of Microprocessors

BASIC ANALOG MODULES 207
For peak performance, it may be necessary to trim the resistors to better
match the diode array characteristic to the 3080 input characterisric.
Essentially this same predistortion circuit can also be found inside a
CA3280 dual OTA. The diode predistortion network is connected directly
between the + and - inputs and is floating; thus, it is not necessary to
ground one of the inputs as in the previous circuit. The output and control
structures are the same as the 3080, and the diode circuit has a variable bias
current control. This is accomplished by adding a terminal called ID, which
sinks current into the negative supply the same as Ie. In practice, the
magnitude of ID determines the average impedance of the diode network,
which, when combined with a series input resistor, determines the signal
level applied to the variable gain cell. The diode network impedance is 70/ID
ohms where ID is given in milliamps and is usually set in the 0.1 rnA range.
Note that the input series resistor value should be at least 20 times the diode
network impedance for best predistortion action. Since all of the input
components are inherently matched in the 3280, adjustment of the
predistortion circuit is not required, and output distortion can be expected to
be less. Noise level has also been reduced (the original 3080 was not
specifically designed for audio use) to less than 2 fLV rms, which makes
signal-to-noise ratios over 80 dB possible.
Gilbert Multiplier
Another predistortion circuit is shown in Fig. 6-18. This circuit is
termed a Gilbert multiplier after its inventor. Diodes D1 and D2 actually do
the predistortion and receive the input signal as currents, 11 and 12. For
greater convenience of use, transistors Q3 and Q4 convert a conventional
differential input voltage to the differential current required by the rest of the
circuit. The two resistors set the input voltage range, which can be made as
large as standard 5-V signal levels. The output is a differential current as
before. Performance is even better than the diode bridge predistorter, offering
an additional 6-dB improvement in noise level and distortion reduction
to O. 1%. Last but not least, the circuit automatically temperature compensates
the gain cell, making it essentially ideal.
Unfortunately, all of the components must be carefully matched to
obtain such good performance, normally a difficult task with discrete circuitry.
Recently, however, a linear IC having two of these circuits along with
3080-style differential-to-single-ended converters has been introduced by a
company appropriately named Solid State Music. Unlike the 3080, this IC
was designed specifically for audio VCA applications. Figure 6-19 shows a
simplified schematic of the IC, which at the time of writing is known by the
type number SSM 2020. The inputs can accept signals up to 5-V peak
directly, while the output is a current up to 1-mA peak. Note the inclusion
of two pairs of exponential converter transistors and even a temperaturecompensating
resistor for them.