-Transistor Cascode Current Source for Voltage-Margin 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 composed of transistors with shared characteristics and a resistive element to disconnect and reconnect nodes appropriately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-phase CMOS inverting amplifier is used, then the circuit is simple and power consumption is low, but the amplifier is susceptible to common mode noise and may oscillate due to in-phase components
Solution Approach 1:
The amplifier is divided into two phases: a first phase for integrating the input signal and a second phase for amplifying the integrated signal. This segmentation allows the circuit to process differential signals effectively, rejecting common mode noise while maintaining simplicity and low power consumption.
2Area of stationary object
If a transistor-based current source is used for bias current, then the circuit area is reduced, but the output impedance decreases and drain current increases with output voltage due to channel length modulation effect
Solution Approach 1:
A compensation transistor is introduced as an intermediary element between the current source transistor and the amplifier. This compensation transistor is controlled by a control signal that counteracts the channel length modulation effect, thereby maintaining constant bias current and high output impedance while using a compact transistor-based current source.
Solution Approach 2:
The compensation transistor implements a feedback mechanism where the control signal adjusts the bias condition of the current source transistor to compensate for variations in output voltage. This feedback loop maintains stable current source characteristics despite the inherent channel length modulation in miniaturized transistors.
3Use of energy by moving object
If the power supply voltage is reduced, then power consumption decreases, but the power supply voltage margin becomes insufficient leading to potential oscillation and reduced output amplitude
Solution Approach 1:
The circuit employs dynamic phase switching between integration and amplification phases, allowing optimal utilization of the reduced power supply voltage margin. During the amplification phase, the differential configuration maximizes the available voltage swing, ensuring sufficient output amplitude even with low power supply voltage.
Solution Approach 2:
The integrated amplifier continuously operates by seamlessly transitioning between integration and amplification phases, ensuring that the useful action (signal processing) continues without interruption. This continuous operation maintains stable performance and prevents oscillation despite reduced power supply voltage margin.
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.