Differential Amplifier Biasing Without AC Coupling Capacitors

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Solution Overview

Problem

Differential amplifiers with cascoded transistors require different DC bias voltages due to mismatched input common-mode voltages, leading to inefficiency and large area consumption with traditional AC coupling capacitors.

Innovation Solution

The amplifier design incorporates cascoded P-type and N-type transistors with resistors and capacitors to control DC voltages, reducing the need for AC coupling capacitors and optimizing DC biasing, thereby improving efficiency and reducing area.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If AC coupling capacitors are used to couple between the gate of the cascoded P-type transistor and the gate of the N-type transistor, then the amplifier can operate with different DC bias voltages, but the area consumption increases significantly

Engineering Contradiction:

Improveability to operate with different DC bias voltagesVSAvoidarea consumption

Core Design Contradiction:

Adaptability or versatilityVSArea of stationary object

Solution Approach 1:

The patent removes the AC coupling capacitor from the circuit by extracting its coupling function and replacing it with a direct connection between the gates of the cascoded P-type and N-type transistors. This eliminates the area consumption of the capacitor while maintaining the ability to handle different DC bias voltages through the inherent transistor characteristics and biasing network

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent makes the direct connection between transistor gates serve multiple functions: it provides DC bias voltage adaptation, enables signal coupling, and establishes the operating point for both transistors simultaneously. This multi-functional approach replaces the single-function AC coupling capacitor

Inventive Principle:

Principle #6Universality (Multi-functionality)

2Reliability

If different DC bias voltages are required for cascoded P-type and N-type transistors, then proper operation is ensured, but the amplifier efficiency decreases

Engineering Contradiction:

Improveproper operationVSAvoidamplifier efficiency

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent changes the DC bias voltage parameters of the cascoded transistors to be equal, eliminating the voltage mismatch. This is achieved through the direct gate connection and optimized biasing network, which simultaneously improves amplifier efficiency by reducing power consumption while maintaining reliable operation through proper biasing

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS12401333B2Amplifier

Publication Date: 2025.08.26 REALTEK SEMICON CORP

AI summary

The present application discloses an amplifier, including: a positive-end PMOS; a negative-end PMOS; a positive-end NMOS, having a drain coupled to a drain of the positive-end PMOS and outputting a negative-end output signal; a negative-end NMOS, having a drain coupled to a drain of the negative-end PMOS and outputting a positive-end output signal; a first resistor, coupled between a gate of the negative-end NMOS and a negative-end input signal; a second resistor, coupled between a gate of the negative-end NMOS and the positive-end output signal; a third resistor, coupled between a gate of the negative-end PMOS and the negative-end input signal; and a fourth resistor, coupled between a gate of the negative-end PMOS and the positive-end output signal.