STUDIO

:AUDIO(- )INPUTTRITR2AUDIOOUTPUTAUDIO( +)INPUTTAILCURRENTVAMPLIFINPIERBIAS UTFIG 3: LM3080 operational transconductance amplifierQ for a given distortion level. SNR is improved inturn, but still remains around the level of cassettereplay systems (circa 60 dB). And without inputattenuation, fairly unnacceptable levels of THDpersists for signals over 70 mV (-20 dBu). Alltold, NSC's 3080, 13700 and related chips fromRCA were a godsend for the synthesisers andconsumer -grade audio of the '70s and early '80s,but not much else.In higher performance circuits, based on BarryGilbert's classic `current -ratioing' (or `current -steering') transconductance cell', non -linearity isreduced by driving the active devices with currentonly. Figs 6 and 7 illustrate typical circuits. Theimmediate trade -offs are added complexity, andthe need for multiple, matched transistors (T1 toT4). The key is to keep the ratio of the currentsin the cell's transistor -pairs constant and equal tothe corresponding pair of external currents underall conditions. In effect, the cell transistors needto be dynamically matched, a multi -dimensionalheadache. The linear input range is expandedwith a mixed bag of techniques like offsetting,pre -distortion networks, cross -coupling, basestoppers and emitter degeneration. Achieving lowTHD then hinges on the cell transistors' life beingkept as constant as possible over a scale ofcollector currents that's as broad as the desiredcontrol range, ie 10' for 100 dB. Subject to designfinesse, transconductance cells of this genre canexhibit respectable audio specifications.rThey are3k11i156kf7.particularly noted for wide bandwidth irrespectiveof gain or attenuation, and good isolation betweenaudio (x) and the control signal (y).In audio VCA parlance, good isolation isdescribed as low `control feedthrough'. The effectis a deviation from 0 v in the DC level at theVCA's output, usually referred to in millivolts.When defined with suitable reference to ZOL (ieDISTORTION vs DIFFERENTIALINPUT VOLTAGE100V515VD1..di0.01FIG 4: LM13700 operational transconductanceamplifier with linearising diodesM11111111..1R( = 10Kf1nnnn IIABC 1mAIN1111__UII/ /E11111 IKT 1ABC+,OUTVIN(PEAK (nE.`snnusac:'iiii'ABC -,OUTí wo 11momnB11.i11111111i1111P111111-M"::::: ":: -FAUNMOM 4'Ó IáAIIIM1111111.: :=I IM..IIIi1Ì 2 Inu .unn1MmI iM11111111111111111111M1111111110 100DIFFERENTIAL INPUT VOLTAGE (rnVpp)1000FIG 5: Operationaltransconductance amplifier withand without predistortion+15Vdc_15.6kROUTPUTas minus so many dBs) it helps the designeridentify VCAs which don't go thump, pop, or clickwhen the gain shift is rapid, which can beimportant in processors, and is vital forautomated muting, but less so for automatedfaders. In the debit column, current -ratioingtranscondtictance VCAs are primarily attenuatòrsor dividers. They have to be tricked intoproducing gain, and in turn, their performancesuffers. Ultimately, their clean operating range isconstrained by imperfections in the cell'stransistors, notably finite base -emitter resistance,VBE mismatch and differences in the saturationcurrent of individual transistors, as well aslimitations in the topology, leading to inadequatecommon -mode gain at the input'.Log -antilogequationBeyond variable transconductance cells, there aremany other ways of implementing VCAs. Most ofthen are now obsolete, since audio isn't the onlyactivity that can benefit from topologies thatprovide wide dynamic range, bandwidths beyond20 kHz, and low distortion or 'error'. When thesefactors take precedence, there's only one otherspecies in the race. The circuit in Fig 8 exploitsthe almost perfect logarithmic relationshipbetween a bi -polar transistor's base -emitterRSw501,(7 12Okf1OUTP,D6.5VTR3TR4X2 Ti Y25Okf7)(¿2DIFFERENTIAL AUDIO INPUTY1,2DIFFERENTIAL CONTROL INPUTI47kf724kf71TR,, TR, TRATR, TRS MATCHEDTRANSISTOR ARRAY6kR15 ddc0.3mÁ+02mÁVS-15VFIG 6: Four -quadrant variable transconductancemultiplierFIG 7: Four- quadrant variable transconductancemultiplier87