Boosted Tail-Current Circuit for Higher Differential Pair Slew Rate
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The finite, substantially constant value of the class-A current source in differential-pair circuits limits the slew rate of the circuit, as the differential current is maximally equal to the tail current, restricting the driving capability of succeeding stages.
Innovation Solution
The boosted tail-current circuit incorporates a second input-voltage-controlled class-B current source, which provides an additional tail current that is non-zero and non-quiescent, enhancing the collective tail current and overcoming the slew rate limitation by increasing the differential current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a class-A current source is used to provide tail current to the differential pair, then the circuit maintains stable quiescent operation, but the slew rate is limited due to the finite constant current value
Solution Approach 1:
The patent applies dynamics by transitioning from a static class-A current source to a dynamic class-B current source that responds to input voltage changes. The class-B current source remains off during quiescent conditions and activates dynamically when the input differential voltage exceeds a threshold, providing additional current during large-signal transitions to improve slew rate while maintaining stable quiescent operation.
Solution Approach 2:
The patent changes the current parameter dynamically by combining a constant class-A current source with a variable class-B current source. The total tail current varies based on input conditions: it remains at the class-A current level during small-signal operation but increases when the class-B current source activates, thereby improving slew rate without compromising quiescent stability.
2Speed
If the tail current is increased to improve slew rate, then the driving capability of succeeding stages is enhanced, but the quiescent power consumption increases
Solution Approach 1:
The patent applies periodic action by using the class-B current source that activates only during specific conditions (when input differential voltage exceeds the threshold) rather than continuously. This allows the circuit to draw high current only when needed for large-signal transitions, improving slew rate while maintaining low quiescent power consumption during normal operation.
Solution Approach 2:
The patent changes the tail current parameter dynamically based on operating conditions. The class-B current source adjusts the total tail current from a low quiescent value to a higher active value when needed, achieving high slew rate during transitions without the penalty of continuous high power consumption.
3Speed
If a class-B current source is added to provide additional tail current, then the differential current and slew rate are enhanced, but the circuit complexity increases
Solution Approach 1:
The patent merges a class-A current source and a class-B current source into a unified tail current provision system. The class-B current source is configured to work in conjunction with the existing class-A current source, combining their outputs to provide the total tail current. This merging approach enhances differential current capability while reusing existing circuit structures to minimize complexity increase.
Solution Approach 2:
The class-B current source is designed with multi-functionality, serving both as a slew rate booster during large-signal transitions and as a complementary current source that works with the class-A current source during normal operation. This universal design allows a single additional component to address multiple performance requirements without proportionally increasing complexity.
Data Source
AI summary
A circuit includes a pair of input transistors configured as a differential pair and having input terminals configured to receive an input voltage. The circuit also includes a first current source connected to and configured to provide a first tail current to the pair of input transistors, the first tail current being a class-A current having a non-zero quiescent value. The circuit also includes a second current source connected to and configured to provide a second tail current to the pair of input transistors, the second tail current being a class-B current having a zero quiescent value and a non-zero non-quiescent value. The second current source is configured to provide the second tail current as a function of the input voltage.