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#1
02-03-2010, 06:24 AM
 kachung30 New Member Join Date: Feb 2010 Posts: 6
HOW TO design antilog/log circuit?

l need to do multiplier by OP-AMP. But l don't know suitable ic value . ie transistor, resisitor ,etc
thank you
#2
02-03-2010, 10:03 AM
 t06afre Senior Member Join Date: May 2009 Posts: 5,939

I have an old book from Burr Brown at home. I shall see if I can find it. But you should be aware of that is many problems with such circuits. As an example they do not work very well at low inputs level. What are your trying to make. It could that is better solution to your problem.
#3
02-03-2010, 01:44 PM
 kachung30 New Member Join Date: Feb 2010 Posts: 6

I am trying to design an voltage multipliers to calculate output voltage. output is a^b

Last edited by kachung30; 02-03-2010 at 01:54 PM.
#4
02-03-2010, 01:55 PM
 Papabravo Senior Member Join Date: Feb 2006 Location: Michigan, USA (GMT-5) Posts: 5,848

As you are no doubt aware, multiplication is a non-linear operation. The construction and use of non-linear circuits have numerous problems including the need to scale the output to prevent saturation.

The use of a logarithmic amplifier is straight forward, but the inverse operation of raising to a power has the problems of saturation and output scaling.

Check application notes from National Semiconductor for ideas on these beasties.
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#5
02-03-2010, 05:10 PM
 MikeML Senior Member Join Date: Oct 2009 Location: AZ27 Posts: 1,461

some light reading. Analog Devices makes log, anti-log, multiplier, and divider ICs.
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#6
02-03-2010, 05:17 PM
 jans123 Junior Member Join Date: Jan 2010 Location: Sweden Posts: 20

Quote:
 Originally Posted by Papabravo As you are no doubt aware, multiplication is a non-linear operation. The construction and use of non-linear circuits have numerous problems including the need to scale the output to prevent saturation. The use of a logarithmic amplifier is straight forward, but the inverse operation of raising to a power has the problems of saturation and output scaling. Check application notes from National Semiconductor for ideas on these beasties.
Right, National was my "tutor" many years ago, long before the application note http://www.national.com/an/AN/AN-311.pdf was published. Look at it, it might give some hints.
#7
02-03-2010, 05:39 PM
 steveb Senior Member Join Date: Jul 2008 Location: New England, USA Posts: 2,431

Quote:
 Originally Posted by kachung30 I am trying to design an voltage multipliers to calculate output voltage. output is a^b
What frequency bandwidth, (or response time) do you need? If you need high speed (MHz for example) you will likely need an analog approach as discussed above. To avoid the difficult design work, you can buy an existing chip, but they are expensive.

However, if you don't need high bandwidth, you can use a cheap microprocessor with built in A/D and D/A converters. All types of nonlinear operations (including multiplication) can be easily handled in programming.
#8
02-03-2010, 06:05 PM
 KL7AJ E-book Developer Join Date: Nov 2008 Location: North Pole, Alaska Posts: 1,546

Might also look at a Gilbert cell. These are true multipliers, but they have limited dynamic range, so need careful signal conditioning beforehand.

Gilbert cells can be made with six transistors.

eric
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#9
02-04-2010, 05:08 AM
 kachung30 New Member Join Date: Feb 2010 Posts: 6

The op-amp must be used uA741.and l want to know the model of diode and value of resistor.
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#10
02-04-2010, 12:33 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

As long as you are using obsolete op amps, you can use the great old National Semiconductor op amp circuits collection - www.national.com/analog/amplifiers/application_notes- select AN-31

Quote:
 l want to know the model of diode and value of resistor
Just about any old diode will work. Try a 1N914. The resistance value has to do with the end point of the input voltage. As the current through the resistor has to be matched by current through the diode, a reasonable resistance will keep the current below the output current limit of the op amp at the upper range of the input voltage.

Be aware that the log response of your circuit is temperature dependent. For analog computation to have any repeatability, you will need to use something like the op amp collection circuit with temperature compensation. Notice there is an analog multiplier circuit in there, too.
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Last edited by beenthere; 02-04-2010 at 12:42 PM. Reason: replaced bad link

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