Hi all,

For a project, I'm trying to amplify a very low signal. From around 100uV to 1V. So I'm looking into making a 3 stages amplifier with gain = 100 each.

Now I've tried with an LM741 but it saturates at such a gain (it work well with gain = 10 but it's too low and I don't want to use more than three stages... because noise + offset etc). I've been told about instrumentation amplifiers and others but I'm looking for a simple solution and parts that'll be easy to find - just a replacement for the LM741 that wouldn't saturate at G=100.

Any suggestions?

Thank you!

 

Is this a DC signal?
If not, what is its frequency?
Where do you live, so I know what parts to recommend?

 

AC signal @60 hz.

 

For a project, I'm trying to amplify a very low signal. From around 100uV to 1V. So I'm looking into making a 3 stages amplifier with gain = 100 each.

Now I've tried with an LM741 but it saturates at such a gain (it work well with gain = 10 but it's too low and I don't want to use more than three stages... because noise + offset etc). I've been told about instrumentation amplifiers and others but I'm looking for a simple solution and parts that'll be easy to find - just a replacement for the LM741 that wouldn't saturate at G=100.

I think you have done a miscalculation. 1 volt divided by 100 uV is 10,000. Three stages with a gain of 100 each is 1,000,000. This is why the 741 saturates.

Try lower gains on the three stages (about 20 each) or use two stages with gains of 100 each.

 

You're totally right, it'll be two stages of gain = 100. I tried only one stage with G=100 and it saturates so there's no way I can do that with two stages.

 

You're totally right, it'll be two stages of gain = 100. I tried only one stage with G=100 and it saturates so there's no way I can do that with two stages.

I dont see why the amplifier saturates with a gain of 100. What power supply voltages are you using?

 

What do you mean by "saturate"? A steady DC output at near either the positive or negative rail? A square wave that looks like a heavily clipped sine wave? Something else? Without a schematic we have no clue, and without an image of the output waveform (hand sketches are fine), we'll still be guessing 20 posts from now.

ak

 

Well I tested the opamp with a sinewave of a 1mV amplitude. I mounted the opamp as an inverter with G = 100 and the output signal is clipped on the top. When I lower the gain, I get a perfect sinewave.

The clipped signal looked like this:

So I figured the amp couldn't take a gain of 100.

 

I don't think the image uploaded correctly so I'll just post the direct link: http://www.hostingpics.net/viewer.php?id=900992IMG20161109150339.jpg
http://www.hostingpics.net/viewer.php?id=900992IMG20161109150339.jpg
Thanks

 

And here's a little schematic http://img15.hostingpics.net/pics/868963achematic.jpg

 

The AC signal needs to be AC-coupled into the amplifier, or the input offset voltage will cause your problems.

https://www.circuitlab.com/circuit/z7vfwk/ac-coupled-op-amp-non-inverting-amplifier/

Also, this circuit needs a capacitor between C2 and GND. This prevents offset error from being amplified.

ak

 

You only need AC gain so you can AC couple and reduce the DC gain to avoid saturation from offsets at high gain.
The simplest is to put a large capacitor (≥10μF) from the bottom of R1 to ground.
That way the AC gain is unaffected but the DC gain is reduced to 1.

Note that the 741 is a noisy op amp and may be a problem at such high gains.
You should go to a lower noise op amp, such as an OP07.

 

Thank you Analog & Crutschow, I appreciate the help. I'll be trying a different op-amp tomorrow + add a capacitor to the schematic and I'll let you know how it went.