198 Chapter 7
For very low noise systems, operating at circuit impedance levels that have been
deliberately chosen to be as low as practicable—such as in moving coil PU head
amplifi ers—bipolar junction transistors are still the preferred device. These will either
be chosen to have a large base junction area or will be employed as a parallel-connected
array, as, for example, in the LM194/394 “ super-match pair ” ICs, where a multiplicity
of parallel-connected transistors are fabricated on a single chip, giving an effective input
(noise) impedance as low as 40 ohms.
However, recent designs of monolithic-dual J-FETs, using a similar type of multiple
parallel-connection system, such as the Hitachi 2SK389, can offer equivalent thermal
noise resistance values as low as 33 ohms and a superior overall noise fi gure at input
resistance values in excess of 100 ohms.
At impedance levels beyond about 1 kilohm there is little practical difference between
any devices of recent design. Earlier MOSFET types were not so satisfactory because
of excess noise effects arising from carrier-trapping mechanisms in impurities at the
channel/gate interface.
7.12 Output Voltage Characteristics ..............................................................................
Since it is desirable that output overload and signal clipping do not occur in audio
systems, particularly in stages preceding the gain controls, much emphasis has been
placed on the so-called “ headroom ” of signal handling stages, especially in hi-fi
publications where the reviewers are able to distance themselves from the practical
problems of circuit design.
While it is obviously desirable that inadvertent overload shall not occur in stages
preceding signal level controls, high levels of feasible output voltage swing demand the
use of high voltage supply rails, which, in turn, demand the use of active components that
can support such working voltage levels.
Not only are such devices more costly, but they will usually have poorer performance
characteristics than similar devices of lower voltage ratings. Also, the requirement for
the use of high voltage operation may preclude the use of components having valuable
characteristics, but which are restricted to lower voltage operation.