I have a circuit which converts it's input to the log of it's bipolar peak.

It uses Vbe matched small signal transistors, thermally bonded, as the
logging element. If kept within a thermal cocoon, it can exhibit an
accuracy of within 1dB over an 80dB range. This was something I built long
ago as a noise meter without range switching; the meter face spans an 80dB
range  which can be switched to give RMS, peak, or average responding.

I can prepare a schematic if anyone's interested.

Michael

At 07:23 PM 8/11/99 -0400, you wrote:
>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
>
>
>
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