Differential Amplifier Driver Circuit With Reduced Common-Mode Bandwidth
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Solution Overview
Problem
Differential amplifier driver circuits face instability when handling both differential and common-mode input signals, particularly in DSL applications, due to high internal power dissipation and limited full power bandwidth, and existing solutions suffer from impractical circuitry and high power dissipation.
Innovation Solution
The proposed solution involves a differential amplifier driver circuit design with current mirrors having multiple outputs, where the additional outputs are used to oppose common-mode signals, reducing common-mode bandwidth without affecting differential-mode bandwidth, and using a 2:1 current mirror ratio to manage signal currents effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the differential amplifier driver circuit is optimized for high gain (gain of ten) to maximize differential-mode bandwidth, then the differential-mode bandwidth is improved, but the circuit becomes unstable when handling common-mode signals because it operates at unity gain for common-mode inputs
Solution Approach 1:
The patent segments the feedback path into two independent loops: a differential-mode feedback loop that maintains high gain for differential signals, and a common-mode feedback loop that provides unity gain for common-mode signals. This is achieved by using separate feedback resistors (RFB1, RFB2) for each amplifier, allowing independent optimization of gain and stability for each mode without compromising the other.
Solution Approach 2:
The patent applies different feedback configurations to different signal modes. The differential-mode path uses high gain feedback network (RFB1, RG) optimized for bandwidth, while the common-mode path uses unity gain feedback network (RFB2) optimized for stability. Each amplifier's feedback network is locally tailored to handle its specific signal type effectively.
2Reliability
If conventional compensation methods are used to stabilize common-mode operation, then common-mode stability is improved, but the circuit requires impractical additional inverting amplifiers and experiences high internal power dissipation
Solution Approach 1:
The patent extracts the common-mode feedback function from the differential feedback path by introducing a separate dedicated common-mode feedback loop. This eliminates the need for complex inverting amplifier stages required by conventional methods, reducing circuit complexity while maintaining stability. The common-mode feedback is handled independently through RFB2 without interfering with the differential-mode path.
Solution Approach 2:
The patent enables each amplifier to self-regulate its common-mode gain through local feedback resistors (RFB1, RFB2) connected to its own output. This self-service mechanism allows each amplifier to independently maintain unity gain for common-mode signals without requiring external compensation circuits or additional active components, thereby reducing overall circuit complexity.
3Reliability
If the circuit operates at unity gain for common-mode signals to maintain stability, then common-mode stability is improved, but the full power bandwidth is limited and power dissipation increases
Solution Approach 1:
The patent implements dynamic feedback where the feedback factor changes based on the signal mode. For differential-mode signals, the feedback network provides high gain (10x) for maximum bandwidth. For common-mode signals, the feedback network automatically switches to unity gain for stability. This dynamic adaptation allows the circuit to optimize performance for each signal type without compromise.
Solution Approach 2:
The patent changes the feedback parameter (gain) based on the signal mode being processed. The feedback resistors are configured to provide different effective gain values: high gain for differential-mode operation and unity gain for common-mode operation. This parameter change allows the circuit to maintain stability for common-mode signals while preserving full power bandwidth for differential-mode signals.
Data Source
AI summary
An amplifier driver circuit (10) includes first (11-1) and second (11-2) feedback amplifiers including first (14-1) and second (14-2) upper current mirrors, respectively, and first (16-1) and second (16-2) lower current mirrors, respectively, first (12-1) and second (12-2) amplifier input stages receiving a common mode input signal, and first (18-1) and second (18-2) amplifier output stages coupled to outputs of the first and second amplifier input stages, respectively. Each current mirror has an input (IN) and first (OUT1) and second (OUT2) outputs. Upper bias terminals of the first (12-1) and second (12-2) amplifier input stages are coupled to the inputs (IN) of the first (14-1) and second (14-2) upper current mirrors, respectively, and are cross-coupled to the second outputs (OUT2) of the second (16-2) and first (16-1) lower current mirrors, respectively. Lower bias terminals of the first (12-1) and second (12-2) amplifier input stages are coupled to the inputs (IN) of the first (16-1) and second (16-2) lower current mirrors, respectively, and are cross-coupled to the second outputs (OUT2) of the second (14-2) and first (14-1) upper current mirrors, respectively, to oppose signals at the inputs (IN) of the current mirrors in response to the common mode input signal.