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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 02-03-2010, 06:24 AM
kachung30 kachung30 is offline
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Default 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
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Old 02-03-2010, 10:03 AM
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t06afre t06afre is offline
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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.
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Old 02-03-2010, 01:44 PM
kachung30 kachung30 is offline
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I am trying to design an voltage multipliers to calculate output voltage. output is a^b

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Old 02-03-2010, 01:55 PM
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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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Old 02-03-2010, 05:10 PM
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some light reading. Analog Devices makes log, anti-log, multiplier, and divider ICs.
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Old 02-03-2010, 05:17 PM
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Quote:
Originally Posted by Papabravo View Post
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.
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Old 02-03-2010, 05:39 PM
steveb steveb is offline
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Quote:
Originally Posted by kachung30 View Post
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.
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Old 02-03-2010, 06:05 PM
KL7AJ KL7AJ is offline
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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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Old 02-04-2010, 05:08 AM
kachung30 kachung30 is offline
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The op-amp must be used uA741.and l want to know the model of diode and value of resistor.
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Old 02-04-2010, 12:33 PM
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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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