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OpAmp help

 
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epp



Joined: 04 Mar 2004
Posts: 264
Helped: 8
Post14 Oct 2004 18:43   OpAmp help
Hello!
How could I get only positive pulses at opamp output? I mean Look at the picture attached. Supply of opamp are: +5V and -0 gnd.
At input V+ is sinus 18*sin(2pi*50*t) and at V- is DC 10V. I want positive output of opamp only when positive sinus i greater than 10VDC.
Thanks.

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flatulent



Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
Post14 Oct 2004 19:16   Re: OpAmp help
You are driving the amplifier beyond its allowable input range. You should use a comparator and then a resistive voltage divider to cut the input signal level to the allowable range specified for the comparator.
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nicleo



Joined: 06 Sep 2004
Posts: 717
Helped: 60
Post15 Oct 2004 1:10   OpAmp help
You can use 'half-wave' rectifier or a diode before the V+. So, only POSITIVE Sinewave will go into your opamp.
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devrimaksin



Joined: 13 Oct 2004
Posts: 93
Helped: 9
Location: Dallas, Texas, USA
Post15 Oct 2004 2:50   OpAmp help
I guess the DC input and the sinewave are the
two input of your opamp and the small square
wave is the output. The unexpected behavior
in the plot is that the opamp output goes to high
when the input pass through ground level (normally it shouldnt). This suggest that when you go below ground level, something going on within the opamp.
Is the input pair MOSFET?
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epp



Joined: 04 Mar 2004
Posts: 264
Helped: 8
Post15 Oct 2004 16:04   Re: OpAmp help
Thank you guys!
Finally, I used opamp TL082 and my problems disappeared. Smile)
After this I think that I would'n use AD opamps anymore.
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glenjoy



Joined: 01 Jan 2004
Posts: 1021
Helped: 47
Location: Philippines
Post15 Oct 2004 17:12   OpAmp help
I have a question here, his Vcc is +5V and Vee is 0V, how come is he expecting a negative output? His rails is only postive and returns to ground, therefore the ouput is only the difference of the two inputs of course amplified by the OP AMP's gain.
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