# Signal multiplier

Discussion in 'The Projects Forum' started by Closedloop, Jun 27, 2014.

1. ### Closedloop Thread Starter New Member

Jun 27, 2014
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Hello guys, I'm working on a closed loop control. I need to take a signal and multiply it by itself as many times as I want (exponentiation) using an analog circuit.
Do you know if anything like this exists?
Thank you very much for your support

2. ### alfacliff Well-Known Member

Dec 13, 2013
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wouldnt that be called an amplifier?

3. ### wayneh Expert

Sep 9, 2010
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Do you mean you want to increase a DC voltage? That's called a variable gain amplifier. Depending on the specs you need, it could be as simple as a basic op-amp circuit. Note that your power supply has to provide the voltage you need at the output.

Oh, I missed the exponentiation part.

4. ### Closedloop Thread Starter New Member

Jun 27, 2014
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No guys I need more than a simple amplifier. I don't want to multiply the signal by a constant value but I want to multiply it by itself.
I'm sorry if the explanation wasn't clear

Dec 13, 2013
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6. ### Closedloop Thread Starter New Member

Jun 27, 2014
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Thank you for your answer alfacliff, i can probably use it for my purpose. Anyway what I was interested in knowing is if I could do something like this.

Take as input the signal X and generate as output X^33

7. ### crutschow Expert

Mar 14, 2008
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You do realize that with such a huge exponentiation factor you will rapidly reach near zero volts or saturate the output for any voltage that isn't very near 1V(?). Why on earth do you want such a huge factor?

Aug 21, 2008
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9. ### Brownout Well-Known Member

Jan 10, 2012
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log-1(logx*A) = x^A, right? So you need log/anti-log and an amplifier.

Last edited: Jun 27, 2014
10. ### Closedloop Thread Starter New Member

Jun 27, 2014
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@crutschow I need an algoritm that amplifies alot values far from 0 and less values near 0. Analysing my signal I realized that I need high factors to do that. Probably there are other ways to do it like e^(n*x) where x is my signal and n is a constant anyway i want to compare them.

@DickCappels I need to do a very fast closed loop control (microseconds), so A to D is not the best choice in my case

@Brownout that's very clever, exactly what I was looking for

Thank you all for your support guys

11. ### MrChips Moderator

Oct 2, 2009
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You need an anti-log circuit:

12. ### crutschow Expert

Mar 14, 2008
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Below is a simulation of MrChips' circuit. Note that the gain change is logarithmic with a large change is gain (bottom plot has a logarithmic scale). It takes about .34V In to give 10mV Out but .65V In gives 7.9V Out. I added an inverter stage so both input and output are positive.

Note that this only works for input positive signals.

If you want a little more gain at lower input signals you could add a resistor in parallel with the diode.

Is that sort of what you are thinking of?

13. ### Brownout Well-Known Member

Jan 10, 2012
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I think we decided we needed a chain: log amplifier - constant gain amplifier - anti-log amplifier to acheive the OP's required function.

However, be aware that these simple log/anti-log amps suffer sever temperature instability. Better to use intgegrated solutions, but they appear to be very expensive.

14. ### Closedloop Thread Starter New Member

Jun 27, 2014
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Exactly what I needed crutschow.
I'll build a test circuit and I will let you know the results.
Thank you everyone