That is a very good question which I cannot answer until the circuit is built and tested.
I'm only presenting ideas for you to work with. Remember this is your homework, not mine.

 

That is a very good question which I cannot answer until the circuit is built and tested.
I'm only presenting ideas for you to work with. Remember this is your homework, not mine.

Absolutely! Thank you!

 

This might be a possible solution.

 

This might be a possible solution.View attachment 75125

Thank you! I will now simulate your suggestion...will take a while to play with...

 

I've never thought of this one before but I would start thinking about a transconductance amplifier.

Start by thinking about Ohm's Law.

I = V/R

If you can keep V constant but change R as a of function V

then I = k/V

Now it is relatively easy to convert I to an output voltage.

That was my first idea, however, I have little to no expierence with TA.... will keep the idea for later use!

 

On reflection I suspect there needs to be a unity gain buffer between Q2 & Q3 emitters. So unfortunately one more Op amp is needed.
It will be interesting to hear how your professor constructed the solution with just two amplifiers.

 

OK guys, I am gratefull to you all for your help,

here is one (tested in NI Multisim) solution:

Using AD633 (multiplier), according to the reference circuit in the datasheet for the AD633, 'Configuration for division operation'. Slightly adjusted to offer for a better linearity of the feed function. As the AD633 scales the output on 10V scale, the output will be Vout=-10*(Vy/Vx). If we keep Vy constant, i.e. =1V, we'll have exactly 1/x dependence at the output. The LOG-ANTILOG converters are already withing the AD633. It also provides Z input for the offset correction, Vy becoming actually a scaling factor.

The linearity of the AD633 is 0.5% at the best (the offset is finely tuned for the better scale).

Will now try to simulate other solutions you kindly offered!

Regards,
S.

 

A two op-amp solution possibly....???

 

A two op-amp solution possibly....???
View attachment 75178

Yes, Vi is always positive. Thank you!

 

A two op-amp solution possibly....???
View attachment 75178

Dear t_n_k,

Forgive me my ignorance, but can you explain further your circuit... Q1-Q2 + R set are there as a current source only? If so, can we use some specialized IC instead, t.ex. LM431? And then, the I ref should also provide some 'V out offset' regulation? Given Vi >0, and both the op-amps operting from bipolar supply (I prefer LM7101 for low offset), will this circuit allow Vi to approach Vground (=0) level to : a) true zero b) as cloase as op-amps allow (typ.15-20 mV) c) typical 'transistor diode' offeset (0.6V)?

Thank you!

 

Yes - the arrangement is somewhat basic biasing of Q1 & Q2 to preset Iref. Perhaps a constant current source might improve accuracy / linearity.
Keep in mind this idea is purely a thought experiment on my part. I've neither built nor simulated the circuit. Make of it what you may.

 

Yes - the arrangement is somewhat basic biasing of Q1 & Q2 to preset Iref. Perhaps a constant current source might improve accuracy / linearity.
Keep in mind this idea is purely a thought experiment on my part. I've neither built nor simulated the circuit. Make of it what you may.

Naturally.... thanks... the circuit appears to me a basic Log-amp (first op-amp), which is inverting as well. The second should be anti-log, if I am not mistaken. Thus, in terms of functions, we have LOG-Invert-Anti-LOG, right? But why you used differential amplifier (for a reason, which is?).

Regards, S.

 

Yes - you have the general idea.
Since this is homework I'll leave finer details for you to nut out.