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Old 19th February 2014, 07:36 AM   #1
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Default Opamp Having Oscillation

Hi everyone,

I am running off a single rail 12V and playing around with this type of differential output design, but the the opamp got oscillated badly. a SAW oscillation wave at 250kHz +/-. The opamp is 4558. What's actually goes wrong

P.S. I tried to ground both of the input, but it still oscillate.
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Last edited by chchyong89; 19th February 2014 at 07:42 AM. Reason: Adding more sign of the problem
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Old 19th February 2014, 08:20 AM   #2
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This thing looks to be an oscillator by design. You are taking the output signal, inverting it, and feeding it back amplified a lot. If that's not an oscillator I don't know what is.

Look up BTL outputs again.
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Old 19th February 2014, 08:26 AM   #3
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Somewhat, it just simulate well with results.
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Old 19th February 2014, 08:58 AM   #4
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Err, uh, I think I just got the negative and postive feedback stuff excatly the wrong way round... My bad.

Anyway, a feedback setup like this, as neat as it looks in simulation, is potentially prone to parasitic oscillation. What do you use for rail decoupling in the practical circuit? The simulation gives you perfect, stiff supplies and ground after all, which is not necessarily the case in practice...
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Old 19th February 2014, 09:10 AM   #5
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Running an op-amp open loop inside a feedback loop is not good design. Sometimes simulation does not capture all parameters...
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Old 19th February 2014, 09:13 AM   #6
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Remove R7 and R8 and connect the - input of U2 to the outout of U2.
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Old 19th February 2014, 09:17 AM   #7
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There are many connection errors. You should give up the U2 because, as you draw schematic, +U1 and +U3 should be connected to GND. In this case, U2 makes no sense.
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Old 19th February 2014, 09:19 AM   #8
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Quote:
Originally Posted by Mooly View Post
Remove R7 and R8 and connect the - input of U2 to the outout of U2.
This doesn't work, actually I intended to make it looks like a fully differential opamp. Like Opa1632 etc...
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Old 19th February 2014, 09:21 AM   #9
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U2 is a virtual ground generator. Its just not connected correctly. Alter it as I suggest above and it should all work correctly.
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Old 19th February 2014, 09:23 AM   #10
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If you want made fully differential opamp, then give up your schematic and try to develop something like this:
Click the image to open in full size.

Later Edit: @Mooly, U2 wants to be a virtual ground generator but can not perform this function if is powered unbalanced. So, you need a supply voltage like + /-15Vdc, not only +12Vdc.
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Last edited by donpetru; 19th February 2014 at 09:28 AM.
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