# Variable gain based on input

Discussion in 'The Projects Forum' started by trherzfeld, Jul 17, 2015.

1. ### trherzfeld Thread Starter New Member

Jul 2, 2015
8
0
Hello, I'm trying to replace a signal in a circuit with a different one that is much lower. So I was just going to amplify the signal at first. The problem is that there are different variable resistors changing the range and value of the signals differently, so my needed gain is different in several cases.

For example:

Having my pot set to "1" gives me a voltage of -0.035, which I need to amplify to 0.449, but having my pot set to "7" gives me a voltage of -0.065, which I need to amplify to 4.6 volts. The pot settings just correspond to the number on a dial, I was just using it as a reference. But as you can see, the gain needed is different depending on where the pot is. And those values can change because there are other variable resistors changing them.

So my question is, can I use a variable gain amplifier of some sort that is based on input voltage to do this? How would you guys go about this? Thanks so much for any help.

2. ### crutschow Expert

Mar 14, 2008
13,469
3,356
Are you looking for a logarithmic (or inverse logarithmic) type of gain response from the ampliier?

3. ### trherzfeld Thread Starter New Member

Jul 2, 2015
8
0
I attached a graph of the values I got.

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4. ### crutschow Expert

Mar 14, 2008
13,469
3,356
I don't see how that applies to your original question.

5. ### trherzfeld Thread Starter New Member

Jul 2, 2015
8
0
That's just a graph of the different gain values I need to obtain.

6. ### #12 Expert

Nov 30, 2010
16,656
7,296
Let's start the 20 questions game!
What kind of signal? What frequency range? What voltage range? What is the impedance of the source of the signal? What is the output supposed to accomplish? How accurate do you need it? How much voltage range is required of the output? How much current is required of the output?

(That leaves 12 questions available for follow up inquiries.)

AnalogKid likes this.
7. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292
Yes; it's called a Logarithmic Amplifier, or LogAmp. Check with Analog Devices web page for lots of great info on LogAmps