Half-wave rectifier opamp in single supply
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Posted by MRW on June 22, 2007, 4:27 pm
Please log in for more thread options Good day! I have the following circuit: http://www.bayimg.com/NabJOAABE Just looking at the configuration on paper, the voltage at R18's terminal next to D2 would have given me V2 if Vin < V2. I'm not quite sure why my spice simulation is showing 1.683V. Any insights? Thanks! | ||||||||||||||||
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Posted by Eeyore on June 22, 2007, 5:30 pm
Please log in for more thread options MRW wrote: Yes. The op-amps 2 input terminals aren't at the same voltage so for some reason the feedback's not working. Is it a rail-to-rail output op-amp ? Graham | ||||||||||||||||
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Posted by MRW on June 22, 2007, 5:43 pm
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wrote: > Yes. The op-amps 2 input terminals aren't at the same voltage so for some
reason
> the feedback's not working.
> > Is it a rail-to-rail output op-amp ? > > Graham Hi Graham, according to TI, it's supposed to be rail-to-rail: http://focus.ti.com/docs/prod/folders/print/tlv2252.html I decided to change the opamp, and I'm getting similar results on paper. Thanks! | ||||||||||||||||
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Posted by John Popelish on June 22, 2007, 5:45 pm
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MRW wrote: > Good day! I have the following circuit:
> http://www.bayimg.com/NabJOAABE > > Just looking at the configuration on paper, the voltage at R18's > terminal next to D2 would have given me V2 if Vin < V2. I'm not quite > sure why my spice simulation is showing 1.683V. Is V2 defined as 2.5 volts, somewhere? The output should go positive enough to supply current through D1 and R14, to ring the right end of R14 up to 2.5 volts. However, that takes (2.5-1.1)/1k= 700uA from the opamp output. Take a look at the data sheet: http://www.ee.siue.edu/~gengel/robotics/pdf/tlv2254.pdf page 27, figure 18. The output current limits with more than the whole supply dropped across the pull up fet before the current gets to 700 mA. Change the 2k resistors to 20k, and it should start working. | ||||||||||||||||
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Posted by John Popelish on June 22, 2007, 6:07 pm
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John Popelish wrote: > MRW wrote:
>> Good day! I have the following circuit:
>> http://www.bayimg.com/NabJOAABE >> >> Just looking at the configuration on paper, the voltage at R18's >> terminal next to D2 would have given me V2 if Vin < V2. I'm not quite >> sure why my spice simulation is showing 1.683V. >
> Is V2 defined as 2.5 volts, somewhere? > > The output should go positive enough to supply current through D1 and > R14, to ring the right end of R14 up to 2.5 volts. However, that takes > (2.5-1.1)/1k= 700uA from the opamp output. > > Take a look at the data sheet: > http://www.ee.siue.edu/~gengel/robotics/pdf/tlv2254.pdf > page 27, figure 18. > > The output current limits with more than the whole supply dropped across > the pull up fet before the current gets to 700 mA. That's 700 uA. > Change the 2k
> resistors to 20k, and it should start working. | ||||||||||||||||
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> http://www.bayimg.com/NabJOAABE
>
> Just looking at the configuration on paper, the voltage at R18's
> terminal next to D2 would have given me V2 if Vin < V2. I'm not quite
> sure why my spice simulation is showing 1.683V.
>
> Any insights?