Fs/4 Class D Amplifier With Envelope Tracking for Wider RF Bandwidth
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Solution Overview
Problem
Conventional Class D amplifiers are limited in bandwidth, particularly for Radio Frequencies, restricting their application beyond the AM band due to the switching speed of transistors operating in saturated or cutoff modes.
Innovation Solution
A Class D amplifier design utilizing Fs/4 modulation and envelope tracking power supplies, including a digital I/O generator, envelope tracker, and Fs/4 modulator stage with sign bits to prevent negative voltage issues, allowing for higher frequency operation and broader bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional Class D amplifiers use binary pulse width modulation with transistors operating in saturated or cutoff modes, then power efficiency is improved, but bandwidth is limited and cannot extend beyond AM band frequencies
Solution Approach 1:
The modulator is divided into multiple segments (first modulator and second modulator) that operate at different frequency ranges. The first modulator handles lower frequencies while the second modulator handles higher frequencies up to the bandwidth limit, allowing the system to maintain efficiency across a broader bandwidth than a single-stage design could achieve.
Solution Approach 2:
The patent employs a nested modulation architecture where the output of the first modulator serves as an input to the second modulator. This nested structure allows successive frequency multiplication and expansion, enabling the system to extend bandwidth beyond what a single modulation stage could achieve while preserving the efficiency benefits of Class D operation.
2Speed
If transistors operate at high switching frequencies to increase bandwidth, then frequency range is improved, but switching speed limitations prevent operation above AM band
Solution Approach 1:
The patent uses periodic modulation actions at different stages, where the first modulator operates at a lower frequency with a first period and the second modulator operates at a higher frequency with a second period. This staged periodic action allows the system to achieve high frequency range without requiring any single transistor to switch at prohibitively high speeds, thereby maintaining reliability.
3Adaptability or versatility
If the amplifier is designed for broader bandwidth to support FM band and HD Radio, then adaptability is improved, but device complexity increases with additional modulator stages
Solution Approach 1:
The dual-modulator design serves multiple functions: the first modulator handles baseband to intermediate frequency conversion while the second modulator performs intermediate to radio frequency conversion. This multi-functional architecture enables the same system to support multiple broadcast standards (AM, FM, HD Radio) without requiring completely different hardware designs for each standard.
Data Source
AI summary
A high power class D amplifier/modulator for use in Radio Frequency ranges that is capable of digital modulation schemes at high efficiencies is disclosed. The new amplifier design features an envelope tracker unit that uses digital pulse-width modulation (PWM) generation to create analog I and Q phase high voltage signal components and sign bits. The I and Q phase signal components and sign bits are fed into an Fs/4 modulator stage to produce an analog output that is an high power RF signal modulated by the analog input signal.