Amplifiers – Signal feedback – Frequency responsive feedback means
Reexamination Certificate
2001-07-24
2003-07-29
Lam, Tuan T. (Department: 2816)
Amplifiers
Signal feedback
Frequency responsive feedback means
C330S103000, C330S104000
Reexamination Certificate
active
06600367
ABSTRACT:
This invention relates to both a low distortion amplifier and to a method of achieving low distortion in an amplifier.
This invention has particular application to amplifiers whose power output stage intrinsically produce most distortion at low frequencies, and in particular, at audio frequencies.
BACKGROUND ART
There has been considerable human effort into attaining low distortion in amplifiers of many applications at all frequencies. In 1950, the best audio power amplifiers produced distortion of about 0.1% at 1 kHz, and in the 1990s, this has been reduced to about 0.001% at 1 kHz, and about 0.02% at 20 kHz, although one manufacturer claims 0.0025% at 20 kHz.
The vast majority of commercial audio amplifiers more or less follow well established standard designs.
There are some exceptions: a Technics SE-A1 amplifier which is known of in some countries incorporates an A-class output stage supplied by a floating low voltage high current power supply. This power supply is connected to a B-class high voltage output stage.
An LT1166 integrated circuit is primarily intended to control the quiescent bias feeding output transistors in audio amplifiers. The LT1166 consists of a low gain transconductance differential amplifier (gain of 0.125 mho) with an inverting and a non-inverting input. The circuitry has a local negative feedback path connecting an output of the power stage to the inverting input of the transconductance amplifier. The input of the output stage is the non-inverting input of the transconductance amplifier. Two local dominant poles necessary for stability are formed by the use of shunt capacitors to ground from the transconductance amplifier's outputs. The Linear Technology application circuitry promises distortions no better than many commercial products.
In the Journal of Audio Engineering Society, vol 29, no 1/2, January/February 1981, pages 27-30, M. J. Hawksford, discloses as a mere paper publication a theoretical means of cancelling distortion in any amplifier stage, including an output stage. This is achieved by subtracting the signals feeding the output power transistors inputs from the amplifier output, and then adding this signal back into the signal driving the output transistors'inputs.
Iwamatsu in U.S. Pat. No. 4,476,442 again as a mere paper publication disclosed circuitry based on the principles of Hawksford. In one embodiment described in this patent specification, Iwamatsu discloses floating power supplies supplying the adding and subtracting circuitry. These floating supplies follow a voltage equal to the sum of the output signal plus a signal linearly proportional to current flowing through an output load. However, Iwamatsu's circuits do not include local dominant poles.
Robert R. Cordell in “MOSPOWER APPLICATIONS”, Siliconix inc. ISBN 0930519-000, 1984, 6.6.3 discloses an audio power amplifier essentially the same as one of the Hawksford's circuits, but including local dominant poles required for stability. This circuit has however no provision for thermal stability, nor floating power supply rails.
The current inventor Bruce H Candy previously in U.S. Pat. No. 5,892,398 as a paper publication only, disclosed an amplifier also utilising the principles of Hawksford, but including local dominant poles required for stability, thermal tracking circuitry for thermal stability, floating power supplies which track the output signal, rather than to the sum of the output signal plus a signal linearly proportional to the current flowing through the output load as in the case of Iwamatsu. Candy also disclosed an output stage input current source load which is also supplied by power from the floating power supplies. It was possible with such an arrangement according to my experiments which were not published to attain a distortion in the order of only 1 part per million at 20 kHz at several hundred watts of output.
Williamson et al. In U.S. Pat. No. 5,396,194 describes as a mere paper publication a switch mode amplifier containing floating low voltage high current power supplies which supply an A-class amplifier. This is similar to the technics SE-A1 except that the drive circuitry is switch-mode rather than class-B and that the power supplying the A-class amplifier is derived from the switch mode power supply rather than a separate power supply.
In one of the Williamson paper descriptions there was described floating power supplies to supply small signal operational amplifiers which were connected as servo loops to control the current flowing through the output devices. There are two feedback paths containing a capacitor which form two local dominant poles which are essential for stability.
The current inventor Bruce H Candy in U.S. patent application 09/054070 describes an amplifier consisting of at least one operational amplifier, a first error correction amplifier, connected up as a servo loop to control the output voltage, as opposed to the output current as in the case of Williamson et al. These operational amplifiers are supplied by power from floating power supplies which track the output voltage.
Candy further describes a local dominant pole required for stability, and the advantages of using wide band operational amplifiers, with gain bandwidth products of more than 100 MHz. In addition, I described a second error correction amplifier, consisting of another operational amplifier, also preferably wideband, connected up as a servo loop to control the output voltage stage which includes the first error correction amplifier. In other words, I described a 2nd order local dominant pole formed by the signal path being amplified by two error correction stages in series.
This also is supplied by the floating power supplies. I further described the advantages of implementing high gain stages with local negative feedback and the attendant local dominant poles required for stability in other stages of the amplifier to reduce distortion. This arrangement does not require the precise setting of the adding and subtracting electronics disclosed by Hawksford and related circuits.
Audio power amplifiers, or operational amplifiers, Usually consist of three definable stages, an input stage, voltage amplifier stage and output stage. In power amplifiers, the output stage, sometimes called the power output stage, usually produces most distortion. However, the distortion of the power output stage maybe substantially reduced by some of the concepts described herein.
Compared to these distortion reduced power output stages, the lowest distortion conventional input stages, voltage amplifier stages may produce substantially greater distortion.
Conventional low distortion input stages are usually a differential voltage to current converter which produce a differential output current. In these low distortion traditional architectures, the differential current output of this input stage is connected to a current mirror, and the output node of the differential current output of the input stage and current mirror is connected to a common emitter cascode amplifier; said common emitter amplifier sometimes being a Darlington. The amplifier's dominant pole is set by a network including a capacitor connected between the output and input of this common emitter cascode stage. Details of these stages are described in a review by Douglas Self in a series of articles in “Electronics World+Wireless World” from August 1993 to January 1994, and also in his book, ISBN 0-7506-2788-3, “Audio power amplifier design handbook”, Newness, Reprinted 1997/1998, and the second edition ISBN 0 7506 4527 X, 2000. Another review is given by Ben Duncan, High Performance Audio Power Amplifiers, ISBN 0 7506 2629 1, Newness 1996.
An object of this invention is to provide a further circuit arrangement which assists in construction of even more accurate amplifiers or at the least, provides the public with a useful alternative.
DISCLOSURE OF THE INVENTION
In one form of this invention although this may not be the only or indeed the broadest form of this there is proposed an electronic am
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Rosenthal & Osha L.L.P.
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