TANH: Hyperbolic tangent transfer function

## 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).