%$Id: tanh.tex,v 25.95 2006/08/26 01:26:53 al Exp $ -*- LaTeX -*-
% man behavior tanh .
% Copyright (C) 2001 Albert Davis
% Author: Albert Davis <aldavis@ieee.org>
%
% This file is part of "Gnucap", the Gnu Circuit Analysis Package
%
% This program is free software; you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation; either version 2, or (at your option)
% any later version.
%
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this program; if not, write to the Free Software
% Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
% 02110-1301, USA.
%------------------------------------------------------------------------
\section{{\tt TANH}: Hyperbolic tangent transfer function}
%------------------------------------------------------------------------
\subsection{Syntax}
\begin{verse}
{\tt TANH} {\it gain limit}\\
{\tt TANH} {\it args}
\end{verse}
%------------------------------------------------------------------------
\subsection{Purpose}

Defines a hyperbolic tangent, or soft limiting, transfer function.
%------------------------------------------------------------------------
\subsection{Comments}

There is no corresponding capability in any SPICE that I know of, but
you can get close with POLY. 

For capacitors, this function defines {\em charge} as a function of
voltage.  For inductors, it defines {\em flux} as a function of
current.

For fixed sources, it defines voltage or current as a function of
time, which is probably not useful.

This function describes a hyperbolic tangent transfer function similar 
to what you get with a single stage push-pull amplifier, or a simple
CMOS inverter acting as an amplifier.
%------------------------------------------------------------------------
\subsection{Parameters}

\begin{description}

\item[{\tt GAIN} = {\it x}] The small signal gain at 0 bias.  (Required)

\item[{\tt LIMIT} = {\it x}] Maximum output value (soft clipping).
(Required)

\end{description}
%------------------------------------------------------------------------
\subsection{Example} 

\begin{description}

\item[{\tt E1 (2 0 1 0) tanh gain=-10 limit=2 ioffset=2.5 ooffset=2.5}] 
This gain block has a small signal gain of -10.  The input is
centered around 2.5 volts.  The output is also centered at 2.5 volts.
It ``clips'' softly at 2 volts above and below the output center, or
at .5 volts ($2.5 - 2$) and 4.5 volts ($2.5 + 2$).

\end{description}
%------------------------------------------------------------------------
%------------------------------------------------------------------------
