Balanced Current Amplifier - PDF document

1 Balanced Current Amplifier In 1931 Loy B
Balanced Current Amplifier In 1931 Loy Barton, a research worker employed byDavid Sarnoff , unearthed the paradigm that has domi-nated all electronic power amplifiers used for audio re-production and industrial power to this present day. Manyincremental additions have embellished Loy’s original 1 Balanced Current Amplifier is decreased by the same amount. Both pulses remaincentered on each other or balanced in time. The resultis that the difference ripple current has a minimum fre-quency which is twice the operating frequency of theThe frequency doubling character of the output is re-Barton’s goal of more from less. The switching lossesare effectively halved by this property as it is only nec-theoretical maximum of N (the number of switches) x fs(the switching frequency). This is a full factor of two fasterThe modulation process makes two decisions per switch-ing cycle for each switch, as both the turn-on time andthe turn-off time are independently controlled by themodulator. A 250KHz Crown BCA design thus has oneis required for full bandwidth audio operation. Previousto the BCA the conventional wisdom correctly held thatThe result of the new paradigm is a convection cooledis approximately an order of magnitude larger am

2 plifierwithout any cooling fan. With no
plifierwithout any cooling fan. With no fan there is no need forthe unit resulting from normal use.The Crown K2 amplifier has all of the nearly ideal powerreactive loads such as loudspeakers are easilydriven. The reactive energy returned from thereoutput with little loss. Non-switchingamplifiers are forced to dissipate allmore (the latter ratio is a function ofthree fold or more.Difficult loads are driven with graceand ease. Current overload isvoltage overload. Thermal overloadis rendered a thing of the past as ittackling the same difficult loads be-amounts of distortion or shutting off entirely. The result isthat a BCA output Watt is operationally larger than thatof previous amplifier designs.ers reveals that rated bench Watts and distortion ratingstained under normal field conditions by the typical userless and will outperform the best large studio amplifierswith a low-speed fan has fan noise. The K2’s over 100dBof electrical signal to noise (A-weighted) is not renderedWith a low-frequency damping factor of over 10,000 andOne final footnote: One (the larger) of Loy Barton’s 1931design examples was a 2.5KW amplifier. Ironically thereare some things that do not change, no matter whichcentury’s paradigms are in force

3 .(1920’s) and can be described as a
.(1920’s) and can be described as an amplifier in whichthere are two similar signal branch circuits operating indifference (summing) circuit to produce an increased 2 Balanced Current Amplifier nals at a single circuit node. This is the method used inall power stages that are referred to in the jarformer with a center-tapped primary, to perform thedifferencing. Transformers had been in use previouslysuch a high cost that it was rarely permissible to oper-Push-pull operation using a transformer with a center-tapped primary as the combiner was particularly attrac-tive as it also solved a problem implicit to transformer-cent bias currents in the primary. The output transformerLarger low-frequency outputs were possible before coredocumented until the 1940’s. Vacuum tubes were stillcircuit design were continuously becoming more diverse.Fundamental things were slowly but surely coming tothe fore.tion of the active devices used to effect the push-pullstage being a signal statistic and not an expression ofthe immediate state of the power devices used to createthe signal statistic. Such is the case within the CrownNote that push-pull concepts in no way imply the oper-ating efficiency of a power stage. The first push-pull am-plifi

4 ers were very inefficient because they w
ers were very inefficient because they were class-Alinear designs which by nature have large quiescentpower losses. Loy Barton’s class-B designs wereeffect the power output of the stage. Largethey dissipated (wasted) much power.stage, it is not necessary to re-tain the class-B paradigm to re-So then is there a class desig-a possible designator. Such athat the Crown BCA is implicitlytechnology where interleaving is taken to its logical con-clusions has been named OCIA (Opposed Current In-terleaved Amplifier) technology.The following figures show the fundamental operation ofthe basic Crown BCA power amplifier. The switch com-forms and the Vp and Vn waveforms are the switchedvoltages which are input to the main output inductors Lpand Ln. The currents Ip and In are the currents flowingin Lp and Ln respectively. 3 Balanced Current Amplifier cycle). When the Sp and Sn switches are both closedduring interval Tp (or Tn) the main output inductor cur-rent magnitudes increase at a rate of Vcc/L whereL=Lp=Ln. When the switches are both off during the in-terval following Tp (equal to Tp=Ts-Tp), the inductor volt-age is reversed and the output currents ramp down atthe same magnitude. Ip and In are of equal magnitudebut of opposite polarity

5 , thus the output currents are saidto be
, thus the output currents are saidto be balanced and therefore the name Balanced Cur-rent Amplifier or BCA.The converter is always biasedsuch that current continuouslyor its companion free-wheeling di-ode (Dp or Dn) are conducting. Theconverter is said (in power conver-(continuous current mode). Assuch the Vp and Vn nodes are ei-There is an equivalent circuit thatcan enlighten one’s understandingof the circuit’s operation. There areTwo are states with one FET on, onewith no FETs on and one with both FETs on. The equiva-lent circuit is composed of a three position switch whichcan either output +Vcc, ground or -Vcc to an output in-ductor of value L/2 where L=Lp=Ln. This equivalent cir-Sn switches are in the same state (on or off). If only oneswitch which is on. Balanced Current Amplifier voltage, the equivalent switch will always be at the groundbeing on or off with a 50% duty cycle. In other wordsTp=Tn=Ts/2 where Ts is the inverse of the switching fre-quency, the switching period. If Tp is the period that Spis on and Tn is the period that Sn is on, then Tp+Tn=Ts.When the output is programmed to be positive, the Spswitch. The width of Sp (Tp) increases as the width ofis still set to 100% (Tp+Tn=Ts). When the Sp switch ist

6 here be a case of having the Sn switch o
here be a case of having the Sn switch on and not theSp switch. Therefore when producing a positive output,erating frequency.When a negative output is to be produced, there will notthe output will be a unipolar negative double frequencyThe suggested equivalent circuit reveals the low outputripple current occurs at zero output, the very state whichsignals. The effective output frequency is doubled with-out operating any switch at double frequency. Bridgedoutput circuits are commonly understood to double theobtain a doubling of frequency. When the BCA’s outputstages are used in a full-bridge, the output ripple fre-channels in phase quadrature which allows the balancedmonaural output to have a 1MHz ripple frequency, yetgiven effective output pulse has two modulation deci-sions (one on each edge), the effective control samplingeffective rate rises to 2Ms/S.of the Crown BCA arepower converter de-plored in the largertronics. A classificationdescribing most cir-cuits. The Crown BCA’spower converter.Crown has chosen to Positive-Programmed Output Balanced Current Amplifier designs of power converters. OCA is the acronym forOpposed Current Am-short for OpposedCurrent InterleavedAmplifier. The OCIAconverters have theerty that conv

7 erterwith increased con-verter size. As
erterwith increased con-verter size. As addi-are paralleled, theincreasing propor-switches. This re-low-pass filtering re- switching signal fromcurrents simultaneouslyease in frequencywhile decreasing in am-plitude, the filtering re-quirements are multiplyrelaxed.many balanced currentpower stages are inter-leaved, total ripple re-at zero signal. Addi-intermediate to full Positive-Programmed Output Negative-Programmed Output 6 Balanced Current Amplifier 1998 Crown International, Inc.BCA™ and Balanced Current Amplifier™ are trademarks and Crown isa registered trademark of Crown International, Inc. D designs which use interleaving there are (N+2)/2 ripplenulls. Thus N/2 more ripple nulls result from OCIA de-Power converters that use OCIA design principles canbe used for both inverter and rectifier applications. Thisis the natural consequence of being a full four-quadrantpower converter. Much more could be said, however itstate-of-the-art amplifier design. Negative-Programmed Outputstandard amount of resultant information, it has two high-speed comparators driven from three signals, Verr, -Verrand Vtr. The same triangle waveform source can be usedPO Box 1000 Elkhart, IN 46515-1000FAST FACTS. 800-294-4094www.crownaudio.com 7