WO2000070754A1

WO2000070754A1 - Distortion reduction technique for inductive boost amplifier

Info

Publication number: WO2000070754A1
Application number: PCT/US2000/013934
Authority: WO
Inventors: Larry Kirn
Original assignee: Jam Technologies, Llc
Priority date: 1999-05-19
Filing date: 2000-05-19
Publication date: 2000-11-23
Also published as: AU5149800A

Abstract

A distortion reduction technique finds particular utility in inductive boost amplifiers (107). In contrast to existing arrangements, the invention uses the boost voltage itself, rather than the output voltage(s), as feedback to linearize the output (114). As such, the boost voltage is not subject to the severe time delay of the output filter (113, 115, 116). The invention is applicable to bridged and non-bridged configurations using analog and/or digital circuitry.

Description

DISTORTION REDUCTION TECHNIQUE FOR INDUCTIVE BOOST AMPLIFIER

Field of the Invention

This invention relates generally to switching amplifiers and, in particular, to a

distortion reduction technique for inductive boost amplifiers.

Background of the Invention

Inductive boost amplifiers yield high efficiency and reasonably low distortion

when driving essentially resistive loads. A representative example of such an

amplifier is shown in U.S. Patent No. 5,610,553 entitled "Switching Amplifier with

Impedance Transformation Output Stage," the teachings of which are incorporated

herein by reference.

When driving inductive loads, however, amplifiers of this type tend to produce

excessive voltage due to their inability to detect back-EMF from the load. This leads

to distortion during times of decreasing output voltage.

Use of negative feedback from the amplifier output gives moderate relief from

-this distortion, but is complicated by phase lags inherent to output filters required in

switching amplifiers. Another feedback source is therefore desirable.

Summary of the Invention

The present invention resides in a distortion reduction technique with

particular applicability to inductive boost amplifiers. In contrast to existing

arrangements, the invention uses the boost voltage itself, rather than the output

voltage(s), as feedback to linearize the output. As such, the boost voltage is not subject to the severe time delay of the output filter, yet is sensitive to the effects of

back-EMF from inductive loads, which tend to exist at frequencies well below the

cutoff frequency of the output filter.

Brief Description of the Drawing

FIGURE 1 is a schematic diagram illustrating a preferred embodiment of the

invention.

Detailed Description of the Invention

Figure 1 is a schematic diagram illustrating a preferred embodiment of the

invention. Note that for cost savings and simplicity, a single boost converter is used

in conjunction with switching devices in a bridged configuration to determine the

polarity of the output. This circuit topology yields a single boosted voltage source,

applicable to either polarity of the output. It will be appreciated that the invention is

equally applicable to non-bridged configurations. In addition, although the technique

is described in conjunction with analog circuitry, all-digital or semi-digital

implementations will be readily apparent to those of skill in the art.

Incoming signal 100 is applied to a full-wave rectifier 101, which yields

absolute value, and a comparator 102, which yields sign or polarity. The output of

full-wave rectifier 101 is applied to the non-inverting input of error amplifier 103,

which in turn drives pulse-width converter 105 triggered by clock source 104. The

output of pulse-width converter 105 is a series of pulses of widths directly

proportional to its input, and used to drive switching device 106. - J

Switching device 106 stores charge in inductor 107. which is released through

diode 109 and filtered by capacitor 110. Diode 108 maintains a known voltage

available during the charging period. The output of diodes 108 and 109 is available to

switching devices 111 and 117. In the case of a positive input, switching device 1 17

is turned on and switching device 118 is turned off by inverter 119, being driven by

comparator 102 mentioned previously.

Concurrently, switching device 111 is turned off and switching device 112 is

turned on by comparator 102. In the case of a negative input, switching devices 11 1

and 118 are turned on and switching devices 112 and 117 are turned off by the same

mechanism. The output of diodes 108 and 109 is therefore connected through either

switching device 111 or 117 and inductor 113 or 115, respectively, to one terminal of

the load 114, filtered by capacitor 116. The second terminal of load 114 is connected

to V+ through either inductor 115 or 113 and switching device 118 or 1 12.

respectively, to allow current flow.

The output of diode 109 is a voltage proportional to the pulse-width developed

by pulse- width converter 105, summed with any back-EMF presented by the load 1 14.

^"This voltage is presented to the inverting input of error amplifier 103. In that the non-

inverting input receives the desired voltage, any deviations in the boost voltage are

reflected as signed corrective actions at the output of error amplifier 103. thereby

nulling consequent distortion.

I claim:

Claims

1. An amplifier output stage with reduced distortion, comprising:

an input representative of a desired output signal;

a circuit for determining a deviation between the input and a feedback signal;

a pulse-width converter fully or partially controlled by the circuit;

a boost converter coupled to the output of the pulse-width converter for

generating a boost voltage for application to a load through gated switches; and

wherein the feedback signal is derived from the boost voltage.

2. The amplifier output stage of claim 1 , wherein a single boost converter

is used in a bridge configuration.

3. In an inductive boost amplifier wherein a boost voltage is coupled to a

load through gated switches, the improvement comprising:

use of the boost voltage as feedback to linearize the output of the amplifier.

4. An amplifier output stage with reduced distortion, comprising:

an input feeding one side of two-input error amplifier;

a pulse-width converter coupled to the output of the error amplifier;

a boost converter coupled to the output of the pulse-width converter for

generating a boost voltage for application to a load tlirough gated switches; and

wherein a signal representative of the boost voltage is delivered to the other

input of the error amplifier.