@Roff: My bad, sorry. I misread your first post and I see what you're saying now. I was taking the steps ("log of the inverse") too literally without actually understanding completely what I was doing. You have just made my life a great deal easier actually
Thank you! I do know about most of the limitations and expectant input conditions of the circuits to keep them stable. Though there might be another forum thread on this in the future if\when I get stuck
This next part actually goes outside the scope of the example. The purpose of the example was to show that some operations that might appear trivial, are actually quite complex. It was never supposed to get into the actual circuitry side. But now curiosity has me and Im famous for going off on tangents in my department.
The first question is about the adding part then. Would I have to negate ln(dx/dt) before the summing opamp? Or could I get away with applying ln(dx/dt) to the inverting input and ln(dy/dt) to the non-inverting input?
The second questions is what would actually happen if I tried to ln a negative number in this environment? Its very tempting to haul out my copy of ngspice and see except Im still learning how to write and use netlists.
Third and last question, which Id probably end up posting at some point after Id started simulating and breaking the simulation. How do you find the absolute value of a voltage? I wont be able to +√(x²) as, in all likelyhood, the circuit will exceed Vmax under certain circumstances. And to be honest at 0130 AM... Im not thinking straight. Could I use a variation of a bridge rectifier? 'variation' because Id have to keep the ground common wouldnt I? Perhaps at this time of morning I should just ask if I could do it with diodes...
Just thought of another question...sorry. If I take the absolute value of the inputs... how would I preserve the sign of the output? both of you got me thinking when you mentioned '4-quadrant' and 'log of a positive number'...
@crutschow: The textbooks Ive read over the last 4 months have all been published between 1955 - ±1977 back when this stuff was more widely known and used. Ive only just come across Hans Camerzind's book available from
Designing Analog Chips by Hans Camenzind recently. If I remember it was published in 2000 or 2004. So my knowledge to date is apparently out of date and limited to the atomic functions. But since a small part of the dissertation is about designing analogue circuits, I think that the atomic building blocks are important. I believe I should try understand them and their limitations. But it will be useful to add that there are these pre-designed IC's. Thank you for the heads up on this! To make an analogy to computers, people ask me why I program in Assembler when I have tools like C# (a 4th generation programming language) available to me. I have more control and understanding of the environment for which Im creating a program, tweaking and optimizing that program thus become easier and faster.
Sorry for the rambling but this is generally how I think. Some of those questions above Ill have to sort out for myself, I just thought of them and had to write them down before I forgot them.
Thank you, both of you, for your replies. You have definitely answered the original post by now, and more!