US 6,587,670 B1 · Dual mode class D amplifiers

Abstract

Pulse width modulated (“Class D”) amplifiers and controllers switching in the 50 khz to 2 MHz range generate harmonics which interfere with AM radio reception. This has precluded wide spread acceptance of class D amplifiers in products with an AM radio. The amplifiers described here use a modified modulation technique when operating with an AM radio to avoid generating harmonics at the receiver selected frequency or its intermediate (IF) frequency. One embodiment uses a harmonic avoidance modulator. A second embodiment uses two or more oscillators and tests for the least distorted output signal. A third embodiment switches the mode of operation to a class AB amplifier when an AM signal is amplified.

Claims2

Dependent claims

A circuit for avoiding AM radio interference in a class D amplifier, said class D amplifier having a switching frequency, said circuit comprising:

means for comparing a first signal representative of the frequency of an AM signal to one or more reference signals representative of different frequencies;
means for generating comparator signals representative of whether the first signal is greater than or less than or equal to the reference signals;
means for generating a frequency divisor number, said frequency divisor number dependent at least in part upon the comparator signals;
means for modifying said switching frequency, said means dividing the frequency of the AM signal by the frequency divisor signal to derive a modified switching frequency;
and
a counter measuring the frequency of the AM signal and generating a digital signal representative of the AM frequency, said reference signals comprising digital reference signals.

3 more elements

Dependencies are inferred from the claim text.

Dual mode class D amplifiers

Life of this patent

Dual mode class D amplifiers

Life of this patent

Abstract

Claims2