Cascode Source Follower Circuit for Higher PSRR and SNR
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Solution Overview
Problem
Conventional source follower circuits have limited power supply noise suppression capabilities, which restricts their ability to enhance the signal-to-noise ratio (SNR) due to ineffective rejection of power supply noise.
Innovation Solution
The proposed source follower circuit employs a cascode configuration with transistors and capacitors to form voltage divider circuits, isolating power supply noise from the gate of the main source follower transistor, thereby increasing the power supply rejection ratio (PSRR) and SNR by ensuring that noise is not reflected in the output signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional source follower circuit is used, then the circuit structure is simple, but the power supply rejection ratio (PSRR) is limited
Solution Approach 1:
The source follower circuit is segmented into two transistors (M1 and M2) arranged in a cascode configuration. Transistor M1 serves as a buffer between the power supply and M2, separating the noise rejection function from the signal following function. This segmentation allows M1 to suppress power supply noise while M2 maintains the source follower operation, thereby improving PSRR without significantly complicating the overall circuit structure.
Solution Approach 2:
Transistor M1 acts as an intermediary element between the power supply and the main source follower transistor M2. It mediates the transmission of power supply noise by providing a low-impedance path to ground for noise signals, preventing them from reaching M2 and degrading the output signal quality. This intermediary approach effectively improves PSRR while maintaining circuit simplicity.
2Measurement precision
If power supply noise suppression is enhanced in a source follower, then the signal-to-noise ratio (SNR) improves, but the circuit complexity increases
Solution Approach 1:
The circuit segments the noise suppression function into a dedicated buffer transistor M1, which handles power supply noise independently. This allows M2 to focus on signal following with high SNR performance. The segmentation achieves improved SNR without requiring complex filtering or shielding circuits, as the noise rejection is inherently provided by the cascode structure.
Solution Approach 2:
The cascode configuration enables the circuit to self-service noise suppression through the natural impedance characteristics of the transistors. M1's low output impedance automatically suppresses power supply noise without requiring external noise filtering components. This self-service approach improves SNR while avoiding additional circuit complexity from external noise suppression components.
Data Source
AI summary
The present application discloses a source follower circuit, arranged for generating output signal according to input signal. The circuit includes: a first transistor having a drain coupled to a first reference voltage; a second transistor having a drain coupled to a source of the first transistor, and the first transistor and the second transistor both have polarization of a first type; a first capacitor, coupled between a gate of the first transistor and the input signal; and a first resistor, coupled between the gate of the first transistor and a first bias voltage; wherein a gate of the second transistor is coupled to the input signal, and a source of the second transistor outputs the output signal.