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{solved} What's wrong with my op amp buffer??
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Beaufays



Joined: Dec 14, 2018
Posts: 4
Location: Belgium

PostPosted: Fri Dec 28, 2018 9:10 am    Post subject: {solved} What's wrong with my op amp buffer??
Subject description: something strange is going on with the simplest of opamp buffer circuits
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Hey,

I'm was building a buffered multi and noticed something strange on the outputs, so I went and breadboarded the simplest non-inverting buffer schematic, and look what I get... I'm at a loss here...I used tl072 and tl074 and both give the same output...
what am I doing wrong? is this because I buy the cheapest opamps on ebay? Confused


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buffer problem
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Last edited by Beaufays on Sat Dec 29, 2018 10:13 am; edited 1 time in total
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JovianPyx



Joined: Nov 20, 2007
Posts: 1988
Location: West Red Spot, Jupiter
Audio files: 224

PostPosted: Fri Dec 28, 2018 10:02 am    Post subject: Reply with quote  Mark this post and the followings unread

It looks like the positive portion of the signal is being clipped just above zero volts. Measure Vss and Vdd at the opamp power pins. It's possible that a bad connection prevents the full positive voltage from entering the opamp. I'm assuming that this circuit is driving nothing. If it's driving some circuit, disconnect the load and measure the waveform again.

I can't say that it won't or doesn't or can't happen, but a bad opamp is probably not the problem. If you have known good opamps, try one of those to be sure.

I won't buy electronic parts from e-bay. But then I always avoid e-bay because I just don't trust it. It is possible to get cheated, but your milage may vary.

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Beaufays



Joined: Dec 14, 2018
Posts: 4
Location: Belgium

PostPosted: Fri Dec 28, 2018 10:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey, yeah the circuit is not driving anything, but that doesn't seem to affect it anyway..
will measure power input, and buy some chips at the local shop to compare.
thx
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Grumble



Joined: Nov 23, 2015
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PostPosted: Fri Dec 28, 2018 4:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
It looks like the positive portion of the signal is being clipped just above zero volts.

That’s hard to tell because the scope is set at ac, so the voltage could be positive, negative or around 0volt.

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JovianPyx



Joined: Nov 20, 2007
Posts: 1988
Location: West Red Spot, Jupiter
Audio files: 224

PostPosted: Fri Dec 28, 2018 4:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

I agree that there's no way to guarantee that the OP set zero on the scope properly, however it is standard practice for some of us. There is most definitely clipping going on there. One of the simplest ways to account for clipping in this extremely simple circuit is for a supply line to be severely degraded with resistance or disconnect. It was only this intuition that caused me to suggest what I did. And I did not rule out a bad opamp, a blown output driver transistor could cause this as well.
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Beaufays



Joined: Dec 14, 2018
Posts: 4
Location: Belgium

PostPosted: Sat Dec 29, 2018 10:12 am    Post subject: Reply with quote  Mark this post and the followings unread

hey,

think I found the problem. setting the scope to DC showed me the OSC was oscilating at around 12V... hence the clipping...
thanks for helping look in the right direction! Wink
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