Author Topic: Amplifier help!  (Read 2738 times)

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Offline CommanderbobTopic starter

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Amplifier help!
« on: July 13, 2008, 04:38:48 PM »
I have a PIR sensor that I am messing with but the output only changes from ~650mv to 700mv. I need to amplify this to 0-5V. How would I do this? I could not find much on amplifiers. I did manage to get it to be 0-5V with four NPN transistors but I would like a better solution. Is there an IC or something that can boost the output by say 1000x?
Thanks,
Justin

Offline izua

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Re: Amplifier help!
« Reply #1 on: July 13, 2008, 06:22:10 PM »
You're looking for an opamp. 741 is the most common one.
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Offline Afroman

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Re: Amplifier help!
« Reply #2 on: July 13, 2008, 09:20:41 PM »
Another common one that performs a lot better than the 741, is the LM324.  You can find one in any radio shack.
You will want to configure it as a non-inverting amplifier
http://www.play-hookey.com/analog/non-inverting_amplifier.html
It won't get you 0-5V, but with a gain of ~5 it'll probably give you a more useable operating range.
« Last Edit: July 13, 2008, 09:22:04 PM by Afroman »

Offline CommanderbobTopic starter

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Re: Amplifier help!
« Reply #3 on: July 13, 2008, 11:49:36 PM »
Can you put two amplifiers after one another to boost it even more?

EDIT:The LM324 has 4 opamps. You could get quite a bit more out of it right. If each made the signal 5x larger then you would have, after four stages, 625x the original right?

Thank a lot for the help,
Justin
« Last Edit: July 14, 2008, 12:19:38 AM by Commanderbob »

Offline izua

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Re: Amplifier help!
« Reply #4 on: July 14, 2008, 01:25:44 AM »
you could make a single opamp with a gain of 625, since they have huge gains, in the range of 10000-100000
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Offline CommanderbobTopic starter

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Re: Amplifier help!
« Reply #5 on: July 14, 2008, 01:32:48 AM »
OK. Thanks a lot for the help. I am going to go with a 741 as it is a lot smaller then the 324. I'll get it tomorrow.
Justin

Offline CommanderbobTopic starter

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Re: Amplifier help!
« Reply #6 on: July 14, 2008, 05:50:31 PM »
OK, new problem. I got the Op Amps. I am using the 324 as it can run off 5V. The problem is the input from the sensor is around 500mv-750mv. Not 0v-250mv. So what happens when I try to amplify it to get any reasonable change the op amp already maxes out. Is there a way to amplify only the middle of the signal?

I don't know if this makes since. So let me see if I can explain it a little better. The input from the sensor right now is 700mv-800mv. I need to amplify the middle 50mv. So from 725mv to 775mv. Any ideas?
Thanks,
Justin

Offline izua

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Re: Amplifier help!
« Reply #7 on: July 14, 2008, 06:19:45 PM »
I'm not exactly sure how it's done, but you're supposed to put the middle value to one of its inputs. Since (I assume) you're using an inverting opamp, you are biasing it towards ground.
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Offline CommanderbobTopic starter

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Re: Amplifier help!
« Reply #8 on: July 14, 2008, 06:58:51 PM »
I don't understand your post. Here is the problem, I need a large gain say 150x for a good signal. What happens is since the lowest value is 0.7V I always get the maxed out voltage as 0.7V*150=105V. It works fine with a little gain and I can get the signal to change about 0.5V, but I would like a couple of volts.
Thanks,
Justin

Offline izua

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Re: Amplifier help!
« Reply #9 on: July 14, 2008, 08:08:00 PM »
You need to get more into opamps to understand what I ment. The opamp returns you the difference between its two inputs, multiplied by gain. One of your inputs is referenced at ground. You want your lowest value to be ground.
So, you need to take from somewhere 0.7v (hint hint resistor network) and reference your signal to this value. So, for example, signal goes to +, reference goes to -.

This way, when your signal will be at 0.7v, the difference between your signal (0.7v) and your reference (0.7v) will be 0, and that's what your opamp will output. From this point, the gain sets the maximum point.
« Last Edit: July 14, 2008, 08:08:51 PM by izua »
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Offline CommanderbobTopic starter

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Re: Amplifier help!
« Reply #10 on: July 14, 2008, 10:31:40 PM »
Ok, I think I got it. How does this look?

Thanks again for the help,
Justin

EDIT: Is it normal for the op-amp to max out at 3.8V when it is powered by 5?
« Last Edit: July 14, 2008, 11:22:00 PM by Commanderbob »

Offline izua

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Re: Amplifier help!
« Reply #11 on: July 15, 2008, 06:41:11 AM »
yes. you will need either a large supply voltage, or an rail-to-rail opamp. sadly, 741 is not rail to rail.
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Offline CommanderbobTopic starter

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Re: Amplifier help!
« Reply #12 on: July 15, 2008, 10:27:41 AM »
Thanks a lot everything is working great. I hope to have it tracking my hand today!
Justin

 


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