184 Chapter 7
7.8 Input Switching .....................................................................................................
The comments made about input connections are equally true for the necessary switching
of the input signal sources. Separate, but mechanically interlinked, switches of the push-
on, push-off type are to be preferred to the ubiquitous rotary wafer switch, in that it is
much easier, with separate switching elements, to obtain the required degree of isolation
between inputs and channels than would be the case when the wiring is crowded around
the switch wafer.
However, even with separate push switches, the problem remains that the input
connections will invariably be made to the rear of the amplifi er/preamplifi er unit,
whereas the switching function will be operated from the front panel so that the internal
connecting leads must traverse the whole width of the unit.
Other switching systems, based on relays, or bipolar or fi eld effect transistors, have
been introduced to lessen the unwanted signal intrusions, which may arise on a lengthy
connecting lead. The operation of a relay, which will behave simply as a remote switch
when its coil is energized by a suitable DC supply, is straightforward, although for
optimum performance it should either be hermetically sealed or have noble metal contacts
to resist corrosion.
7.8.1 Transistor Switching
Typical bipolar and FET input switching arrangements are shown in Figures 7.13 and
7.14. In the case of the bipolar transistor switch circuit of Figure 7.13 , the nonlinearity
of the junction device when conducting precludes its use in the signal line; the circuit
is therefore arranged so that the transistor is nonconducting when the signal is passing
through the controlled signal channel, but acts as a short-circuit to shunt the signal path to
the 0-V line when it is caused to conduct.
In the case of the FET switch, if R 1 and R 2 are high enough, the nonlinearity of the
conducting resistance of the FET channel will be swamped, and the harmonic and other
distortions introduced by this device will be negligible (typically less than 0.02% at 1 V
rms and 1 kHz).
The CMOS bilateral switches of the CD4066 type are somewhat nonlinear and have a
relatively high level of breakthrough. For these reasons they are generally thought to be