Regarding the discussion of the use of non-linear amplifiers for
compression of seismic output.

I have tried a circuit which uses a feed-back amplifier with multiple
diodes and resistors in the feedback path.  If germanium diodes are
used, the output can be switched smoothly at each .5 volt point to any
gain desired.  It can be used for both posative and negative signals.
In a feedback instrument this cannot be in the feedback to the
instrument, otherwise the system stability is affected.  It must be in a
separate output channel.

The circuit is any opamp with the primary feedback resistor having
direct connection and chosen to give the sensitivity needed for small
signals.  In parallel with this is a pair of diodes in parallel back to
back which are in series with a smaller resistor.  This smaller resistor
defines a smaller gain when the diodes conduct in either direction.  The
diode pairs can be continued with ever smaller resistors to define other
switching points and scale factors.  The output is a smooth curve in
both polarities.

The probem is that the diode resistance change is not sharp, so that the
output curves must be converted by use of a curve approximation in a
calculation or table look-up if precision is desired.

Has anyone tried something similar?

George Harris



_____________________________________________________________________

Public Seismic Network Mailing List (PSN-L)