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1/x function with op amps or multiplier or something else...

Well, a voltage doubler rectifier (one capacitor, two diodes) into ground is a frequency-current converter, so it only takes that and a transimpedance amp for its load.

A ramp-up/reset-at-V oscillator can be made to do the voltage-to-period part. A comparator, one-shot, current source, timing cap will do (can't use the '555 direct, it has the wrong thresholds).

Big problem is, with variable frequency, your lowpass filter has to deal with the worst case... which limits the input range. 0,0 V_in wasn't ever gonna work, anyhow.

Reply to
whit3rd
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See...

I leave it as an exercise for the student to convert the output current from C4 to the appropriate voltage needs of your system ;-)

Note that node IN is essentially a virtual ground, considering your loose tolerance requirements... so you could drive your incoming

0.4-2.5V thru a resistor into this node.

Note also that there is no multiplier in a feedback loop issue/hang... both OpAmps are used simply to force nodes C1 and C3 to be zero.

AND: It's temperature compensated ;-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

Cool circuit! MAT04AD? Hmm...I bet this shit is expeeeeensive...

Reply to
bitrex

I've never bought one, just designed them in many times.

Phil Hobbs would know the price... I don't think it's outrageous. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

You _could_ roll your own... the MAT-04 is just 4 matched transistors... just sort thru your stockpile of 3904's. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

Right, that's basically what I was talking about.

Except the 2N3904 idea....a 1K temperature error makes an 8% current error.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Even better, force the left-side diode...

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

So? Convert voltage into time:

- Create a periodic log ramp. Start with a sawtooth oscillator, make it linear, and feed that ramp to a log amp. Then take the derivative.

- Meanwhile, compare the input voltage to the ramp. This generates a delayed rising edge.

- Trigger a S&H by the edge. You now have a sampled 1/x function, with variable sampling proportional to the input.

Simply run it fast enough that you don't mind the steps from the S&H.

All told, this only needs a dual comparator, a quad op-amp, and a handful of transistors (a CCS, something for the log, and maybe a sampling JFET or analog switch), not too terrible.

:-)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

The MAT04, of course, is long gone. When it was still available, it cost more than a multiplier IC. Looking at AoE-III Table 8.1b, page 502, we see the only matched quad NPN is the THAT 300, about $10.

The Analog Devices AD633 "low-cost" multiplier IC is $9.15. My favorite AD734 costs $28. But oops, besides $$$, neither part can run on +/-5 volts.

We're again forced to regret that Barrie Gilbert and Analog Devices haven't chosen to update their accurate, medium-speed analog multiplier designs.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Their HQ is right down the street from me, I can go over on Monday and yell at 'em if you like...

Reply to
bitrex

Wonder how well matched the devices are in an MPQ2222?

But, as multipliers go, this application doesn't need an exotic multiplier. Wonder what the golden oldies MC1594-95-96 go for now-a-days? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

Here, see if this fits, I linked it where I can post attachments...

formatting link

Steve

Reply to
sroberts6328

My posting of...

is a bit obscure, given the scrambled pin-out of the MAT-04.

So I have drawn up an easier to follow schematic along with an explanation of how it works.

This explanation is on the S.E.D/Schematics Page of my website as "OneOverV_Explained.pdf" ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

Thanks, Steve!

I had forgotten all the CA parts from the past. The CA3086 should work just fine in my circuit.

And there's also the CA3046.

I have Spice models for those devices. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

"OneOverV_Explained.pdf", on the S.E.D/Schematics Page of my website, has been updated, showing it working with CA3046 NPN's. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

"OneOverV_Explained.pdf", on the S.E.D/Schematics Page of my website, has been further updated, showing it working with CA3046 NPN's, AND with voltage-input/voltage-output... with an added benefit, the voltage at the collector of Q4 is now held at zero volts... improving the matching. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

Thanks for all the responses, a lot of good info, it's going to take me a while to wrap my head around some of it! Dang smart people in this forum! Much thanks for the OneOverV pdf!

Reply to
Fibo

Fibo, You are quite welcome! (It was fun for me ;-) ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

formatting link
| 1962 | The touchstone of liberalism is intolerance

Reply to
Jim Thompson

Yes, I think the OP said the signals were dc or slow varying so a PWM based multiplier could be a very practical method and avoids all the tempco and matching problems of Vbe based log/exp.

piglet

Reply to
piglet

Here is a sketch of the concept I had in mind:

If a 2.5V or similar reference is available then the comparator PWM maker levels can be referenced to that and a wider duty cycle range obtained and less gain needed in the output servo - should improve the accuracy.

piglet

Reply to
piglet

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