Op Amp Trouble 2nd Stage...
Op Amp Trouble 2nd Stage...
(OP)
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!!





RE: Op Amp Trouble 2nd Stage...
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Op Amp Trouble 2nd Stage...
RE: Op Amp Trouble 2nd Stage...
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Op Amp Trouble 2nd Stage...
RE: Op Amp Trouble 2nd Stage...
RE: Op Amp Trouble 2nd Stage...
Is this for school?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Op Amp Trouble 2nd Stage...
WCPGW???
RE: Op Amp Trouble 2nd Stage...
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Op Amp Trouble 2nd Stage...
RE: Op Amp Trouble 2nd Stage...
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.
RE: Op Amp Trouble 2nd Stage...
Zahid
RE: Op Amp Trouble 2nd Stage...
RE: Op Amp Trouble 2nd Stage...
Keith Cress
kcress - http://www.flaminsystems.com