OPA187: Simple buffer circuit with OPA187 does not work (voltage follower)

Bulent Kupelı

Part Number: OPA187
Other Parts Discussed in Thread: OPA171, TLV2186, OPA333, OPA388, OPA325, OPA191

Hi all, I have a question about that using opa187 as buffer. I designed a simple buffer circuit with opa187 but my circuit does not work regular. For example, my supply voltage is 5 volt for opamp and my input signal is 1 volt DC, I can see 1 volt in my output of opamp. Input signal is 2 volt, my output is 2 volt. Input signal is 3 volt, output is 3 volt. Input is 4, output is 4 but if my input signal is bigger than 4,2 volt, my output signal is happening 4,99 volt directly. This is my problem. Why can not I see 4,2 volt in my output of opa187 . If I make supply voltage of my opamp as 7 volt, I can see 4,2 volt in my output but I want to see it while supply is 5 volt. I try my circuit in simulation and also it is not work. In simulation, while my input is bigger than 4 volt, my output show 3,97 volt.I can not understand it.Generaly, at simple general purpose opamp, if you set simple buffer circuit, your output signal will be your input signal and its output range will be beetwen your opamp supply voltages. I set my circuit as thinking so but it does not work. Do you have used opa187 before ? and do you have seen this problem ? Can you help me ?

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Bulent,

you violate the common mode input voltage range:

Your input voltage must stay 2V below the positive supply voltage.

Better take a RRIO (input output rail-to-rail) OPamp. "Rail" here means "supply voltage" :-)

Kai

0 Raymond Zhang1 over 4 years ago in reply to kai klaas69

TI__Mastermind 48076 points

Hi Bulent,

Kai answered your question. If your input will go as high as 4V, you will need to have rail to rail op amp for the application.

Best,

Raymond

0 Bulent Kupelı over 4 years ago in reply to kai klaas69

Prodigy 160 points

Hi Kai,

Thank you for answer but I tried my circuit with opa171 and it works. opa187 and opa171 are rail to rail op amp and has same CMRR. Why opa187 does not work like opa171 ?. I can not understand this.

0 Bulent Kupelı over 4 years ago in reply to Raymond Zhang1

Prodigy 160 points

Hi Raymond,

Thank you for answer. OPA187 is rail to rail op amp.

0 Raymond Zhang1 over 4 years ago in reply to Bulent Kupelı

TI__Mastermind 48076 points

Hi Bulent,

Please see the follow up reply.

Best,

Raymond

0 Raymond Zhang1 over 4 years ago in reply to Raymond Zhang1

TI__Mastermind 48076 points

Hi Bulent,

I did not answer your question. My bad. The datasheet is ok. Here are the issues.

The input voltage range in OPA187 is from (V- -1) to (V+ -2), the output swing still can be Rail to Rail output, if there is a gain, see the image below. When you input 4V, it violates the common mode input voltage requirements, hence the op amp is no longer operating in a linear region.

In OPA187, the input is not rail to rail input, the output can only follow the input in an unity gain configuration. If a circuit has a gain of 2, e.g., then it will output the rail to rail, see the captured image and attached Tina simulation below.

/cfs-file/__key/communityserver-discussions-components-files/14/OPA187-E2E.TSC

Best,

Raymond

0 kai klaas69 over 4 years ago in reply to Bulent Kupelı

Guru 140200 points

Hi Bulent,

the OPA171 isn't working either:

Kai

0 Raymond Zhang1 over 4 years ago in reply to kai klaas69

TI__Mastermind 48076 points

Hi Bulent,

Thanks for Kai's added comments in OPA171. It is the same reason as in OPA187.

If you are looking for Rail-to-Rail Input and Output Op Amplifier (RRIO), please check out TLV2186.

If you have other questions, please let us know.

Best,

Raymond

0 Marek Lis over 4 years ago in reply to kai klaas69

TI__Guru 60610 points

Use instead RRIO OPA333, OPA388, OPA325, or OPA191.

0 Bulent Kupelı over 4 years ago in reply to kai klaas69

Prodigy 160 points

Hi Kai,

Thank you for answer. My circuit works with opa171 in real. I get the results that I want from my circuit. I have not tried my circuit with opa171 in simulation yet.

0 Bulent Kupelı over 4 years ago in reply to Raymond Zhang1

Prodigy 160 points

Hi Raymond,

Thank you for answer. My circuit is a simple voltage follower or non-inverting amplifier circuit.In voltage follower circuit, my problem is that when V+ is bigger than 4 volt in opa187,the output of opa187 is happening 4.99 volt in real. İf I try my circuit in simulation, when V+ is bigger than 4 volt, the output is happening 3.97 volt and the output does not change while voltage of V+ is between 4 and 5 volt. I try my circuit with opa171 and it works. Also, both of the opa187 and opa171 are rail to rail output.

0 Bulent Kupelı over 4 years ago in reply to Raymond Zhang1

Prodigy 160 points

Hi Raymond,

Thank you for suggestion.

0 Bulent Kupelı over 4 years ago in reply to Marek Lis

Prodigy 160 points

Hi Marek,

Thank you for suggestion.

0 Marek Lis over 4 years ago in reply to Bulent Kupelı

TI__Guru 60610 points

Your OPA171 circuit only seems to "work" because of your constant dc signal. But if you apply ac signal you'll see the distortion as shown below.

Violating the input common-mode voltage range decreases current biasing the input differential pair, Itail, which in turn results in a lower bandwidth (BW=~Itail), lower slew rate (SR~Itail/Cc) and much higher input voltage noise (Vnoise=1/Itail) where Cc is a Miller compensation cap inside the op amp.

Vcm Range.pptx

0 Bulent Kupelı over 4 years ago in reply to Marek Lis

Prodigy 160 points

Hi Marek,

The input signal is DC and it is smaller than 1 kHz for AC signal in my circuit. I think that the opa171 will work for my circuit and I dont think that the opa171 will cause problems for my circuit.

Thank you for information.

Bulent

0 kai klaas69 over 4 years ago in reply to Bulent Kupelı

Guru 140200 points

Hi Bulent,

believe me, you WILL get problems, in some way or other...

Kai

0 Bulent Kupelı over 4 years ago in reply to kai klaas69

Prodigy 160 points

Hi Kai,

Thank you for answer. We solved this problem. The op opamp should have both rail to rail input and output for voltage follower circuit. :)

Bulent

0 kai klaas69 over 4 years ago in reply to Bulent Kupelı

Guru 140200 points

Good decision :-)

Kai

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OPA187: Simple buffer circuit with OPA187 does not work (voltage follower)