Fully Differential Amplifier Design

Fully Differential Amplifier schematic Design

A Fully Differential Amplifier (FDA) amplifies the voltage difference between two input signals while maintaining a differential output. Unlike single-ended amplifiers that amplify the voltage with respect to a common reference point, FDAs amplify the voltage between two input terminals. This differential configuration allows FDAs to reject common-mode signals (signals that are common to both inputs) and amplify only the signal difference between the inputs. 

Fig: Typical Fully Differential Amplifier


In a Fully Differential Amplifier (FDA), Common Mode Feedback (CMFB) is a mechanism employed to regulate and control the common-mode voltage at a desired common mode level. It involves sensing the output common-mode voltage (average of output signals), comparing it to the reference level, amplifying the difference through an error amplifier, and using the resultant signal to adjust the circuit parameters, ensuring that the common-mode voltage remains within a specified range. CMFB operates in a feedback loop, continuously monitoring and correcting the common-mode voltage to enhance the amplifier's common-mode rejection capabilities. This is crucial in applications where maintaining low common-mode voltage and high common-mode rejection is essential.

Fig: Common Mode Feedback Block Diagram


Fig: FDA with CMFB schematic

Fig: FDA differential TB


Fig: FDA Differential LG Sims


Fig: FDA common mode TB

Fig: FDA common mode LG Sims

Fig: FDA as integrator

Fig: FDA as Single Ended to Differential Convertor

Fig: FDA as SE to Differential Convertor: Step response


Fully Differential Amplifier Layout

Fully Differential Amplifier’s layout was done meticulously, with proper common centroid matching at each and every sub-section. This is important because the signal integrity of the entire chain depends on the performance of FDA. Two key blocks - Single Ended to Differential Integrator and Fully differential Integrator control the signal flow and error correction of the entire differential Audio signal path.


Fig: Dummies in the FDA block

Fig: Full Top level Layout of the FDA

The above figure shows the various Cap matching


Fig: All NMOS current mirrors common centroid matched

Fig: All PMOS current mirrors common centroid matched


Fig: Input pair matching


Fig: CMFB Error Amp matching

Fig: First stage PMOS load matching


Fig: Second stage PMOS matching




Fig: Fully Differential Integrator Layout



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