Folded Cascode Amplifier Biasing for Low-Power Slew Boost
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Solution Overview
Problem
Folded cascode amplifiers face issues with current starvation and stability due to high current consumption, which worsens slew rate and input-referred voltage noise, especially when dealing with large input signal swings or tilts.
Innovation Solution
A cascode amplifier input stage that dynamically adjusts current consumption based on the magnitude and polarity of the differential input signal, using a bias stage with differential transistor pairs of varying device areas to generate a cascode control signal independent of the transconductance of the first differential transistor pair, and a slew boost circuit to enhance slew performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the tail current source magnitude is increased to enhance slew rate, then the slew rate improves, but the power consumption increases and stability is compromised
Solution Approach 1:
The patent implements dynamic current consumption adjustment by using a bias stage with differential transistor pairs that respond to the magnitude and polarity of the differential input signal. The cascode control signal is dynamically adjusted based on input conditions, allowing the amplifier to consume more current during large signal swings (improving slew rate) and less current during small signal operation (reducing power consumption), thus resolving the contradiction between slew rate and power consumption
Solution Approach 2:
The patent changes the current consumption parameter dynamically based on input signal conditions. By using differential transistor pairs with varying device areas in the bias stage, the circuit adjusts the cascode control signal to optimize current distribution, enabling the amplifier to achieve high slew rate when needed while maintaining low power consumption during normal operation
2Reliability
If the quiescent current in cascode transistors is increased to prevent current starvation, then circuit stability improves, but power consumption increases
Solution Approach 1:
The patent uses dynamic adjustment of the cascode control signal based on the differential input signal magnitude. During large signal swings, the circuit provides sufficient current to prevent current starvation and maintain stability. During small signal operation, the current consumption is reduced, thus resolving the contradiction between maintaining circuit stability and reducing power consumption
3Adaptability or versatility
If the voltage drop across cascode resistors is increased to accommodate larger input swings, then input range improves, but the collector voltage increases causing saturation and phase reversal
Solution Approach 1:
The patent adjusts the cascode control signal parameter dynamically based on input signal conditions. By using differential transistor pairs with different device areas, the circuit optimizes the voltage distribution across cascode resistors, enabling the amplifier to accommodate larger input swings while preventing collector voltage saturation and phase reversal through controlled parameter adjustment
Data Source
AI summary
An amplifier circuit can be configured to receive a differential input signal having a common mode component that can extend to at least one power supply rail for the amplifier circuit. The amplifier circuit can include an input stage, such as having a first differential transistor pair, and the input stage can receive the differential input signal and in response conduct a differential first current to a cascode output stage. The cascode output stage can include or use a cascode control signal that is adjusted in response to the differential input signal. The cascode control signal can be independent of a transconductance of the first differential transistor pair. In an example, the amplifier circuit includes a slew boost circuit configured to source or sink current at an output of the amplifier based on a magnitude and polarity of the differential input signal.