Hi there -
I recently came across this article on building a diode function generator.
They talk about a relatively simple circuit, with two zener diodes facing each other, in the feedback loop of an inverting op-amp.

Since I'm not very good at the math side of this, I was hoping someone could help me understand what is happening.

My understanding is that as you approach the zener value, you slightly attenuate the voltage, so that it's not a hard "limiting" function, but rather a smooth, eased-into limiting. Is that accurate?

Apparently, when the zener voltage is reached, the gain drops to (||)/. I'm not sure what || means, though and I'd apprecite if someone could explain it in terms of this calculation.

 

The two vertical lines are a shorthand notation for parallel. When the diodes present a "low impedance", then Ra || Rf. In other words, Ra will be in parallel with Rf. One thing you know about resistors in parallel is that the total resistance will be strictly less than either of the individual resistances. A lower resistance in the feedback path means a lower gain.

 

Below the voltage where the diodes conduct, the gain is Rf / Ri.

After the diodes start conducting the gain is Rp / Ri, where Rp is the parallel value of Rf and Ra.

 

Hi and thank you, @Papabravo ..just to make sure I understand: if Rf = 100k and Ra = 10k, Rf||Ra = 9.1k?

 

Hi and thank you, @Papabravo ..just to make sure I understand: if Rf = 100k and Ra = 10k, Rf||Ra = 9.1k?

Yes,
\( R_f||R_a\;=\;\cfrac{100K\Omega\times 10K\Omega}{100K\Omega + 10K\Omega}\;=\;9.0901K\Omega \)

There will be small deviations due the finite resistance of the diodes.

 

Here's a simulation of the waveform with 5V Zeners:

 

This is a logarithmic function circuit.


https://en.wikipedia.org/wiki/Log_amplifier