Hyperbolic tangent transfer function

I answered somewhere else in this thread, the LM194 would be one of these dual NPN, with a very low rbb, high beta and very low offset. It would certainly make a very close approximation of the TANH function at the proposed 100uA current through the long tailed pair. The differential input Win proposed will IMHO not improve the curve, but allow for a higher input voltage. Since for values over 10Vt the output is 0.99991, we can put a limiter in front to allow for higher input voltages.

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ciao Ban
Bordighera, Italy

True, but an isolated signal ground allows better following of small inputs. That's one purpose. Oft-times the common- mode voltage can usefully be negative, that's another purpose.

Good. Increasing the output resistors by 10x will also help increase the common-mode range, shown above.

With respect to using this scheme as a precision nonlinear attenuator for large signals to increase dynamic range, offset voltage errors in the input pair can have a dramatic degrading effect when attempting to calculate the original voltage from the attenuated output signal - small shifts in apparent input voltage correspond to large changes in the actual voltage for large voltages. E.g., near zero volts a 1mV shift corresponds to 1mV, but at 120mV it corresponds to a much higher voltage.

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 Thanks,
    - Win

In article , Ban wrote: [snip]

You can make the collector voltages more favourable by swapping the collector-opamp connections. As below.

+-----+---o | | +Vcc .-. .-. | | | | 1k_ | |10k| | +--|___|--+ '-' '-' | | | | | |\\ | +-----)------------+----|-\\ | | | | >-+---o | +------------+----|+/ Vout | | | |/ Vin |/ \\| .-. o------|Q1 Q2|--+ | |1k |>