Half-wave rectifier opamp in single supply

[MRW]

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!

 

[Eeyore]

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?

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

 

[MRW]

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!

 

[John Popelish]

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.

 

[John Popelish]

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.

 

[MRW]

That's 700 uA.

Thanks, John! I appreciate the additional insight.