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[SOLVED] division of two values with op amps

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d123

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Hi,

I've been doing a little searching for an op amp division circuit that has a numerator and a denominator and a "result" output. All I've found so far are things related to log anti-log circuits and this block diagram (it's the 'a' picture) and a multiplier/divider in the Op Amp Circuit Collection AN-31 (aka snla140c), neither are particularly what I am looking for.

multiplers and dividers block diagram.JPG

multiplier divider snla140c aka an31.JPG

The AN-31 multiplier/divider circuit seems to require (a*b)/c = d. I only want/need a/b = c and do not wish to modify a circuit to fit figure 70 as simulations are promising for my needs, so I do not want to stray from where I've reached so far in the design.

By any small chance is anyone familiar with an op amp-based circuit that meets this specific requirement? Or, worst case, to an experienced eye, does figure 70 look adaptable with a bit of patient tinkering? Or, is it theoretically as simple as making the input to 'e2' 0V i.e. (e1*(e2 = 0V))/e3 = eOut?

Thanks.
 
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Or, is it theoretically as simple as making the input to 'e2' 0V i.e. (e1*(e2 = 0V))/e3 = eOut?
Not e2=0, but e2 =1, respectively a suitable constant value.
 
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Not e2=0, but e2 =1, respectively a suitable constant value.

Lapse of reason there, op amps don't alter elementary mathematics :oops:

Thanks!
 

Hi,

I know some regulation circuits from the 1970ies with analog math...
Nowadays I'd use a digital soluton.

What's your idea behind using analog circuitry?

Klaus
 
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Hi Klaus,

I know some regulation circuits from the 1970ies with analog math...
Nowadays I'd use a digital soluton.

What's your idea behind using analog circuitry?

Klaus

No special reason, just another "the journey is as good as the destination and what you learn along the way can be very enlightening." I've had a brilliant idea ;) about how to measure that 1G resistor that would avoid (deludedly insane) attempts at making a 2nA current source at home...

I know a little code in e.g. Python would make this far quicker and easier but first I'd like to see how inaccurate the a) simulation results are, b) how inaccurate an actual circuit is compared to the theory. Unusual approach but whatever, keeps me out of trouble.

What do you mean you know some regulation circuits from the 70s with analog math?
 

Hi,

What do you mean you know some regulation circuits from the 70s with analog math?
I mean: analog math is outdated for most applications.

Klaus
 
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Hi,

Outdated meaning inaccurate and taking up unnecessary PCB space?

I'm finding the conceptual/design stage interesting because it helps to understand old mistakes in previous circuits with unrealistic expectations and with the little I understand about real circuits and components and especially user ability nowadays*, the "ideal" (and e.g. BJT identical) models in the simulator make a - for example - measurement circuit appear to be in theory far "better" than practice/reality would ever reproduce, especially in a home-brew circuit.

*Somewhere in their educational material, Bourns have a small section pointing out that a trained, experienced technician would take far less time to set a trimpot as required than a less experienced person. To that I could add that trimpots seem to have an elusive sweet spot that one could spend hours twiddling the screw slightly more clockwise, slightly more couterclockwise and back and forth again and again and never quite seem to get to the exact position/value searched for... - Which I feel is directly related to your comment(s) and the reason for exploring this circuit a little - it's good to try to consider the "invisible" problems (e.g. the "perfect" resistor was 10k before soldering it and then applying a voltage to it and then the temperature went up, and now it is certainly not the "10k" measured with a +-5% DMM) before wasting €50 and 50 hours on a pile of disappointing silicon junk.
 

Outdated meaning there are newer, better, more accurate, more stable methods.
 
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Hi,

In figure 70 multiplier/divider:

What's Z1 there for? Is it to hold Q1 and Q3 base voltages at a set level?

Why is it on the negative rail/Why is the cathode on ground?
 

The original AN30/AN31 multiplier/divider circuit uses ground potential for the respective transistors. Vcb = 0 should be fine for most log/antilog circuits,
 

I think of doing analog multiplication by starting with a given voltage (to be the first number), then applying gain (to be the second number) via transistor.

There are circuits which use a jfet to vary gain by changing bias voltage.

So the first number is a variable supply voltage, or else a resistance in the drain or source leg.
The second number is a voltage at the gate which adjusts gain.
 
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Hi,

Thanks, guys.

An-31 and the other one snla140c are not quite identical implementations and tried a single supply version of snla140c multiplier divider circuit but the simulator was not amused - 30mV output whatever the input... Thank goodness for the quick-fix part that is the 7660... And, saw yesterday that Zeners can be replaced by the TL431, which is great as the simulator agreed with the substitution and made no changes to the results with the TL431 as Zener replacement.

Anyway, did as you suggested, FvM, E2 is "1" and in simulations, get the result I would expect, more than acceptable/better than expected (so I'm subtracting 2% or so accuracy-wise for real-world version), may even make this in the end if the breadboard stage doesn't throw up horrors of reality grossly divergent from pretty simulation results.

Thanks, everyone.
 

Accuracy of log/antilog analog computing circuits is mainly limited by non ideal transistor characteristics, type variations and intra pair junction temperature differences. Increasing Vce above the necessary saturation margin will immediately increase thermal mismatching effects by larger transistor power dissipation, thus I wonder if the revised circuit is well considered.
 
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Hi FvM,

Preface: lengthy answer is not confrontational, intended to add to what I think you meant. It's well-considered because I just want to check simulation to reality differences for now.

Accuracy of log/antilog analog computing circuits is mainly limited by non ideal transistor characteristics, type variations and intra pair junction temperature differences.

Thanks, hadn't even considered such a point. Not expecting miracles here, just having a look to see if it's worth proceeding further (unlikely) to make an SMD proto board. Using matched dual soics (shame have no matched quad package here) and at present a TH quad OA.

Increasing Vce above the necessary saturation margin will immediately increase thermal mismatching effects by larger transistor power dissipation, thus I wonder if the revised circuit is well considered.

Increasing Vce above the necessary saturation margin... - it would take me a while or more of studying BJT basics (again) to even genuinely pretend to understand that in practical terms and apply it diligently.

thus I wonder if the revised circuit is well considered.

This is "Mr. hobbyist" we're talking about, so I think we both can assume not well considered nor well understood are givens.

In case you're interested, which I doubt much - from simulation to breadboard "rough sketch" of only the multiplier divider circuit:
V+ = 4.82V (should be +4V)
Using SOT-23 ref (LM4041) with 10k TH trimmer to get 1V for E1 and E2
TO-92 ref (TL431) a) downstream of TH TC7660 to buffer it (get -3.98V) and b) another in place of Zener (another meaning TL431)
two SOIC dual matched BJTS (NSS40301) and a TH quad OA (LMC6464)
replaced 1k resistors with 10k resistors (all measure between 9.91k and 9.93k) and 10R trim with 1k trimmer (measures a miserable 937R total)
Used 100k trimpot to get input from 0.99V to 1.01V (small range is the area of interest to measure/check a 1% 1GOhm resistor someday hopefully for another circuit where that +-1% would ruin low value capacitor measurements)
A few nasty-looking but unavoidable 5cm antenna wires here and there...

= Output/Result is surprisingly faithful to simulation but a couple of mV off in one direction.
Some of the nodal voltages are quite different to the simulator's nodal voltages.
Simulation is precise with 1V in +-100mV. Breadboard starts to be 30mV to far worse not far away from the 1V +-10mV range.

Conclusion: better than expected (thought it would be grossly different to simulation) (very unlike a discrete current/voltage source that was brilliant in simulations and verging on the criminal in reality and a waste of time and components); about as inaccurate as expected as well; needs a lot more exploring to see where discrepancies are coming from. Won't be going into mass production yet.

Not sure what you meant by "true potentiometer circuit" - Besides sounding suspiciously like "This will be right with just one, or two or three or more components/sub-blocks", do you mean a logarithmic potentiometer?
 

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