Amplifiers – Signal feedback – Frequency responsive feedback means
Reexamination Certificate
2002-09-06
2004-09-28
Choe, Henry (Department: 2817)
Amplifiers
Signal feedback
Frequency responsive feedback means
C330S260000
Reexamination Certificate
active
06798285
ABSTRACT:
This invention relates to both an amplifier and to a method of achieving low distortion in an amplifier.
This invention has particular application to audio amplifiers.
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 was reduced to about 0.001% at 1 kHz, and about 0.02% at 20 kHz, although one manufacturer claims 0.0025% at 20 kHz.
A majority of commercial audio power amplifiers more or less follow standard designs.
Details of some examples of these are given 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 1997/8 and a second edition ISBN 0 7506 4527 X, also Newness, 2000. Another book containing a comprehensive review of amplifiers, is authored by Ben Duncan called “High performance Audio Power Amplifiers,” Newness ISBN 0 7506 2629 1, 1996, reprinted 1997/8.
There are some exceptions to these designs: 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 B-class High Voltage Output Stage.
An LT1166 integrated circuit is primarily intended to control 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 output 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 for stability are formed by the use of shunt capacitors to ground from the transconductance amplifier's outputs. This Linear Technology application circuitry promises distortions no less than many currently existing commercial products.
In 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 the 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, 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 the current flowing through the output load. However, Iwamatsu's circuits do not include local dominant poles.
Robert R. Cordell in “MOSPOWER APPLICATIONS,” Siliconix Inc. ISBN 0-930519-0, 1984, 6.6.3 discloses an audio power amplifier essentially the same as one of the Hawksford's circuits, but including the essential local dominant poles required for stability. This circuit has no provision for thermal stability, nor floating power supply rails, which are rare in amplifiers.
The current inventor Bruce H Candy previously in U.S. Pat. No. 5,892,398 as a mere paper publication only, discloses an amplifier also utilizing 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 discloses an output stage input current source load which is also supplied by power form the floating power supplies. Candy claims that it is possible with this arrangement to attain a distortion of the order of 1 part per million at 20 kHz at several hundreds of watts 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. All the claims are concerned with the switching power saving technique.
In one of the Williamson paper descriptions there was described floating power supplies to supply small signal operational amplifiers which are 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 has considered 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 would be supplied by power from floating power supplies which track the output voltage.
Candy further has considered a local dominant pole being required for stability, and the advantages of using wide-band operational amplifiers, with gain bandwidth products of more than 100 MHz. In addition, Candy has considered a second error correction amplifier, consisting of another operational amplifier, also preferably wide-band, connected up as a servo loop to control the output voltage stage which includes the first error correction amplifier. In other words, Candy has considered a 2
nd
order local dominant pole formed by the signal path being amplified by two error correction stages in series.
This also would be supplied by the floating power supplies. Further considered are 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 usually consist of three definable stages: an input stage, voltage amplifier stage and output stage. Sometimes, the amplifier input stage and the voltage amplifier stage together are called the amplifier input 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 may be substantially reduced by some of the concepts considered by me previously. Compared to these distortion reduced power output stages, the lowest distortion conventional input stages and 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; the 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.
In his second edition, Douglas Self disclosed the advantages of a second order global dominant pole, consisting of splitting the integrating capacitor in the voltage amplification stage, that is the said dominant pole setting capacitor, and conn
BHC Consulting Pty., Ltd.
Choe Henry
Osha & May L.L.P.
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