## 4.13 `TANH`: Hyperbolic tangent transfer function

### 4.13.1 Syntax

`TANH` *gain limit*

`TANH` *args*
### 4.13.2 Purpose

Defines a hyperbolic tangent, or soft limiting, transfer function.
### 4.13.3 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 *charge* as a function of
voltage. For inductors, it defines *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.
### 4.13.4 Parameters

`GAIN` = *x*- The small signal gain at 0 bias. (Required)

`LIMIT` = *x*- Maximum output value (soft clipping).
(Required)

### 4.13.5 Example

`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).