Op Amp Trouble 2nd Stage... 3

[crthompson]

I am trying to amplify an extremely small signal up to around 5VAC to eventually drive a relay. The front end amplifier is a differential amp. The output of the differential amp is a very clean sine wave around 560mVAC. After this, I try to achieve a gain of 10 through an inverting amp. After I put the second stage in the circuit, my signal goes away... I have also tried a non-inverting amp with a gain of 10 and I get the same result. As soon as I remove the second stage amp, my signal returns. I'm sure that I must be missing something. The op amp I am using is a TL084CN quad amp. I am only using two of the amps. I have moved the second stage around and tried it on the remaining amps with the same results. I have swapped the entire quad amp and I get the same results... Any help is greatly appreciated!! Thanks ahead of time!!

 

[IRstuff]

How are you AC coupling this?

TTFN
faq731-376

 

[crthompson]

I am bringing the signal directly into the first amp. The initial signal is a 5 microvolt 60 cycle signal.

 

[IRstuff]

How did you confirm that there is no DC offset in your output?

TTFN
faq731-376

 

[crthompson]

The output of the first amp? I looked at my waveform with reference to ground on an oscope. The sine wave crossed zero exactly half way between + & - amplitudes...

 

[crthompson]

I need to clarify my last post... I had no offset when the scope was set for AC coupling. I did have an offset when the scope was set for DC coupling. Is this a problem?

 

[IRstuff]

Didn't they teach you in school that an amplifier amplifies both AC and DC? Particularly if it's DC coupled?

Is this for school?

TTFN
faq731-376

 

[MagicSmoker]

Hmmm... let's see. OP wants to amplify a *60Hz* signal in the microvolt range using an op-amp with a Vio spec of something like 6mV.

WCPGW???

 

[MacGyverS2000]

X-10...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[crthompson]

First of all, I am (obviously) not an electronics expert... This is the first time that I have ever had to design something like this from scratch. This is not for school. This is an unfinished project at work that has been assigned to me. I did learn in my electronics class that DC can be amplified as well as AC... What I didn't realize is that the oscilloscope was filtering the dc component. @MagicSmoker: I only switched to this op amp after I ran into a similar problem with the instrument amp(INA126) I was using... The TL084CN gave me roughly the same output as the instrumentation amp. This output was proportional to my input. I think what I need to do first is get rid of my offset with a couple of capacitors and then take it from there... Am I at least headed in the right direction now? Thanks for the help!

 

[MagicSmoker]

OP: where are you located? It makes a big difference, believe me. No practicing engineer in North America would try to amplify a 5uV 60Hz signal (unless, maybe, he was trying to make a touch-operated switch) because 60Hz signals are ubiquitous in the environment here. If you really need to amplify a 60Hz signal that is *not* related to the AC mains here then you need to use a differential amplifier, preferably an instrumentation amp. Like the one you stopped using.

Next you need to divide your gain among more stages, with AC coupling between each one, so that the input offset voltage from each stage doesn't swamp its output. Think about it: the DC gain applies to both your 5uV signal *and* the 6mV of input offset. If you multiply by 100x you get 600mV of offset and 500uV of signal. AC couple to the next x100 stage and you get 600mV of offset again but now your signal is 50mV. AC couple again, etc.

Or choose a better op-amp to begin with!

You were also exceeding the gain-bandwidth product for the TL084, which is 2.5MHz. This means you can theoretically multiply a 2.5MHz signal by 1, or a 250kHz signal by 10, or a 25kHz signal by 100... at a gain of 100,000 your signal has to be 25Hz or less (and that's for a sinusoidal signal, not a trapezoidal to square one).

Finally, this is really basic stuff which one need not be an electronics expert to know. Not trying to be harsh, just pointing out that when you sign up here with "electrical" as your classification you shouldn't be asking questions like this.

 

[Gehr]

Dear CRThompson, all you require is AC coupling. The DC offset saturates your driven stage.
Zahid

 

[crthompson]

Sorry it has taken me so long to get back to everyone. Things have been very busy around here lately and I haven't had much of a chance to work on this project but I just wanted to thank everyone and let you know your suggestions were very helpful and solved my problem. I now have my circuit doing what I wanted it to do. I went back to the instrumentation amp, added an AC coupling capacitor to the output of the Instr. amp, provided a DC path to ground, and after that I was able to amplify my signal to the limits of the TL084CN quad amp. Again, thank you all for the very helpful advice and education.

 

[itsmoked]

Thanks for the feedback. Glad you got it working.

Keith Cress
kcress -

http://www.flaminsystems.com