analogoperational-amplifier
While studying analog circuits and conducting experiments with op-amps, I couldn't help wondering why would I need an analog circuit to add/multiply/integrate electrical signals. In modern times, computers are quite cheap, and it seems a lot easier letting a computer calculate anything you want (it's fast, versatile, reliable…)
So, why use op amps and analog circuits?
Don't expect to build analog computers.
They are mainly useful for custom reconfigurable analog filters & some schemes with opamps, that's it.
Can't suggest any resources except datasheeds on your chips :-)
If your variable is constrained to be > 0 you can use a simple log-antilog circuit which would likely cost a lot less than a purpose-built multiplier chip. For example, this one in an older TI app note.
For the transistors you can use matched duals which cost less than 20 cents each, and the amplifier could be a quad op-amp.
In this case E2 would be your input voltage. and E1 and E3 can be connected to a reference voltage.
Best Answer
The moral of the story is digital electronics need an interface to the outside world. Analog electronics are necessary to get the outside world's signals into a form that can be digitized. For example, how do you get that \$\frac{50\mu V}{degree }\$ from a thermocouple into a signal large enough to put into an ADC? Use an instrumentation amplifier.
Here's an example of something I built a while back:
It implements the equation \$V_{out}=1.552V_{in}-0.000560V_{in}^3\$ That is an approximation for sine shaping, the and circuit itself shaped the triangle wave into a sine wave with less than .05% THD. It could have been done in the digital domain, but:
the input signal was \$20V_{pp}\$, way to big for immediate conversion without some kind of zero and span, and
the signal was already analog and it would have made no sense to convert to digital just to convert back to analog.
On the subject of taking signals digital, an analog anti-aliasing filter is essential before any ADC. This filter is just a low pass filter to ensure \$f_{sig}<f_{Nyquist}\$. That has to be done in the analog domain. Solid analog circuitry is essential to the operation of any embedded system.